Abstract:
An image forming device includes a casing, an image forming arrangement, a supplying control unit, a temperature detecting unit, a warm-up control unit, and an image forming control unit. The image forming arrangement is provided in the casing and includes a medium passage and a motor configured to drive in response to an image-forming signal or a warm-up signal. The supplying control unit allows a recoding medium to be supplied in the medium passage in response to the image-forming signal, and prevents a recording medium from being supplied in the medium passage in response to the warm-up signal. The temperature detecting unit detects a temperature inside or around the casing. The warm-up control unit controls the motor to drive at a first speed in response to the warm-up signal if a temperature detected by the temperature detecting unit is equal to or higher than a first temperature, and to drive at a second speed lower than the first speed in response to the warm-up signal if a temperature detected by the temperature detecting unit is lower than the first temperature. The image forming control unit controls the motor to drive at a third speed in response to the image-forming signal if a temperature detected by the temperature detecting unit is equal to or higher than a second temperature, and to drive the motor at a fourth speed lower than the third speed in response to the image-forming signal if a temperature detected by the temperature detecting unit is lower than the second temperature.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2006-234179 filed Aug. 30, 2006. The entire content of each of these priority applications is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image forming device which has a motor capable of varying its driving load in accordance with temperature. 
         [0004]    2. Description of the Related Art 
         [0005]    In conventional various equipment having a motor for driving a load, such as an image forming device, the driving load increases due to thermal contraction of an object to be driven or increase in lubricant viscosity, in the case where such equipment is left at a low temperature. 
         [0006]    In an device which is assumed to operate at a low temperature, therefore, a large motor enough to obtain a required torque, or a motor driven by a large current is used, so as to obtain the driving load required at the lowest temperature to ensure operation. 
         [0007]    However, the torque, adjusted for the device to operate at a low temperature, is too high within a normal temperature range. This prevents the motor cost from being reduced, and the image forming device from being miniaturized. 
         [0008]    In response to this, taking advantage of the relation that a motor torque becomes higher as an RPM (revolutions per minute) decreases as shown in FIG. 3, Japanese Unexamined Patent Application Publication No. 06-121592 proposes that a motor is driven at a low RPM so as to compensate for insufficient torque occurred at a low temperature in the case where an operating temperature of the motor is lower than the preset temperature. 
         [0009]    However, in such a conventional device, where the operational speed of the motor varies simply with temperature, it is difficult for the device to form a steady image. One is because the device is not warmed up properly. Another is because the operational speed (RPM) of the motor changes at an inappropriate timing during image formation. 
       SUMMARY OF THE INVENTION 
       [0010]    In view of the above-described drawbacks, it is an objective of the present invention to provide an image forming device capable of varying the operational speed (RPM) of a motor in accordance with temperature at an appropriate timing, for steady image formation and efficient operation. 
         [0011]    In order to attain the above and other objects, the present invention provides an image forming device including a casing, an image forming arrangement, a supplying control unit, a temperature detecting unit, a warm-up control unit, and an image forming control unit. The image forming arrangement is provided in the casing and includes a medium passage and a motor configured to drive in response to an image-forming signal or a warm-up signal. The supplying control unit allows a recoding medium to be supplied in the medium passage in response to the image-forming signal, and prevents a recording medium from being supplied in the medium passage in response to the warm-up signal. The temperature detecting unit detects a temperature inside or around the casing. The warm-up control unit controls the motor to drive at a first speed in response to the warm-up signal if a temperature detected by the temperature detecting unit is equal to or higher than a first temperature, and to drive at a second speed lower than the first speed in response to the warm-up signal if a temperature detected by the temperature detecting unit is lower than the first temperature. The image forming control unit controls the motor to drive at a third speed in response to the image-forming signal if a temperature detected by the temperature detecting unit is equal to or higher than a second temperature, and to drive the motor at a fourth speed lower than the third speed in response to the image-forming signal if a temperature detected by the temperature detecting unit is lower than the second temperature. The first speed is equal to or different from the third speed. The second speed is equal to or different from the fourth speed. The first temperature is equal to or different from the second temperature. 
         [0012]    Another aspect of the present invention provides an image forming including a casing, an image forming arrangement, a supplying control unit, a temperature detecting unit, and a warm-up control unit. The image forming arrangement is provided in the casing and includes a medium passage and a motor configured to drive in response to an image-forming signal or a warm-up signal. The supplying control unit allows a recoding medium to be supplied in the medium passage in response to the image-forming signal, and prevents a recording medium from being supplied in the medium passage in response to the warm-up signal. The temperature detecting unit detects a temperature inside or around the casing. The warm-up control unit controls the motor to drive at a first speed in response to the warm-up signal if a temperature detected by the temperature detecting unit is equal to or higher than a first temperature, and to drive at a second speed lower than the first speed until a temperature detected by the temperature detecting unit has reached a second temperature equal to or higher than the first temperature in response to the warm-up signal if a temperature detected by the temperature detecting unit is lower than the first temperature. 
         [0013]    Another aspect of the present invention provides an image forming device including a casing, an image forming arrangement, a supplying control unit, a temperature detecting unit, a warm-up control unit, and an image forming control unit. The image forming arrangement is provided in the casing and includes a medium passage and a motor configured to drive in response to an image-forming signal or a warm-up signal. The supplying control unit allows a recoding medium to be supplied in the medium passage in response to the image-forming signal, and prevents a recording medium from being supplied in the medium passage in response to the warm-up signal. The temperature detecting unit detects a temperature inside or around the casing. The warm-up control unit controls the motor to drive at a first speed until a temperature detected by the temperature detecting unit has reached a second temperature in response to the warm-up signal if a temperature detected by the temperature detecting unit is lower than a first temperature lower than the second temperature. The image forming control unit controls the motor to drive at a second speed higher than the first speed in response to the image-forming signal after the warm-up control unit controls the motor to drive at a first speed until a temperature detected by the temperature detecting unit has reached a second temperature. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a side sectional view schematically showing the structure of a laser printer according to the present invention; 
           [0016]      FIG. 2  is a block diagram schematically showing the structure of a control unit; 
           [0017]      FIG. 3  illustrates characteristics of a motor; 
           [0018]      FIG. 4  is a flowchart showing a main process according to a first embodiment; 
           [0019]      FIG. 5  is a flowchart showing details of an operational speed setting; 
           [0020]      FIG. 6  illustrates the relation between the motor characteristics and a lowest temperature/a first temperature/second temperature; 
           [0021]      FIG. 7  is a graph showing an in-device temperature and a heat-up characteristic of an object to be warmed up; 
           [0022]      FIG. 8  is a flowchart showing a main process according to a second embodiment; 
           [0023]      FIG. 9  is a flowchart showing details of a warm-up control and a half speed warm-up control; and 
           [0024]      FIG. 10  is a flowchart showing a main process according to a third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The terms “upward”, “downward”, “upper”, “lower”, “above”, “below”, “beneath” and the like will be used throughout the description assuming that the ink jet printer is disposed in an orientation in which it is intended to be used. 
         [0026]    Hereinafter, embodiments will be described with reference to the accompanying drawings. 
       First Embodiment  
       [0027]      FIG. 1  is a side sectional view schematically showing the structure of a laser printer  1  as an image forming device according to a first embodiment. In the following description, the right side of  FIG. 1  is defined as the front side. 
         [0028]    &lt;Overall Structure of Laser Printer&gt; 
         [0029]    The laser printer  1 , which is a direct transfer tandem-type color laser printer, includes a substantially box-like main body case  3  as shown in  FIG. 1 . On the top surface of the main body case  3 , a discharge tray  5  on which sheets on which image formation has been formed is placed. 
         [0030]    The main body case  3  covers a feeder unit  20  which feeds a sheet on which an image is formed; a feed system  30  which further feeds the sheet fed by the feeder unit  20 ; an image forming unit  10  which forms an image on the sheet fed by the feed system  30 ; and a control unit  40  (see  FIG. 2 ) which drives and controls these components. Hereinafter, the feeder unit  20 , the feed system  30 , and the image forming unit  10  are collectively referred to as an “image forming system”. 
         [0031]    &lt;Feeder Unit&gt; 
         [0032]    The feeder unit  20  includes the following components: a feed tray  21  attached to a lowermost portion of the main body case  3  so as to be pulled out forward, on which a sheet on which an image is formed is placed; a pickup roller  22  disposed above the front end of the feed tray  21  so as to take the sheet out of the feed tray  21 ; a separation pad  23  which applies conveyance resistance to the sheet taken by the pickup roller  22  out of the feed tray  21  so as to separate one sheet from another; and feed rollers  24  and  25  which feed the sheet sent from between the pickup roller  22  and the separation pad  23 , to the feed system  30 . 
         [0033]    &lt;Feed System&gt; 
         [0034]    The feed system  30  includes the following components: a drive roller  31  which rotates in synchronization with the operation of the image forming unit  10 ; a follower roller  32  rotatably provided at a position far from the drive roller  31 ; and a conveying belt  33  extended between the drive roller  31  and the follower roller  32 . Driven by the drive roller  31 , the conveying belt  33  circulates in the counterclockwise direction in  FIG. 1  to convey the sheet mounted on its top surface rearward. 
         [0035]    A belt cleaner  34  for removing toner, paper powder and the like adhered to the conveying belt  33  is provided under the feed system  30 . 
         [0036]    &lt;Image Forming Unit&gt; 
         [0037]    The image forming unit  10  includes the following components: a scanner unit  60  which forms a latent image on the surface of a photoconductive drum  71  (to be described later); a process unit  70  which develops the latent image formed by the scanner unit  60  with toner, and transfers the developed image to the sheet fed by the feed system  30 ; and a fixing unit  80  which fixes the image transferred on the sheet by the process unit  70 . 
         [0038]    The process unit  70  includes four process cartridges  70 K,  70 Y,  70 M, and  70 C which perform development with toner corresponding to colors of black, yellow, magenta and, cyan, arranged from the upstream side, in the conveying direction of the conveying belt  33 . As is generally known, the process cartridges  70 K,  70 Y,  70 M, and  70 C have the same structure, including a photoconductive drum  71 , a charger  72 , a transfer roller  73 , and a developer cartridge  74 . 
         [0039]    The scanner unit  60  includes a laser generator, a polygon mirror, a lens, and a reflecting mirror. The scanner unit  60  applies laser beams L of each color based on predetermined image data, to the surface of the photoconductive drum  71  of the corresponding process cartridge at high speed so that a latent image is formed on the surface of the photoconductive drum  71 . 
         [0040]    The fixing unit  80  includes a heat roller  81  provided on the side of the printing surface of a sheet, which feeds a sheet while heating the toner transferred to the sheet; and a pressure roller  82  provided on the opposite side of the heat roller  81  across the sheet, which presses the sheet onto the heat roller  81 . The fixing unit  80  fixes the toner on the sheet by heat for fixation. The sheet which has passed through the fixing unit  80  is discharged to the discharge tray  5  through the discharge slit  7 . 
         [0041]    &lt;Control Unit&gt; 
         [0042]      FIG. 2  is a block diagram showing the structure of part of the control unit  40 , relevant to the present embodiment. 
         [0043]    As shown in  FIG. 2 , the control unit  40 , provided in a space between the scanner unit  60  and the process unit  70 , includes a thermistor  41  (see  FIG. 1 ) which detects the temperature inside the main body case  3  (hereinafter referred to as an “in-device temperature”). The control unit  40  further includes a feed system driving motor  42  which drives the various rollers of the feeder unit  20  as well as the drive roller  31  of the feed system  30 ; a polygon driving motor  43  which drives the polygon mirror of the scanner unit  60 ; a development driving motor  44  which drives the photoconductive drums  71  of the process cartridges  70 K,  70 Y,  70 M, and  70 C; a fix driving motor  45  which drives the heat roller  81  of the fixing unit  80 ; and a motor driver  46  which provides drives these motors with driving power. The control unit  40  still further includes a well-known microcomputer  47  which has a CPU, a ROM, and a RAM as its main components; and a signal processing unit  48  which processes signals from various sensors and units of the device including the thermistor  41  so as to transmit the signals to the microcomputer  47 , having an ASIC for generating various signals under the direction of the microcomputer  47  to transmit the signals to the motor driver  46  and other units. 
         [0044]    The signal processing unit  48  controls input to/output from at least the following components in addition to the thermistor  41  and the motor driver  46 : an operation panel  51  formed of a liquid crystal display panel which includes a cursor key and a switch for entering various settings and directions, and a menu screen for displaying various menu items, so as to display contents entered by a user and various notices to be given to a user; and a network interface (I/F)  53  which accepting a print job request via a communication network such as LAN from an external device (for example, a personal computer). 
         [0045]    As each of the motors  42  to  45  which drives the image forming system, as shown in  FIG. 3 , a stepping motor is employed, in which the torque of its output shaft decreases as the motor operational speed (RPM) increases. 
         [0046]    Hereinafter, the definition “normal speed” means an rpm at which a torque large enough to drive the load of the motor is obtained within its specified operation guarantee temperature range (10° C. to 32.5° C. in this embodiment, see  FIG. 6 ) (1240 rpm in this embodiment). The definition “half speed” means an rpm half of the normal speed (620 rpm in this embodiment). 
         [0047]    &lt;Operation Control&gt; 
         [0048]    Hereinafter, referring to the flowcharts shown as  FIGS. 4 and 5 , a description is given for a main process to be performed by the microcomputer. 
         [0049]    After the laser printer  1  is power-on, and then, each unit of the device is initialized, the main process is performed. 
         [0050]    As shown in  FIG. 4 , the main process starts with S 110  for setting the operational speed of the motor. 
         [0051]    As shown in  FIG. 5 , in order to set the operational speed, the thermistor  41  detects an in-device temperature T in S 310 . Next, it is determined in S 320  whether or not the in-device temperature T is lower than a temperature TL that is the lowest within a preset operating range. If the in-device temperature T is lower than the temperature TL (S 320 : YES), an error process for prohibiting the motor from driving is set in S 360 . Thus, the device is prevented from being damaged due to forced operation. 
         [0052]    In the error process, specifically, a notice is sent via the operation panel  51 , saying that the motor is prohibited from driving. After that, a standby state is maintained until a power-off or reset command is given via the operation panel  51 . 
         [0053]    On the other hand, if it is determined that the in-device temperature T is higher than the temperature TL (S 320 : NO), it is determined in S 330  whether or not the in-device temperature T is higher than a first temperature T 1 . The first temperature T 1  has to be set so that the drive capability obtained when the motor is driven at normal speed is sufficiently larger than the driving load at the first temperature T 1 . 
         [0054]    If it is determined that the in-device temperature T is higher than the first temperature T 1  (S 330 : YES), the operational speed of the motor is set to normal speed in S 340 . On the other hand, if it is determined that the in-device temperature T is equal to or lower than the first temperature T (S 330 : NO), the operational speed of the motor is set to half speed in S 350 . The operational speed setting is then finished. 
         [0055]    As shown in  FIG. 6 , the temperature TL is set to a value equal to or larger than a temperature THL (−2° C. in this embodiment) that is the lowest within temperatures at which the torque of the motor set to the half speed can drive the load. Also, the first temperature T 1  is set to a value equal to or larger than a temperature TNL (5° C. in this embodiment) that is the lowest within temperatures at which the torque of the motor set to the normal speed can drive the load. 
         [0056]    Return to  FIG. 4 , after the operational speed setting (S 110 ) is finished, a warm-up control is performed at a speed set in S 110  over a predetermined period of time in S 120 . 
         [0057]    The “warm-up control” is defined as “to operate all the available motors without forming an image on a sheet”. Specifically, this operation is performed so that the driving force of the feed system driving motor  42  is not transferred to the pickup roller  22  of the feeder unit  20 . The warming-up control is performed when power is turned on, or when the laser printer  1  returns to normal operating state from sleep mode in the present embodiment. The laser printer  1  is provided with a sensor (not shown) for detecting a recording medium fed in a feeding path. The microcomputer  47  controls the image forming unit  10  to form an image on the recording medium, only when the sensor detects the recording medium. 
         [0058]    In the “warm-up control” according to the present embodiment, following operations are performed: the heat roller  81  is heated to reach a predetermined temperature; the conveying belt  33 , the belt cleaner  34 , and the photoconductive drum  71  are driven to remove toner adhered to the conveying belt  33  and the photoconductive drum  71 ; toner accommodated in a toner accommodating chamber (not shown) is agitated; remaining amount of toner agitated is detected; a timing when an image should be formed is checked while driving the polygon motor. However, the pickup roller  22  is not driven in the “warm-up control”. Thus, the motors are driven without forming an image on the recording medium. 
         [0059]    The “predetermined period of time” when the operational speed is set to the half speed is longer than the “predetermined period of time” when the operational speed is set to the normal speed. Specifically, the “period of time” for half speed is longer than a value considerably larger than the “period of time”, that is obtained by a experiment, taken for increasing the in-device temperature T from the temperature T 1  to a second temperature T 2  equal to or higher than the first temperature T 1  by warming up. 
         [0060]    As shown in  FIG. 7 , the heat-up curve of the in-device temperature to be sensed by the thermistor  41  does not necessarily agree with the heat-up curve of an object (for example, toner) to be heated by warm-up control. In consideration of the difference, the second temperature T 2 , specifically, the period of time for half speed, have to be set so that the object is, without failure, heated up to a temperature higher than a desired temperature. 
         [0061]    On the other hand, the “period of time” for normal speed does not have to be set in consideration of the in-device temperature T, since the in-device temperature T is higher than the first temperature T 1 . Only in consideration of the time required for operation-checking each unit, a minimum length of time may be set therefor. 
         [0062]    After the warm-up control is finished, it is determined in S 130  whether or not a preset period of time has elapsed after either the last control of warm-up control in S 120  or print control in S 210  (to be described later) is finished. 
         [0063]    If the period of time has not elapsed yet (S 130 : NO), it is determined in S 140  whether or not a print job has been requested via a network I/F  53  or the like. If no print job has been requested (S 140 : NO), the process returns to S 130  so that S 130  and S 140  are repeated. 
         [0064]    If any print job has been requested (S 140 : YES), the same operational speed setting as in S 110  is performed in S 150  to reset the operational speed of the motor. The process then proceeds to S 210 . 
         [0065]    On the other hand, if it is determined that a predetermined time has elapsed (S 130 : YES), a sleep mode in which power supply is stopped except for units required for accepting a print job is set in S 160 . Subsequently, it is determined in S 170  whether a print job has been requested. If a print job has been requested (S 170 : YES), the sleep mode is cancelled in S 180 . Specifically, power supply to each unit is started so that print control becomes available. 
         [0066]    Then, in S 190 , the same operational speed setting as in S 110  is performed to reset the operational speed of the motor. Subsequently, in S 200 , in the same way as in S 120 , a warm-up control for driving the motor over a predetermined period of time at the speed reset in S 190  is performed. The process then proceeds to S 210 . 
         [0067]    In S 210 , the motor is driven at the speed reset in the operational speed setting performed in S 150  or S 190  to perform a print control for printing on one sheet according to the print job accepted in S 140  or S 170 . 
         [0068]    Subsequently, it is determined in S 220  whether or not all the printing requested by the accepted print job has been completed by printing of one sheet. If not (S 220 : NO), it is determined in S 230  that the present speed is set to which of the normal speed or the half speed. 
         [0069]    If the present speed has been set to half in S 230 , the process returns to S 150 , and an operational speed setting for resetting the operational speed of the motor is performed. A print control is then performed at the reset speed. 
         [0070]    On the other hand, if the present speed has been set to normal in S 230 , the process returns to S 210 , and a print control is performed immediately without resetting the operational speed. 
         [0071]    If it is determined that all the printing requested by the accepted print job has been completed (S 220 : YES), the process returns to S 130 , and a standby state is maintained until a next print job is requested. 
         [0072]    &lt;Effect&gt; 
         [0073]    As described above, in the laser printer according to the present embodiment, if the in-device temperature T at the time of the startup of warm-up control or print control is lower than first temperature T 1 , the motor is driven at half speed. Therefore, the motor torque, which has become relatively insufficient due to an increasing driving load caused by a low temperature, is compensated for, without employing a larger motor or a larger driving current. 
         [0074]    Also in the laser printer  1 , when power is turned on or sleep mode is cancelled, a warm-up control is performed at the operational speed set according to the in-device temperature T over a predetermined period of time. If the temperature does not rise sufficiently (does not reach the first temperature T 1 ), the motor is driven at half speed even for print control. Therefore, printing is started quickly without prolonging warm-up operation than required. 
         [0075]    Furthermore, when the operational speed is set to the half speed, the in-device temperature T is repeatedly detected every time a print control for one sheet is finished, thereby resetting the speed. Therefore, when the in-device temperature T becomes higher enough to drive the motor at the normal speed, the operational speed of the motor is set to the normal speed. Thus, the time required for printing becomes shorter without prolonging printing at the half speed than required. 
         [0076]    Furthermore, the operational speed of the motor is maintained to a fixed value during a period for printing one sheet. Therefore, a steady image is formed successfully without causing uneven printing in one sheet due to speed change. 
         [0077]    Also in the laser printer  1 , if the in-device temperature T is lower than the temperature TL, or equivalently, if the torque does not become sufficient for the driving load even when the motor is driven at half speed, the motor is prohibited from driving. Therefore, the damage caused by driving the motor under excessive load is prevented. 
         [0078]    In the present embodiment, the thermistor  41  is provided inside the main body case  3  so as to detect the in-device temperature T. However, the structure may be such that the thermistor  41  detects the temperature outside the main body case  3 , or equivalently, the ambient temperature of the laser printer  1 . 
         [0079]    The ambient temperature does not change drastically due to warm-up control and print control by the laser printer  1  under ordinary circumstances. Therefore, in the case where the ambient temperature is detected, the ambient temperature may be detected less frequently as well as speed resetting. 
         [0080]    In the present embodiment, in-device temperature T detection and speed resetting are repeated every time a print control for one sheet is finished, if the speed is set to half speed. If a plurality jobs are requested, the detection of the in-device temperature T and resetting the operational speed of the motor may be repeated every time one job is finished. 
       Second Embodiment 
       [0081]    Next, a description is given for a second embodiment. 
         [0082]    In this embodiment, a main process to be performed by the microcomputer  47  is partially different from the main process of the first embodiment. Therefore, a description is given mainly for the difference. 
         [0083]    As shown in  FIG. 8 , the main process starts with S 400  for setting the operational speed of the motor. Note that the operational speed setting is the same as in S 110  according to the first embodiment. 
         [0084]    Subsequently, a warm-up control is performed in S 410 . As shown in  FIG. 9 , for the warm-up control, firstly, it is determined in S 510  whether the operational speed set in S 410  is normal or half. If the operational speed is normal, the motor is driven at normal speed only over a predetermined period of time for warming up in S 520 . The warm-up control is then finished. 
         [0085]    On the other hand, if the operational speed is half, the motor starts to be driven at half speed for warming up in S 530 . Subsequently, in S 540 , the thermistor  41  detects the in-device temperature T. 
         [0086]    Then, it is determined in S 550  whether or not the detected in-device temperature T is higher than the second temperature T 2  (set to be equal to or higher than the first temperature T 1 ). If equal to or lower than the second temperature T 2  (S 550 : NO), it is determined in S 570  whether or not an allowed time has elapsed after starting the warm-up control in S 530 . 
         [0087]    The first temperature T 1  and the second temperature T 2  are set in the same way as in the first embodiment. The allowed time is set, for example, to a value approximately twice to three times as large as an average of time that the in-device temperature T takes to reach the second temperature T 2  from the lowest temperature TL (3 minutes in this embodiment) when the motor is driven at half speed for warming up. 
         [0088]    If it determined that the allowed time has not elapsed (S 570 : NO), the process returns to S 540  so as to repeat the above procedure. On the other hand, it is determined that the allowed time has elapsed (S 570 : YES), an error process is set to prohibit the motor from driving in S 580 . The error process is the same as in S 360  of the first embodiment. 
         [0089]    If it is determined that the in-device temperature T is higher than the second temperature T 2  (S 550 : YES), the warm-up control is finished in S 560 . 
         [0090]    As described above, if the operational speed is set to normal, that is, the in-device temperature T has already been equal to or higher than the first temperature T 1 , the warm-up control is performed at normal speed only over a predetermined period of time. On the other hand, if the operational speed is set to half, the warm-up control is performed at half speed until the in-device temperature T becomes higher than the second temperature T 2 . Thus, the object to be warmed up is heated up to a value equal to or higher than the first temperature T 1  without failure. However, if the in-device temperature T does not reach the second temperature T 2  even though the allowed time has elapsed, the motor is prohibited from driving. 
         [0091]    Return to  FIG. 8 , after the warm-up control (S 410 ) is finished, it is determined in S 420  whether or not a preset period of time has elapsed after either the last control of warm-up control in S 410  or print control in S 490  (to be described later) is finished 
         [0092]    If the period of time has not elapsed yet (S 420 : NO), it is determined in S 430  whether or not a print job has been requested via the network I/F  53  or the like. If no print job has been requested (S 430 : NO), the process returns to S 420  so that S 420  and S 430  are repeated. If any print job has been requested (S 430 : YES), the process proceeds to S 490 . 
         [0093]    On the other hand, if it is determined that a predetermined time has elapsed (S 420 : YES), a sleep mode in which power supply is stopped except for units required for accepting a print job is set in S 440 . Subsequently, it is determined in S 450  whether a print job has been requested. If a print job has been requested (S 450 : YES), the sleep mode is cancelled in S 460 . Specifically, power supply to each unit is started so that print control becomes available. 
         [0094]    Then, in S 470 , the same operational speed setting as in S 400  is performed to reset the operational speed of the motor. Subsequently, in S 480 , in the same way as in S 410 , a warm-up control is performed. The process then proceeds to S 490 . 
         [0095]    In S 490 , the motor is driven at normal speed to perform all the requested print controls according to the print job accepted in S 430  or S 450 . The process then returns to S 420 , and a standby state is maintained until a next print job is requested. 
         [0096]    &lt;Effect&gt; 
         [0097]    As described above, in this embodiment, if the in-device temperature T at the time of power-on or sleep mode cancellation is lower than first temperature T 1 , the motor is driven at half speed. Similarly to the first embodiment, the motor torque, which has become relatively insufficient due to an increasing driving load caused by a low temperature, is compensated for, without employing a larger motor or a larger driving current. 
         [0098]    Also in the laser printer  1  according to this embodiment, a warm-up control is continued at half speed until the in-device temperature T becomes equal to the second temperature T 2 . In performing a print control, therefore, the load to be driven by the motor is reduced sufficiently to a value light enough to be driven by the torque obtained at normal speed without any difficulty. 
         [0099]    In the laser printer  1  according to this embodiment, the motor is stopped driving if the warm-up control at half speed is not finished even when the allowed time has elapsed. When the in-device temperature does not reach the value of the second temperature T 2  even if any number of warm-up control are performed, a warm-up control is prevented from being prolonged wastefully. 
       Third Embodiment 
       [0100]    Next, a description is given for a third embodiment. 
         [0101]    In the present embodiment, a main process to be performed by the microcomputer  47  is partially different from the main process of the above embodiments. Therefore, a description is given mainly for the difference. 
         [0102]    As shown in  FIG. 10 , the main process starts with S 610 . In the S 610 , a warm-up control is continued at half speed until the in-device temperature T becomes higher than the second temperature T 2 . In this half speed warm-up control, specifically, the procedure from S 530  to S 580  defined by alternate long and short dash lines is to be performed. 
         [0103]    Subsequently, it is determined in S 620  whether or not a preset period of time has elapsed after either the last control of warm-up control in S 610  or print control in S 680  (to be described later) is finished. 
         [0104]    If the period of time has not elapsed yet (S 620 : NO), it is determined in S 630  whether or not a print job has been requested via the network I/F  53  or the like. If no print job has been requested (S 630 : NO), the process returns to S 620  so that S 620  and S 630  are repeated. If any print job has been requested (S 630 : YES), the process proceeds to S 680 . 
         [0105]    On the other hand, if it is determined that a predetermined time has elapsed (S 620 : YES), a sleep mode in which power supply is stopped except for units required for accepting a print job is set in S 640 . Subsequently, it is determined in S 650  whether a print job has been requested. 
         [0106]    If a print job has been requested (S 650 : YES), the sleep mode is cancelled in S 660 . Specifically, power supply to each unit is started so that print control becomes available. Subsequently, in S 670 , the same half speed warm-up control is performed as in S 610 . The process then proceeds to S 680 . 
         [0107]    In S 680 , the motor is driven at normal speed to perform all the requested print controls according to the print job accepted in S 430  or S 450 . The process then returns to S 620 , where a standby state is maintained until a next print job is requested. 
         [0108]    &lt;Effect&gt; 
         [0109]    As described above, according to the laser printer  1  of this embodiment, a warm-up control is continued consistently at half speed until the in-device temperature T becomes equal to the second temperature T 2 . In performing a print control, therefore, the load to be driven by the motor is reduced sufficiently to a value light enough to be driven by the torque obtained at normal speed without any difficulty. 
         [0110]    According to the laser printer  1  of this embodiment, a warm-up control is performed consistently at half speed, thereby simplifying the control procedure. 
         [0111]    Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention. 
         [0112]    In above-described embodiments, the feeder unit, the feed system, and the image forming unit are collectively referred to as the image forming system. However, the image forming system may includes another unit that operates in the image forming operation. 
         [0113]    Further, in the above-described embodiments, the laser printer is used as the image forming device. However, the image forming device only have to include a motor for operate at least one of the feed unit, the feed system, and the image forming unit.