Patent Publication Number: US-2003235422-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an image forming apparatus, such as a printer, copy machine, facsimile, complex machine, etc., which is provided with a fixing unit for thermally fixing an image transferred onto a paper sheet.  
       [0003] 2. Description of the Related Art  
       [0004] An image forming apparatus is known which is provided with a fixing unit for fixing an image transferred onto a paper sheet by heating a to-be-heated object using a heater lamp. In order to prevent an inrush current (flicker) that occurs when a heater lamp is turned on, an apparatus provided with a plurality of heater lamps has been proposed. In such an image forming apparatus, if, for example, two heater lamps are employed, one of the heater lamps is first turned on, and the other heater lamp is turned on a predetermined period after, in order to mainly prevent an inrush current when the heater lamps are turned on. In other words, the two heater lamps are turned on at different timings.  
       [0005] Further, an image forming apparatus has been proposed in which an inrush current is prevented by switching the connection of two heater lamps between a series connection and parallel connection. An image forming apparatus has also been proposed in which switching control is executed so as to equalize the lives of a plurality of heater lamps employed therein.  
       [0006] The specifications for each of the above-described heater lamps are designed to enable a maximum power to be output during a printing operation, and also to shorten the warming-up period. Accordingly, when all the heater lamps are tuned on, the amount of heat generated by them is excessive. This means that if all the heater lamps are turned on, the preset target temperature is reached in a short time. Upon detecting that the target temperature is reached, the heater lamps are turned off. However, since an excessive amount of heat is supplied in a short time, a temperature higher than the target temperature may be reached. This phenomenon is called “overshooting”. As described above, in the image forming apparatuses proposed so far, there is a tendency for the variations in the range of a controlled target temperature, i.e., the temperature ripple, to increase.  
       [0007] Thus, in the above-described image forming apparatuses, although an inrush current can be prevented, using a plurality of heater lamps, a great temperature ripple occurs during temperature control of the to-be-heated object in the warming-up mode, printing mode, standby mode, etc.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008] It is an object of the invention to provide an image forming apparatus in which the temperature ripple is reduced by controlling, in accordance with predetermined conditions, the turn-on and -off of a plurality of heater lamps provided for heating a to-be-heated object that is used to fix an image.  
       [0009] An image forming apparatus according to an aspect of the invention is an apparatus for executing predetermined image forming, and comprises: a to-be-heated object which thermally fixes an image transferred onto a paper sheet; a first heater lamp heats the to-be-heated object with a predetermined rated power; a second heater lamp which heats the to-be-heated object with a rated power lower than the predetermined rated power; a timer section which measures time; a detecting section which detects a temperature of the to-be-heated object; and at least one of a warming-up mode, standby mode and print mode, as an operation mode configured to control the heating of the to-be-heated object.  
       [0010] The warming-up mode is an operation mode in which the to-be-heated object is heated from a cold start temperature to a predetermined temperature. In the warming-up mode, the second heater lamp is turned on and time measurement is started. The first heater lamp is turned on after a predetermined time period t1 elapses, thereby heating the to-be-heated object to the predetermined temperature.  
       [0011] The standby mode is an operation mode in which the to-be-heated object is maintained at a print-enabling temperature, and a request for printing is waited for. In the standby mode, turn-on and turn-off of the second heater lamp is executed preferentially, and the second heater lamp is turned on and time measurement is started. The first heater lamp is turned on if a preset target temperature is not reached even after a predetermined time period t3 elapses, thereby heating the to-be-heated object to a temperature falling within a predetermined range.  
       [0012] The print mode is an operation mode for predetermined image printing, in which the to-be-heated object is controlled to execute thermal fixing. In the print mode, turn-on and turn-off of the first heater lamp is executed preferentially, and the first heater lamp is turned on and time measurement is started. The second heater lamp is turned on if a preset target temperature is not reached even after a predetermined time period t2 elapses, thereby heating the to-be-heated object to a temperature falling within a predetermined range.  
       [0013] Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
     [0014] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
     [0015]FIG. 1 is a schematic sectional view illustrating a printer according to an embodiment of the invention;  
     [0016]FIG. 2 is a view useful in explaining the structure for controlling the heating of a heater roller;  
     [0017]FIG. 3 is a table showing the temperature control logic in warming-up mode;  
     [0018]FIG. 4 is a table showing the temperature control logic in standby mode;  
     [0019]FIG. 5 is a table showing the temperature control logic in printing mode; and  
     [0020]FIG. 6 is a view useful in explaining temperature control. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0021] An embodiment in which an image forming apparatus according to the invention is used in a printer  1  as shown in FIG. 1 will be described with reference to the accompanying drawings.  
     [0022] As shown in FIG. 1, a photosensitive drum  2 , which rotates counterclockwise in the figure, is provided at substantially the center of the cross section of the printer  1 . A charger  3 , exposure unit  4 , developing unit  5 , transfer unit  6 , cleaning unit  7  and de-electrifying LED  8  are provided around the photosensitive drum  2 .  
     [0023] A drawable cassette  9  is housed in the bottom portion of the printer  1 . Paper sheets P are received in the cassette  9 . A conveyance path  10  for the paper sheets P extends from the cassette  9  to a discharge port  12  via the space between the photosensitive drum  2  and transfer unit  6  and via a fixing unit  11  provided at the upper right side of the photosensitive drum  2 . Each paper sheet P discharged from the discharge port  12  is received in a paper receiving section  13 . Each paper sheet P is forwarded to the conveyance path  10  by a pickup roller  14 . Then, each paper sheet is guided between the photosensitive drum  2  and transfer unit  6  by conveyance rollers  15  and  16  opposing each other, with the conveyance path  10  interposed there between.  
     [0024] The charger  3  uniformly charges the periphery of the photosensitive drum  2  with a predetermined charge.  
     [0025] The exposure unit  4  emits a laser beam through the route indicated by the arrow in the figure, thereby scanning the periphery of the photosensitive drum  2  to form an electrostatic latent image thereon.  
     [0026] The developing unit  5  supplies a developing roller  51  with a developer supplied from a developer hopper, thereby converting the electrostatic latent image on the photosensitive drum  2  into a developer image.  
     [0027] The transfer unit  6  transfers the developer image formed on the photosensitive drum  2 , onto each paper sheet P.  
     [0028] The cleaning unit  7  clears the remaining developer from the periphery of the photosensitive drum  2 .  
     [0029] The fixing unit  11  has a heater roller  17  as a to-be-heated object, and a pressure roller  18 . The fixing unit  11  fixes the developer image, transferred to each paper sheet P, by heating the developer image using the heater roller  17 , and pressing it using the pressure roller  18 .  
     [0030] In the printer  1 , while the photosensitive drum  2  having its periphery charged by the charger  3  is rotating, the exposure unit  4  emits a laser beam to form a predetermined electrostatic latent image on the periphery of the photosensitive drum  2 , and the developing unit  5  develops the electrostatic latent image. In the printer  1 , in synchronism with the development of the image, the pickup roller  14  and conveyance rollers  15  and  16  are rotated to convey a paper sheet between the photosensitive drum  2  and transfer unit  6 , whereby the developer image is transferred onto the paper sheet. In the printer  1 , the paper sheet P with the developer image transferred is guided to the fixing unit  11 , where the developer image is thermally fixed on the paper sheet. The resultant paper sheet P is discharged through the paper discharge port  12 . In the above-described manner, the printer  1  prints out a predetermined image on a paper sheet P.  
     [0031] Heating control by the heater roller  17  will now be described. As shown in FIG. 2, a CPU  21 , ROM  22 , RAM  23 , I/O port  24  and ASIC  25  are connected to a system bus.  
     [0032] The ASIC  25  contains an A/D converter control circuit  26 , drive circuit  27 , drive circuit  28  and heater turn-on control circuit  29 .  
     [0033] The heater turn-on control circuit  29  contains a temperature detecting circuit  30  and timer  31 . The heater turn-on control circuit  29  executes heat control on the heater roller  17  in warming-up mode  32 , standby mode  33  or print mode  34 .  
     [0034] A signal line extends from the drive circuit  27  to a photo triac  35 . This signal line is connected to a DC power supply (not shown) via a light emission section  36  provided in the photo triac  35 . The photo triac  35  also incorporates a light receiving section  37  for receiving light emitted from the light emission section  36 .  
     [0035] A signal line extends from the drive circuit  28  to a photo triac  38 . This signal line is connected to the DC power supply (not shown) via a light emission section  39  provided in the photo triac  38 . The photo triac  39  also incorporates a light receiving section  40  for receiving light emitted from the light emission section  39 .  
     [0036] The roller heater  17  is provided with a heater lamp  41  with a rated power of 600 W and a heater lamp  42  with a rated power of 300 W. An end of the heater lamp  41  is connected to an AC power supply  43  via the light receiving section  37  of the photo triac  35 . An end of the heater lamp  42  is connected to the AC power supply  43  via the light receiving section  40  of the photo triac  38 . The other ends of the heater lamps  41  and  42  are connected to the AC power supply  43  via a thermostat  44  for preventing excessive heating. The heater lamps  41  and  42  are connected in parallel to the AC power supply  43 .  
     [0037] Accordingly, if a signal is supplied from the drive circuit  27 , the light emission section  36  of the photo triac  35  emits light. If the light receiving section  37  receives the light, the heater lamp  41  is connected to the AC power supply  43 . As a result, the heater lamp  41  is turned on. If the drive circuit  27  transmits no signal, the light emission section  36  emits no light. At this time, since the light receiving section  37  detects no light, the heater lamp  41  is not connected to the AC power supply  43 . As a result, the heater lamp  41  is not turned on.  
     [0038] Using the same mechanism as the above, the turn-on and -off of the heater lamp  42  is executed. Since the turn-on and -off of the heater lamps  41  and  42  is executed using this mechanism, the heater turn-on control circuit  29  can control the turn-on and -off of the heater lamps  41  and  42 , individually.  
     [0039] A thermistor  45  is provided on a central portion of the heater roller  17 . A thermistor  46  is provided on an end portion located in the longitudinal direction. The thermistors  45  and  46  are connected to the A/D converter control circuit  26 .  
     [0040] The CPU  21  controls the printer  1 . The ROM  22  stores, for example, programs that the CPU  21  executes. The RAM  23  stores, for example, a work area that is necessary when the CPU  21  executes a program stored in the ROM  22 . The I/O port  24  is used for, for example, a connection to a device.  
     [0041] The three modes for the heating control of the heater roller  17  will be described. The warming-up mode  32  is an operation mode in which the heater roller  17  is heated from a cold start temperature to a print enabling temperature. The standby mode  33  is an operation mode in which the heater controller  17  is kept at the print enabling temperature and is waiting for a request for printing. The print mode  34  is an operation mode in which the heater roller  17  is controlled so as to execute thermal fixing for predetermined image printing.  
     [0042] A target temperature range, which is commonly necessary in each mode to enable the heater roller  17  to fix a developer image on a paper sheet P, is set as an upper limit target temperature (Tmax) and lower limit target temperature (Tmin).  
     [0043] The temperature detecting circuit  30  fetches the outputs of the thermistors  45  and  46  via the A/D converter control circuit  26 , thereby detecting the surface temperature of the heater roller  17 .  
     [0044] The timer  31  is started when the heater lamp  41  or  42  has been turned on. Further, the timer  31  is restarted when the temperature detected by the temperature detecting circuit  30  is lower than the lower limit target temperature (Tmin). The timer  31  stops when a time period preset for each of the aforementioned modes has elapsed. Different time periods are set for the aforementioned modes. The time period set for the warming-up mode  32  is t1. The time period set for the print mode  34  is t2. The time period set for the standby mode  33  is t3. The relationship between the thus-set time periods is t1&lt;t2&lt;t3.  
     [0045] A description will now be given of the heater roller temperature control logic used for each of the warming-up mode  32 , standby mode  33  and print mode  34 . In each mode, the turn-on and -off of the heater lamps  41  and  42  are determined on the basis of the time period  t  measured by the timer  31  and the temperature T detected by the temperature detecting circuit  30 .  
     [0046] The temperature control logic for the warming-up mode  32  will be described. As illustrated in FIG. 3, if t&lt;t1 and T&lt;Tmin, the heater lamps  41  and  42  are OFF and ON, respectively (R 1 ). If t1≦t and T&lt;Tmin, the heater lamps  41  and  42  are both ON (R 2 ).  
     [0047] The temperature control logic for the standby mode  33  will be described. As illustrated in FIG. 4, if t&lt;t3 and Tmin&lt;T&lt;Tmax, the heater lamps  41  and  42  are OFF and ON, respectively (R 3 ). If Tmax≦T, the heater lamps  41  and  42  are both OFF irrespective of the value of  t  measured by the timer  31  (R 4 ). Further, even if Tmin&lt;T &lt;Tmax instead of Tmax≦T, if t3≦t, the heater lamps  41  and  42  are both OFF (R 5 ). If t&lt;t3 and T&lt;Tmin, the heater lamps  41  and  42  are OFF and ON, respectively (R 6 ). If t3≦t and T&lt;Tmin, the heater lamps  41  and  42  are both ON (R 7 ).  
     [0048] The temperature control logic for the print mode  34  will be described. As illustrated in FIG. 5, if t&lt;t2 and Tmin&lt;T&lt;Tmax, the heater lamps  41  and  42  are ON and OFF, respectively (R 8 ). If Tmax≦T, the heater lamps  41  and  42  are both OFF irrespective of the value of  t  measured by the timer  31  (R 9 ). Further, even if Tmin&lt;T&lt;Tmax instead of Tmax≦T, if t2≦t, the heater lamps  41  and  42  are both OFF (R 10 ). If t&lt;t2 and T&lt;Tmin, the heater lamps  41  and  42  are ON and OFF, respectively (R 11 ). If t2≦t and T&lt;Tmin, the heater lamps  41  and  42  are both ON (R 12 ).  
     [0049] A description will be given of the turn-on and -off of the heater lamps  41  and  42  when the printer  1  executes printing in the print mode  34  after it operates in the warming-up mode  32  and standby mode  33 .  
     [0050] In the warming-up mode  32 , only the heater lamp  42  is turned on, and the timer  31  continues to count until the period t reaches the set time period t1 (R 1 ). When the time period t measured by the timer  31  has reached the set time period t1, the heater lamp  42  is turned on (R 2 ). Thus, the heater roller  17  is heated so that the temperature T exceeds the lower limit target temperature.  
     [0051] After the heater roller  17  is heated and the temperature T exceeds the lower limit target temperature, the mode is shifted to the standby mode  33 . In the standby mode  33 , the heater lamp  41  is turned off and only the heater lamp  42  is kept ON (R 3 ). Upon the turn-off of one of the lamps, the timer  31  is restarted. When the temperature T has come to be equal or higher than the upper limit target temperature, both the heater lamps  41  and  42  are turned off irrespective of the time period  t  measured by the timer  31  (R 4 ).  
     [0052] If the temperature T is not lower than the lower limit target temperature even after the time period  t  measured by the timer  31  within the target temperature range (between the upper and lower limit target temperatures) has reached the set time period t3, both the heater lamps  41  and  42  are turned off (R 5 ). This operation is executed to prevent the heater lamps  41  and  42  from being kept ON due to a possible malfunction of the temperature detecting circuit  30 .  
     [0053] If the temperature T is lower than the lower limit target temperature, the heater lamp  42  is turned on. If the heater lamp  42  is already turned on, the timer  31  is restarted to continue the ON state of the heater lamp  42  (R 6 ). Where the temperature T is lower than the lower limit target temperature, and only the heater lamp  42  is in the ON state, if the temperature T does not reach the lower limit target temperature even after the time period  t  measured by the timer  31  has reached the set time period t3, the heater lamp  41  is turned on, as well as the heater lamp  42  (R 7 ).  
     [0054] In the standby mode  33 , if the printer  1  has received a request for printing, the mode is shifted to the print mode  34 . In the print mode  34 , if the temperature T is within the target temperature range and the time period  t  measured by the timer  31  is equal to or shorter than the set time period t2, only the heater lamp  41  is kept ON (R 8 ). If the temperature T is equal to or higher than the upper limit target temperature, both the heater lamps  41  and  42  are turned off irrespective of the time period  t  measured by the timer  31  (R 9 ).  
     [0055] If the temperature T is not lower than the lower limit target temperature even after the time period  t  measured by the timer  31  has reached the set time period t2, both the heater lamps  41  and  42  are turned off (R 10 ). This operation is executed to prevent the heater lamps  41  and  42  from being kept ON because of the malfunction of the temperature detecting circuit  30 .  
     [0056] If the temperature T is lower than the lower limit target temperature, the heater lamp  41  is turned on. If the heater lamp  41  is already turned on, the timer  31  is restarted to continue the ON state of the heater lamp  41  (R 11 ). Where the temperature T is lower than the lower limit target temperature, and only the heater lamp  41  is in the ON state, if the temperature does not reach the lower limit target temperature even after the time period  t  measured by the timer  31  has reached the set time period t2, the heater lamp  42  is turned on, as well as the heater lamp  41  (R 12 ).  
     [0057] After printing on a paper sheet is finished, the print mode  34  is ended and again returned to the standby mode  33 , where the aforementioned standby mode operation is executed.  
     [0058]FIG. 6 is a graph illustrating the above-described temperature control logic, the ordinate indicating the time  t , and the abscissa indicating the temperature T. Along the time axis  t , the mode shift is shown, which is executed in the order of the warming-up mode  32 , standby mode  33 , print mode  34  and again standby mode  33 . The temperature axis T indicates the upper limit target temperature (Tmax) and lower limit target temperature (Tmin). Also shown are the measurement start and end of each of the time period t1 set for the warming-up mode  32 , time period t3 set for the standby mode  33 , and time period t2 set for the print mode  34 . Further, in FIG. 6, the vertical lines indicate control switching times.  
     [0059] Furthermore, the lower portion of the graph is a bar chart indicating power W for heating the heater roller  17 . W 0  indicates power used when both the heater lamps  41  and  42  are turned off, i.e., power of 0. W 1  indicates power used when the heater lamp  41  is solely turned on. W 2  indicates power used when only the heater lamp  42  is turned on. W 3  indicates power used when both the heater lamps  41  and  42  are turned on.  
     [0060] As shown in FIG. 6, the power W for heating the heater roller  17  is applied thereto in a stepwise manner in the order of, for example, W 0 , W 2 , W 1  and W 3 . In other words, there is neither the case where the heater lamps  41  and  42 , which are both in the OFF state, are simultaneously turned on, nor the case where the heater lamps  41  and  42 , which are both in the ON state, are simultaneously turned off.  
     [0061] By virtue of this structure, the temperature of the heater roller  17  is prevented from being abruptly increased or decreased, thereby suppressing overshooting or undershooting. As a result, the variations in the temperature control range and temperature ripple can be reduced.  
     [0062] In the above embodiment, since the heater lamp  42  of a lower rated power is first turned on at the cold start, and the heater lamp  41  of a higher rated power is turned on the time period t1 after, the occurrence of an inrush current can be avoided as compared to the case where the heater lamp  41  of the higher rated power is solely turned on at the cold start.  
     [0063] In the standby mode  33 , as the heater lamp  42  of a lower rated power than the heater lamp  41  is turned on in preference to the latter, the power consumption can be reduced.  
     [0064] In the print mode  34 , as the heater lamp  41  of a higher rated power than the heater lamp  42  is turned on in preference to the latter, the time required to increase the temperature to a target temperature range can be reduced.  
     [0065] Further, since the rated power to be applied to the heater roller  17  can be switched in a stepwise manner, the temperature change becomes smooth, thereby reducing the temperature ripple in the warming-up mode  32 , standby mode  33  and print mode  34 . This means that the temperature ripple that occurs during the heat control of the heater roller  17  from the turn-on to the turn-off of the printer  1  can be reduced.  
     [0066] In the above-described embodiment, the image forming apparatus of the invention is a printer. However, the invention is not limited to this, but may be a copy machine, complex apparatus, facsimile, etc., in which a predetermined image is thermally fixed.  
     [0067] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.