Abstract:
In a temperature detector a smoother circuit receives a pulse signal generated by a pulse signal generation circuit to smooth the signal and transmit it to a first node. A comparator compares a level in voltage transmitted on the node with that in voltage on a second node connected to a thermistor element line and outputs a resultant comparison as a detection signal to a heater controller. The temperature detector can have on the first node a reference voltage adjustable by a duty ratio of the pulse signal output by the pulse signal generation circuit. A resistor or similar component can be dispensed with and simply by adjusting the pulse signal&#39;s duty ratio the reference voltage can be adjusted. Temperature can thus be detected with high precision.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to thermal fixation in printers and similar image formation apparatuses and particularly to detecting the temperature of a hot roller.  
         [0003]     2. Description of the Background Art  
         [0004]     Laser printers corresponding to conventional image formation apparatuses receive image data from a host and process the data at an image formation unit. More specifically, a photosensitive material is exposed to laser light to form an electrostatic latent image which attracts charged toner and is developed, and a sheet is brought into close contact with the photoreceptor to receive the toned image which is in turn fused to provide a permanent image.  
         [0005]     In connection with the thermal fixation a conventional laser printer provides thermal fixation at a fuser internally provided with a movable fusing member implemented by a hot roller having a surface with a thermistor attached thereto. The thermistor is a temperature detection device decreasing in resistance as it is heated, and is generally used to detect the hot roller&#39;s surface temperature. It is used to exert control to set the hot roller&#39;s surface temperature for example around 180 to 200° C., which allows efficient thermal fixation.  
         [0006]     In connection with this temperature control, the thermistor or the like provides analog output, which may have its voltage value linearly read for example via a microcomputer or ASIC&#39;s analog port and from the read voltage value the temperature may be detected and the surface temperature may thus be controlled. However, the analog port, having a function reading the voltage value linearly, is controlled in a complicated manner and is also an expensive component. Accordingly, it is suggested to replace the analog port with an AD converter converting an analog output to a digital output and detect temperature from the output of the AD converter.  
         [0007]     For example, Japanese Patent Laying-Open No. 07-055589 discloses an AD converter for temperature control wherein a temperature detected of a temperature measuring thermistor is detected by the AD converter. More specifically the document discloses that a plurality of resistors are used so that a threshold value obtained by resistance division can be used to detect temperature at a plurality of temperature measuring points.  
         [0008]     The AD converter for temperature control that employs a plurality of resistors to measure temperature at a plurality of temperature measuring points, however, has variation between the resistors, and temperature cannot be detected with precision.  
         [0009]     Furthermore, if the number of temperature measuring points is increased a plurality of resistors different in resistance must accordingly be provided. This disadvantageously results in increased number of components and hence increased area for mounting.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention has been made to overcome the above disadvantages and it contemplates an image formation apparatus including a temperature controller capable of detecting temperature with high precision and also contributing to reduced area for mounting.  
         [0011]     The present invention provides an image formation apparatus including: a fuser having a heater used in forming an image to provide thermal fixation; a temperature detector detecting temperature of the fuser; and a temperature controller controlling the heater based on a resultant detection of the temperature detector. The fuser includes a hot roller heated by thermal conduction provided by the heater, a pressurization roller cooperating with the hot roller to pressurize a sheet inserted, and a temperature detection element transmitting a voltage in accordance with a temperature. The temperature controller includes a pulse signal generation circuit operative in response to instruction to output a pulse signal and also adjust duty ratio of the pulse signal, and a first digital port outputting the pulse signal from the pulse signal generation circuit, a second digital port receiving a digital signal corresponding to the resultant detection of the temperature detector. The temperature detector includes a smoother circuit receiving the pulse signal from the pulse signal generation circuit to smooth and output the pulse signal as a reference voltage, and a comparator comparing the voltage transmitted from the temperature detection element with the reference voltage to output a resultant comparison to the temperature controller. The pulse signal generation circuit adjusts the duty ratio of pulse signal in accordance with a mode.  
         [0012]     The present invention provides an image formation apparatus including: a fuser having a heater used in forming an image to provide thermal fixation; a temperature detector detecting temperature of the fuser; and a temperature controller controlling the heater based on a resultant detection of the temperature detector. The fuser includes a temperature detection element transmitting a voltage in accordance with a temperature. The temperature controller includes a pulse signal generation circuit operative in response to instruction to output a pulse signal and also adjust the duty ratio of the pulse signal. The temperature detector includes a smoother circuit receiving the pulse signal from the pulse signal generation circuit to smooth and output the pulse signal as a reference voltage, and a comparator comparing the voltage transmitted from the temperature detection element with the reference voltage to output a resultant comparison to the temperature controller.  
         [0013]     Preferably the temperature controller further includes a first digital port outputting the pulse signal from the pulse signal generation circuit, and a second digital port receiving a digital signal corresponding to the resultant detection of the temperature detector.  
         [0014]     Preferably the fuser further includes a hot roller heated by thermal conduction provided by the heater, and a pressurization roller cooperating with the hot roller to pressurize a sheet inserted.  
         [0015]     Preferably the pulse signal generation circuit adjusts the duty ratio of the pulse signal in accordance with a mode.  
         [0016]     The present image formation apparatus includes a temperature detector having a comparator comparing a voltage transmitted from a temperature detection element with a reference voltage, and a smoother circuit smoothing and outputting a pulse signal as a reference voltage. The pulse signal is output by a pulse signal generation circuit capable of adjusting the pulse signal&#39;s duty ratio. In other words, the temperature detector can have a reference voltage adjustable by the duty ratio of the pulse signal output by the pulse signal generation circuit. A resistor or similar component can be dispensed with and simply by adjusting the pulse signal&#39;s duty ratio a voltage can be adjusted. Temperature can thus be detected with high precision, controllability improved, and the number of components and hence the area for mounting reduced.  
         [0017]     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  shows an external configuration of an image formation apparatus in accordance with an embodiment of the present invention.  
         [0019]      FIG. 2  shows a schematic configuration of a fuser, a temperature detector and a heater controller according to an embodiment of the present invention.  
         [0020]      FIG. 3  is a diagram for illustrating a hot roller&#39;s temperature to be set in a variety of modes of a laser printer.  
         [0021]      FIG. 4  shows a schematic configuration of the temperature detector according to the embodiment of the present invention.  
         [0022]      FIG. 5  is a diagram for illustrating a system allowing the temperature detector to detect temperature according to the embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Hereinafter the present invention in an embodiment will be described with reference to the drawings. In the figures, identical or like components are identically denoted.  
         [0024]     With reference to  FIG. 1 , a personal computer  310  is a device controlling a laser printer  380  corresponding to an image formation apparatus according to an embodiment of the present invention and includes a USB controller  311 , a console  312 , a command output portion  313 , and a portion outputting data to be printed  314 .  
         [0025]     Console  312  receives instruction from a user for printing. Portion  314  is implemented by a program and a CPU executing the program and outputs data to be printed. Command output portion  313  is implemented by a program and a CPU executing the program and outputs a sheet feed command, a print start command, a sheet discharge command and other various commands. USB controller  311  outputs the commands and the data to be printed on a USB cable  315 .  
         [0026]     Laser printer  380  includes a controller  320 , a laser scan unit  332 , a photoconductor drum  333 , a toner cartridge  334 , a sheet feed motor  329 , a fuser  330 , a sheet discharge roller  331 , and a temperature detector  19 .  
         [0027]     Controller  320  includes a CPU  321 , a USB controller  322 , a ROM  323 , and an ASIC  324 . ROM  323  has stored therein a program executed by CPU  321 . USB controller  322  receives the commands, which are a sheet feed command, a print start command, a sheet discharge command and other various commands, and the data to be printed transmitted from personal computer  310  on USB cable  315 .  
         [0028]     ASIC  324  includes a sheet feed motor controller  326 , a print controller  327 , and a heater controller  400  controlling fuser  330 .  
         [0029]     Sheet feed motor controller  326  controls the driving of sheet feed motor  329 . Sheet feed motor  329  drives a roller (not shown) to transport a sheet. As the roller rotates, the roller&#39;s rotation is transmitted to a roller in fuser  330  and to sheet discharge roller  331  and thus rotates them to transport the fed sheet.  
         [0030]     When CPU  321  receives the sheet feed command, CPU  321  instructs sheet feed motor controller  321  to drive sheet feed motor  329 .  
         [0031]     When CPU  321  receives the data to be printed together with the print start command, CPU  321  outputs the data to laser scan unit  332  in response to instruction issued from print controller  327 . Laser scan unit  332  outputs toward photoconductor drum  333  laser light corresponding to an image to be printed. Thus on photoconductor drum  333  a latent image corresponding to the image to be printed is formed. Photoconductor drum  333  with the latent image formed thereon is rotated and thus receives toner thereon from toner cartridge  334 . The toner is attracted on photoconductor drum  333  in a pattern corresponding to the image to be printed. Subsequently, a sheet is brought into contact with photoconductor drum  333  and thus receives the pattern formed on photoconductor drum  333 , when fuser  330  effects thermal fixation by a hot roller, as described above.  
         [0032]     When CPU  321  receives the sheet discharge command, CPU  321  instructs sheet feed motor controller  326  to drive sheet feed motor  329  to discharge a printed sheet.  
         [0033]     With reference to  FIG. 2 , fuser  330  internally includes a hot roller  3  serving as a movable fusing member, a pressurization roller  5  pressed into contact with hot roller  3 , and a heater lamp  4  arranged internal to hot roller  3  and turn on to heat hot roller  3 .  
         [0034]     A thermistor  6  is a temperature detection element decreasing in resistance when it is heated, as has previously been described. Thermistor  6  is attached to hot roller  3  to abut against a surface thereof, and varies in resistance in response to the hot roller  3  temperature and thus detects the hot roller  3  surface temperature. Thermistor  6  is arranged at an end of a synthetic resin member  15 .  
         [0035]     The temperature detected by thermistor  6  is transmitted on a thermistor element line  7  in the form of an analog voltage to temperature detector  19 . Temperature detector  19  compares the received analog voltage with a reference voltage to provide a result and is driven thereby to output a corresponding detection signal to heater controller  400 .  
         [0036]     Heater controller  400  operates in response to the detection signal output from temperature detector  19  to turn on/off heater lamp  4  to control hot roller  3  to have a prescribed temperature.  
         [0037]     In general, as a laser printer has a variety of modes, hot roller  3  should accordingly be set to have different temperatures to correspond to the modes. As has been described previously, for example for a print mode, a high temperature (of approximately 180-200° C.) needs to be maintained, whereas for a standby state or a sleep mode, maintaining a prescribed temperature is desirable as such can contribute to reduced power consumption.  
         [0038]      FIG. 3  is a diagram for illustrating a hot roller&#39;s temperature to be set in a variety of modes of a laser printer.  
         [0039]     A print operation is suspended at a time t 51 . When at the time the print operation ends, the hot roller transitions from the current fusing temperature H 11  to a slightly lower, standby temperature H 12  (a temperature for a standby state that can immediately shift to the print operation).  
         [0040]     When a prescribed period of time elapses with this temperature condition maintained, and a time t 52  is arrived at, the control stops heating the hot roller. The hot roller provides a natural thermal radiation and thus generally decreases in temperature, and finally reaches a sleep temperature H 13  (a temperature attained when residual heat is completely eliminated: substantially equal to ambient temperature).  
         [0041]     In that condition at a time t 53  the print start command is again input. In response the hot roller is again heated and thus reaches fusing temperature H 11  at a time t 54 , when the print operation will start.  
         [0042]     As such, as has been described previously, a temperature to be set should be changed to correspond to modes.  
         [0043]     With reference to  FIG. 4 , the present invention in an embodiment provides temperature detector  19  including resistors  22 ,  24 , a capacitor  23 , and a comparator  20 .  
         [0044]     Comparator  20  has one input node N 2  electrically coupled with thermistor element line  7 , and the other input node N 1  connected to a smoother circuit  21 .  
         [0045]     Smoother circuit  21  includes resistor  22  between nodes N 0  and N 1 , and capacitor  23  between node N 1  and a ground voltage GND. Smoother circuit  21  smoothes or averages a pulse signal output from heater controller  400  via a digital port DP 2 , and transmits the smoothed (or averaged) pulse signal as an input voltage on node N 1 .  
         [0046]     Resistor  24  is provided between node N 0  and a prescribed drive voltage (5V) to pull up node N 0  to a drive voltage level.  
         [0047]     Comparator  20  compares an input voltage corresponding to the pulse signal output from heater controller  400  via a digital port DP 2  smoothed with an analog voltage provided on thermistor element line  7  to output a detection signal DTA to a digital port DP 1 .  
         [0048]     Heater controller  400  includes a pulse signal generation circuit  410  outputting a pulse signal to digital port DP 2 . The pulse signal generation circuit  410  can operate in response to instruction to generate the pulse signal and also adjust the pulse signal&#39;s duty ratio.  
         [0049]      FIG. 5  is a diagram for illustrating a system allowing temperature detector  19  to detect temperature in the embodiment of the present invention.  
         [0050]     In the present embodiment a temperature to be set is adjusted by a duty ratio of a pulse signal.  
         [0051]     With reference to  FIG. 5 , node N 0  receives a prescribed pulse signal from heater controller  400  via digital port DP 2 . This prescribed pulse signal is passed through and thus smoothed (or averaged) by smoother circuit  21 . More specifically, the pulse signal is smoothed (or average) to have a prescribed voltage level based on a duty ratio defining the pulse signal&#39;s high level period Hs and low level period Ls. This prescribed voltage level and the analog voltage received from the thermistor detection element are compared by comparator  20  to output detection signal DTA.  
         [0052]     For example, if an analog voltage level of node N 2  is higher than a prescribed voltage level of node N 1 , detection signal DTA is set to “1”.  
         [0053]     In contrast, if an analog voltage level of node N 2  is lower than the prescribed voltage level of node N 1 , detection signal DTA is set to “0”.  
         [0054]     Heater controller  400  operates in response to detection signal DTA received via digital port DP 1  to perform a prescribed decision operation to control heater lamp  4  in temperature.  
         [0055]     More specifically, if detection signal DTA is “1”, which indicates that an analog voltage level on node N 2  obtained from thermistor detection line  7  is higher than a prescribed reference voltage level on node N 1 , a decision can be made that the temperature is higher than that corresponding to the prescribed reference voltage.  
         [0056]     If detection signal DTA is “0”, which indicates that an analog voltage level on node N 2  obtained from thermistor detection line  7  is lower than the prescribed reference voltage level on node N 1 , a decision can be made that the temperature is lower than that corresponding to the prescribed reference voltage. In accordance with this decision the heater lamp is controlled to turned on/off  
         [0057]     In the print mode, hot roller  3  is continuously held within a prescribed range of temperature, as described hereinafter.  
         [0058]     Initially in the print mode heater lamp  4  is turned on to set a prescribed reference voltage to a voltage level corresponding to a maximum temperature (for example of 200° C.) in the print mode. More specifically, CPU  321  instructs heater controller  400  that the print mode is entered. In heater controller  400  pulse signal generation circuit  410  operates in response to the instruction issued from CPU  321  to adjust the pulse signal&#39;s duty ratio so as to set a voltage level corresponding to the maximum temperature (for example of 200° C.) and output the adjusted pulse signal, and when comparator  20  outputs detection signal DTA of “1” heater controller  400  turns off heater lamp  4  to stop the maximum temperature from further increasing.  
         [0059]     Subsequently, the prescribed reference voltage is set to a voltage level corresponding to a minimum temperature (for example of 180° C.) in the print mode. More specifically, pulse signal generation circuit  410  operates in response to instruction issued from CPU  321  to adjust the pulse signal&#39;s duty ratio so as to set a voltage level corresponding to the minimum temperature (for example of 180° C.) and output the adjusted pulse signal, and when comparator  20  outputs detection signal DTA of “0” heater controller  400  turns on heater lamp  4  to stop the minimum temperature from further decreasing.  
         [0060]     Thus switching the prescribed reference voltage in level and controlling based on a decision operation as described above can repeatedly be performed so that hot roller  3  in the print mode can be held constantly within the range of the maximum temperature (for example of 200° C.) to the minimum temperature (for example of 180° C.).  
         [0061]     In the present embodiment, temperature detector  19  can adjust the prescribed reference voltage in level, as desired, by adjusting a duty ratio of a pulse signal generated by pulse signal generation circuit  410 .  
         [0062]     For example if in accordance with instruction issued from CPU  321  the pulse signal&#39;s high and low level periods are defined by a duty ratio of 1:1 then smoother circuit  21  sets the node N 1  voltage level to 50% relative to the drive voltage (5V), i.e., 2.5V. As an example, if a temperature corresponding to the drive voltage (5V) is 300° C., and the duty ratio of 1:1 is set, then a voltage of 2.5V corresponding to 150° C. can be output from the smoother circuit.  
         [0063]     As described above, as the heater controller  400  pulse signal generation circuit  410  can output a pulse signal with a controlled duty ratio smoother circuit  21  can output an adjusted, smoothed input voltage level on input node N 1 . A reference temperature corresponding to a reference value serving as a reference can thus be adjusted as desired.  
         [0064]     In the present embodiment temperature detector  19  can control a reference voltage, i.e., a corresponding reference temperature by a duty ratio of a pulse signal output from pulse signal generation circuit  410  of heater controller  400 . This can eliminate the necessity of comparing by a comparator by a threshold value using a resistor, as done in the conventional art. A reduced area for mounting, and temperature detection with high precision can be achieved.  
         [0065]     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.