Patent Publication Number: US-10317825-B2

Title: Image formation apparatus, control method, and medium storing program

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2016-229499 filed on Nov. 25, 2016, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Field of the Invention: 
     The present invention relates to an image formation apparatus including a fixing device and a controller, a method for controlling the fixing device, and a medium storing a program that is to be executed on a computer controlling the fixing device. 
     Description of the Related Art: 
     Japanese Patent Application Laid-open No. 2006-171480 discloses a method for controlling a heater of a fixing device. Specifically, when a target temperature is raised from 150° C. to 180° C., temperature adjustment control is switched from PI control to P control to gradually increase the target temperature by 5° C. When a heater temperature reaches a switching temperature (165° C.), the temperature adjustment control is switched to the PI control to gradually raise the target temperature by 5° C. until the target temperature reaches 180° C. 
     SUMMARY 
     According to an aspect of the present teaching, there is provided an image formation apparatus which includes: a fixing device; a heater driver; and a controller. The fixing device includes: a heater; a heating member to be heated by the heater; and a temperature sensor which outputs a signal depending on a temperature of the heater or the heating member. The heater driver includes a power circuit which controls electric power supply to the heater based on control value of the heater. The controller includes an electric circuit and is configured to: obtain a detection temperature from the signal outputted from the temperature sensor; set the control value of the heater based on the detection temperature; and output the control value to the heater driver. In a case that a target temperature of the heater or the heating member changes from a first target temperature to a second target temperature which is lower than the first target temperature and before the detection temperature becomes equal to or lower than a first switching temperature which is equal to or lower than the second target temperature, the controller is configured to set control value by on/off control or proportional control using a deviation between the target temperature and the detection temperature. In a case that the target temperature of the heater or the heating member changes from the first target temperature to the second target temperature and after the detection temperature becomes equal to or lower than the first switching temperature, the controller is configured to set the control value by proportional-integral control using the deviation and an integral value of the deviation. In a case that the target temperature of the heater or the heating member is set to the first target temperature and the detection temperature is lower than the first target temperature and before the detection temperature becomes equal to or higher than a second switching temperature which is lower than the first target temperature, the controller is configured to set the control value by the on/off control or the proportional control using the deviation. In a case that the target temperature of the heater or the heating member is set to the first target temperature and the detection temperature is lower than the first target temperature and after the detection temperature becomes equal to or higher than the second switching temperature, the controller is configured to set the control value by the proportional-integral control using the deviation and the integral value of the deviation. 
     According to the present teaching, when the target temperature changes from the first target temperature to the second target temperature lower than the first target temperature and after the detection temperature becomes equal to or lower than the first switching temperature that is equal to or lower than the second target temperature, the control value of the heater is set by the proportional-integral control. This can prevent the integral value of the deviation from being minus. Thus, it is possible to reduce the undershoot after the detection temperature becomes equal to or lower than the first switching temperature. The present teaching is applicable to a method for controlling the image formation apparatus, a program for controlling the image formation apparatus, and a medium storing the program. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic configuration of an image formation apparatus according to an embodiment of the present teaching. 
         FIG. 2  depicts a configuration for controlling a fixing device. 
         FIG. 3  is a flowchart indicating operation of a controller. 
         FIG. 4  is a timing diagram indicating changes of a target temperature, a detection temperature, an integral value of a deviation between the target temperature and the detection temperature, and a flag, depending on operation of the controller. 
         FIG. 5  is a timing diagram indicating changes of the target temperature, the detection temperature, and the integral value of the deviation, when a switching temperature is equal to a second target temperature. 
         FIG. 6  is a timing diagram indicating changes of the target temperature, the detection temperature, and the integral value of the deviation, when the switching temperature is lower than the second target temperature. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring to the drawings, an embodiment of the present teaching is described below in detail. In the following explanation, directions are defined on the basis of a user who uses an image formation apparatus. Namely, a right side in  FIG. 1  is defined as a “front”, a left side in  FIG. 1  is defined as a “rear”, a near side in  FIG. 1  is a “left”, and a far side in  FIG. 1  is defined as a “right”. An up-down direction in  FIG. 1  is defined as “up and down”. 
     As depicted in  FIG. 1 , a laser printer  1 , which is an exemplary image formation apparatus, mainly includes a body casing  2 , a sheet feeder  3 , an exposure unit  4 , a process cartridge  5 , a fixing device  8 , and a controller  100 . 
     The sheet feeder  3 , which is disposed at a lower portion in the body casing  2 , mainly includes a feed tray  31 , a pressure plate  32 , and a feed mechanism  33 . The pressure plate  32  raises sheets S loaded in the feed tray  31  and the feed mechanism  33  supplies each sheet S to a space between a photosensitive drum  61  and a transfer roller  63  of the process cartridge  5 . 
     The exposure unit  4 , which is disposed at an upper portion in the body casing  2 , includes a light source, a polygon mirror, a lens, a reflecting mirror, and the like (those of which are not depicted in the drawings). In the exposure unit  4 , a light beam (see an alternate long and two short dashes line in  FIG. 1 ) that is emitted from the light source on the basis of image data is scanned on a surface of the photosensitive drum  61  at high speed to expose the surface of the photosensitive drum  61 . 
     The process cartridge  5  is disposed below the exposure unit  4 . The process cartridge  5  is removably attached to the body casing  2  through a front-side opening, which appears when a front cover  21  of the body casing  2  provided on the front side is open. The process cartridge  5  includes a drum unit  6  and a developing cartridge  7 . 
     The drum unit  6  mainly includes the photosensitive drum  61 , a charging unit  62 , and the transfer roller  63 . The developing cartridge  7  is removably attached to the drum unit  6 , and mainly includes a developing roller  71 , a supply roller  72 , a layer-thickness regulating blade  73 , a storage part  74  storing a toner, and an agitator  75 . 
     In the process cartridge  5 , the surface of the photosensitive drum  61  is uniformly charged by the charging unit  62 , then is exposed with the light beam from the exposure unit  4  to form an electrostatic latent image based on image data on the photosensitive drum  61 . The toner in the storage part  74  is supplied to the supply roller  72  while being agitated by the agitator  75 , and then supplied from the supply roller  72  to the developing roller  71 . The toner enters between the developing roller  71  and the layer-thickness regulating blade  73  with rotation of the developing roller  71 , and is carried, as a thin layer having a certain thickness, on the developing roller  71 . 
     The toner carried on the developing roller  71  is supplied to the electrostatic latent image formed on the photosensitive drum  61 . This visualizes the electrostatic latent image (the electrostatic latent image is made as a visual image), and a toner image is formed on the photosensitive drum  61 . Allowing the sheet S to pass between the photosensitive drum  61  and the transfer roller  63  transfers the toner image formed on the photosensitive drum  61  onto the sheet S. 
     The fixing device  8 , which is disposed on a rear side of the process cartridge  5 , mainly includes a heating roller  81  as an exemplary heating member, a pressure roller  82 , a halogen lamp  83  as an exemplary heater, and a thermistor  84  as an exemplary temperature sensor. 
     The heating roller  81  is a cylindrical member made of metal, includes the halogen lamp  83  therein, and is configured to be heated by the halogen lamp  83 . The pressure roller  82  is a member having an elastic layer around a core metal. The pressure roller  82  is provided in pressure contact with the heating roller  81 . 
     In the fixing device  8 , the toner image transferred on the sheet S is thermally fixed during a period during which the sheet S passes between the heating roller  81  and the pressure roller  82 . The sheet S to which the toner image has been thermally fixed is discharged on the discharge tray  22  by conveyance rollers  23  and  24 . 
     The controller  100  controls respective parts or components of the laser printer  1 , such as the fixing device  8 . The controller  100  includes a single electric circuit or multiple electric circuits. Specifically, as depicted in  FIG. 2 , the controller  100  includes a CPU  110 , a ROM  120 , a RAM  130 , an input/output circuit  140 , and the like. 
     The ROM  120  stores programs controlling the respective parts of the laser printer  1  and data including a variety of setting information. The RAM  130  is used as a working area for the CPU  110  that performs each of the programs or a storage area in which data is temporarily stored. The CPU  110  performs a variety of arithmetic processing based on signals output from a variety of sensors such as the thermistor  84 , programs and data read, for example, from the ROM  120 , and the like. 
     The controller  100  outputs a control signal to each part of the laser printer  1  based on the arithmetic result of the CPU  110 , thus controlling each part of the laser printer  1 . In other words, each part of the laser printer  1  is configured to operate or act in response to the control signal output from the controller  100 . 
     The thermistor  84  is a sensor that outputs, to the controller  100 , a signal depending on a temperature of the heating roller  81 . The thermistor  84  is disposed to face a surface of the heating roller  81  with a predefined space therebetween. The controller  100  obtains a detection value (detection temperature T) of a surface temperature of the heating roller  81  from the signal output from the thermistor  84 . 
     When controlling the fixing device  8 , the controller  100  sets control value U of the halogen lamp  83  and outputs the control value U set, to a heater driver  10 . The heater driver  10  is a power circuit that controls power supply to the halogen lamp  83  based on a duty ratio, which is determined from the control value U set by the controller  100 . 
     The controller  100  sets a target temperature TT of the heating roller  81 , and sets the control value U of the halogen lamp  83  so that the detection temperature T obtained from the signal output from the thermistor  84  follows the target temperature TT. 
     Specifically, when the target temperature TT has been set to a first target temperature TT 1  and the detection temperature T is lower than the first target temperature TT 1 , and before the detection temperature T becomes equal to or higher than a switching temperature TS 1 , the controller  100  sets the control value U of the halogen lamp  83  by proportional control using a deviation TT-T (hereinafter referred also to as “ΔT”) between the target temperature TT (TT 1 ) and the detection temperature T. 
     Specifically, the controller  100  calculates the control value U of the halogen lamp  83  by control in which proportional action for changing the control value U in proportion to the deviation ΔT is performed but integral action for changing the control value U in proportional to an integral value I of the deviation ΔT is not performed. In other words, the controller  100  sets the control value U of the halogen lamp  83  by so-called P control. 
     In this embodiment, the controller  100  sets the control value U of the halogen lamp  83  following the equation (1) described below by use of a proportional gain K p  set in advance.
 
 U=K   p ( TT−T )  (1)
 
     The switching temperature TS 1  is set to a temperature lower than the first target temperature TT 1 . The switching temperature TS 1  is previously set through experiment, simulation, or the like to correspond to the first target temperature TT 1 . Specifically, the switching temperature TS 1  is set to a value that does not make overshoot too large. The large overshoot may be caused by rapid temperature increase in the heating roller  81  when the target temperature TT is set to a high temperature (i.e., the first target temperature TT 1 ). 
     When the target temperature TT has been set to the first target temperature TT 1  and the detection temperature T is lower than the first target temperature TT 1 , and after the detection temperature T becomes equal to or higher than the switching temperature TS 1 , the controller  100  sets the control value U of the halogen lamp  83  by proportional-integral control using the deviation ΔT and the integral value I of the deviation ΔT. 
     Specifically, the controller  100  calculates the control value U of the halogen lamp  83  by control in which both the proportional action and the integral action are performed. In other words, the controller  100  sets the control value U of the halogen lamp  83  by so-called PI control. 
     In this embodiment, the controller  100  sets the control value U of the halogen lamp  83  following the equations (2) and (3) by use of the proportional gain K p  and an integration gain K i  set in advance,
 
 U=K   p ( TT−T )+ K   i   I   (2)
 
 I   n   =I   n-1   +Ts ( TT   n   −T   n )  (3)
 
     wherein, n put after each variable indicates that the variable is a current value, n−1 indicates that the variable is a previous value. Ts indicates a time interval of sampling. 
     When the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2 , and before the detection temperature T becomes equal to or lower than a switching temperature TS 2 , the controller  100  sets the control value U of the halogen lamp  83  by proportional control using a deviation ΔT between the target temperature TT (TT 2 ) and the detection temperature T. 
     Specifically, the controller  100  calculates the control value U of the halogen lamp  83  by the control (P control) in which the proportional action is performed but the integral action is not performed. In this embodiment, the controller  100  sets the control value U of the halogen lump  83  following the above equation (1). 
     The second target temperature TT 2  is set to a temperature that is lower than the first target temperature TT 1 . The switching temperature TS 2  is set to a temperature that is equal to or lower than the second target temperature TT 2 . In this embodiment, the second target temperature TT 2  is the same as the switching temperature TS 2 . 
     The first target temperature TT 1 , the switching temperature TS 1 , the second target temperature TT 2 , and the switching temperature TS 2  are set to satisfy the equation (4) described below.
 
 TT 1− TS 1&gt; TT 2− TS 2  (4)
 
     When the target temperature TT has been changed from the first target temperature TT 1  to the second target temperature TT 2 , more specifically, when the target temperature TT has been updated, the controller  100  resets the integral value I of the deviation ΔT calculated so far to a predefined value. In this embodiment, the predefined value is zero. 
     When the target temperature TT has been changed from the first target temperature TT 1  to the second target temperature TT 2 , and after the detection temperature T becomes equal to or lower than the switching temperature TS 2  (the second target temperature TT 2 ), the controller  100  sets the control value U of the halogen lamp  83  by proportional-integral control using the deviation ΔT and the integral value I of the deviation ΔT. 
     Specifically, the controller  100  calculates the control value U of the halogen lamp  83  by performing the control (PI control) in which both the proportional action and the integral action are performed. In this embodiment, the controller  100  sets the control value U of the halogen lump  83  following the above equations (2) and (3). 
     When the target temperature TT has been updated to make the integral value I of the deviation ΔT zero, the controller  100  sets a flag FI to zero. When the flag FI is set to one, the controller  100  calculates the integral value I of the deviation ΔT. When the flag FI is set to zero, the controller  100  stops the calculation of the integral value I. When the target temperature TT has been set to the first target temperature TT 1 , and when the detection temperature T is lower than the first target temperature TT 1  and becomes equal to or higher than the switching temperature TS 1 , the controller  100  performs the integral action. In that case, the controller  100  sets the flag FI to one. When the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2 , and when the detection temperature T is equal to or lower than the switching temperature TS 2 , the controller  100  performs the integral action. In that case, the controller  100  sets the flag FI to one. An initial value of the flag FI is zero. 
     Subsequently, operation of the controller  100  (a method for controlling the fixing device  8 ) is described below with reference to the flowchart of  FIG. 3  and the timing diagram of  FIG. 4 . 
     As depicted in  FIGS. 3 and 4 , when, at a time t 0 , the target temperature TT has been set to the first target temperature TT 1  and the target temperature TT n  is different from the target temperature TT n-1  (S 1 : Yes), the controller  100  resets the integral value I n  to zero, and sets the flag FI to zero. Further, the controller  100  sets a detection temperature T n  at that time, that is, the detection temperature T at the time of setting the first target temperature TT 1 , to an updated temperature TO, and stores the updated temperature TO (S 2 ). 
     After the step S 2 , the controller  100  determines whether the target temperature TT n  is higher than the updated temperature TO (S 4 ). The target temperature TT n  is higher than the updated temperature TO when the target temperature TT has been set to the first target temperature TT 1  at the time t 0  (S 4 : Yes). Thus, the controller  100  determines whether the detection temperature T n  is equal to or higher than the switching temperature TS 1  (S 5 ). 
     Since the detection temperature T n  is lower than the switching temperature TS 1  at the time t 0  (S 5 : No), the controller  100  sets the control value U n  of the halogen lamp  83  in a step S 9 . In that situation, since the integral value I n  is zero, the controller  100  sets the control value U n  of the halogen lamp  83  by the P control (the equation (1)), and outputs the control value U n  to the heater driver  10 . 
     The target temperature TT is not updated between the time t 0  and a time t 1 , and the target temperature TT n  is the same as the target temperature TT n-1  (S 1 : No). Thus, the controller  100  determines in a step S 3  whether the flag FI is set to zero. Since the flag FI is set to zero between the time t 0  and the time t 1  (S 3 : Yes), the controller  100  proceeds to a step S 5  via a step S 4 . Further, since the detection temperature T n  is lower than the switching temperature TS 1  (S 5 : No), the controller  100  sets the control value U n  of the halogen lamp  83  by the P control in the step S 9 . Then, the controller  100  outputs the control value U n  to the heater driver  10 . 
     When the detection temperature T n  becomes equal to or higher than the switching temperature TS 1  at the time t 1  in the step S 5  (S 5 : Yes), the controller  100  sets the flag F 1  to one (S 7 ) and calculates the integral value I n  of the deviation ΔT between the target temperature TT n  and the detection temperature T n  (S 8 ). Since the integral value I n  is not zero at the time t 1 , the controller  100  sets the control value U n  of the halogen lamp  83  by the PI control (the equation (2)) and then outputs the control value U n  to the heater driver  10  (S 9 ). 
     Since the target temperature TT is not updated between the time t 1  and a time t 2  (S 1 : No), the controller  100  executes the step S 3 . Since the flag F 1  is set to one between the time t 1  and the time t 2  (S 3 : No), the controller  100  calculates the integral value I n  in a step S 8 , sets the control value U n  of the halogen lamp  83  by the PI control, and outputs the control value U n  to the heater driver  10  (S 9 ). 
     The target temperature TT n  is different from the target temperature TT n-1  (S 1 : Yes) when the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2  at the time t 2 . Thus, the controller  100  makes the integral value I n  calculated so far zero, sets the flag F 1  to zero, sets the temperature T n  at that time, that is, the detection temperature T at the time of updating the target temperature TT from the first target temperature TT 1  to the second target temperature TT 2 , to another updated temperature TO, and stores the another updated temperature TO (S 2 ). 
     After the step S 2 , the controller  100  executes the step S 4 . When the target temperature TT has been updated to the second target temperature TT 2  at the time t 2 , the target temperature TT n  is equal to or lower than the updated temperature TO (S 4 : No). Thus, the controller  100  determines whether the detection temperature T n  is equal to or lower than the switching temperature TS 2  (S 6 ). 
     Since the detection temperature T n  is higher than the switching temperature TS 2  at the time t 2  (S 6 : No), the controller  100  sets the control value U n  of the halogen lump  83  in the step S 9 . In that situation, since the integral value I n  is zero, the controller  100  sets the control value U n  of the halogen lamp  83  by the P control and outputs the control value U n  to the heater driver  10 . In a section in which the detection temperature T n  is higher than the target temperature TT, a proportional (K p  (TT−T)) is a minus, which makes the control value U n  a minus. When the control value U n  is a minus, the heater driver  10  performs the same control as the case in which the control value U n  is zero, namely, the heater driver  10  controls the power supply to the halogen lamp  83  in a state where the duty ratio is 0%. Specifically, the halogen lamp  83  is turned off. 
     The target temperature TT is not updated between the time t 2  and a time t 3  (S 1 : No), and thus the controller  100  executes the step S 3 . Since the flag FI is set to zero (S 3 : Yes), the controller  100  executes a step S 6  after the step S 4 . Since the detection temperature T n  is higher than the switching temperature TS 2  between the time t 2  and the time t 3  (S 6 : No), the controller  100  sets the control value U n  of the halogen lamp  83  by the P control in the step S 9 . Then, the controller  100  outputs the control value U n  to the heater driver  10 . 
     When the detection temperature T n  becomes equal to or lower than the switching temperature TS 2  at the time t 3  in the step S 6  (S 6 : Yes), the controller  100  sets the flag F 1  to one (S 7 ) and calculates the integral value I n  (S 8 ). Since the integral value I n  is not zero at the time t 3 , the controller  100  sets the control value U n  of the halogen lamp  83  by the PI control and outputs the control value U n  to the heater driver  10  (S 9 ). 
     Since the target temperature TT is not updated after the time t 3  in  FIG. 4  (S 1 : No), the controller  100  executes the step S 3 . Since the flag FI is set to one (S 3 : No), the controller  100  calculates the integral value I n  in the step S 8 , sets the control value U n  of the halogen lamp  83  by the PI control, and outputs the control value U n  to the heater driver  10  (S 9 ). 
     In this embodiment described above, as depicted in  FIG. 5 , when the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2  that is lower than the first target temperature TT 1 , and after the detection temperature T becomes equal to or lower than the switching temperature TS 2 , the PI control is started to set the control value U of the halogen lamp  83 . When comparing this embodiment and a case in which the PI control is started at a temperature higher than the switching temperature TS 2 , the embodiment can prevent the integral value I of the deviation ΔT from being minus. This reduces the undershoot after the detection temperature T becomes equal to or lower than the switching temperature TS 2 . 
     The first target temperature TT 1 , the switching temperature TS 1 , the second target temperature TT 2 , and the switching temperature TS 2  satisfy the relation TT 1 −TS 1 &gt;TT 2 −TS. In other words, the difference between the second target temperature TT 2  and the switching temperature TS 2  is smaller than the difference between the first target temperature TT 1  and the switching temperature TS 1 . Thus, the PI control can be started to set the control value U of the halogen lamp  83  in a state where the deviation ΔT between the second target temperature TT 2  and the detection temperature T is small. This shortens the time required for the detection temperature T to stabilize at the target temperature TT 2 . 
     When the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2 , the integral value I of the deviation ΔT is set to a predefined value (specifically, zero). Thus, when the PI control is started after the detection temperature T becomes equal to or lower than the switching temperature TS 2 , it is possible to prevent the integral value I of the deviation ΔT from being too large. This shortens the time required for the detection temperature T to stabilize at the second target temperature TT 2 . 
     Although the embodiment of the present teaching is described above, the present teaching is not limited to the above embodiment. The above configurations may be changed appropriately as described below without departing from the gist and/or scope of the present teaching. 
     For example, in the above embodiment, the switching temperature TS 2  is equal to the second target temperature TT 2 . The present teaching, however, is not limited thereto. The switching temperature TS 2  may be lower than the second target temperature TT 2 . In that case, as depicted in  FIG. 6 , when the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2 , and after the detection temperature T becomes lower than the second target temperature TT 2  and becomes equal to or lower than the switching temperature TS 2 , the PI control is started. 
     When the P control (proportional control) is continued as is after the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2 , assuming that a minimum temperature for the first undershoot where the detection temperature T is lower than the second target temperature TT 2  is T min , the switching temperature TS 2  lower than the second target temperature TT 2  is set to a temperature higher than the minimum temperature T min . 
     In the above embodiment, the second target temperature TT 2  and the switching temperature TS 2  have the same temperature. This makes it possible to start the PI control in the state where the deviation ΔT between the second target temperature TT 2  and the detection temperature T is smaller than the case in which the switching temperature TS 2  is lower than the second target temperature TT 2 . As a result, as depicted in  FIG. 5 , a time t 5  elapsing after the PI control is started in the state where TT 2 =TS 2  and before the detection temperature T reaches the second target temperature TT 2 , is shorter than a time t 6  elapsing after the PI control is started in the state where TT 2 &gt;TS 2  before the detection temperature T reaches the second target temperature TT 2 . 
     The detection temperature T reaching the second target temperature TT 2  stabilizes at the second target temperature TT 2 . Therefore, by setting the second target temperature TT 2  and the switching temperature TS 2  to the same temperature, it is possible to shorten the time for the detection temperature T to stabilize at the target temperature TT 2 . 
     In the above embodiment, the integral value I of the deviation ΔT is reset to zero as the predefined value, when the target temperature TT has been updated from the first target temperature TT 1  to the second target temperature TT 2 . The present teaching, however, is not limited thereto. For example, the predefined value may be any other value than zero. Or, when the target temperature has been updated, the integral value of the deviation between the target temperature and the detection temperature may be kept as is without being reset to the predefined value. When the next proportional-integral control is started, the integral value kept may be used to set the control value of the halogen lamp. 
     In the above embodiment, the “proportional control” of the present teaching is exemplified by the P control. The present teaching, however, is not limited thereto. For example, the proportional control of the present teaching may be so-called PD control that performs the proportional action and differential action that changes the control value in proportion to a differential value of the deviation between the target temperature and the detection temperature. In the above embodiment, the “proportional-integral control” of the present teaching is exemplified by the so-called PI control. The present teaching, however, is not limited thereto. For example, the proportional-integral control of the present teaching may be so-called PID control that performs the proportional action, the integral action, and the differential action. 
     When the target temperature TT is set to the first target temperature TT 1 , when the temperature of the heating roller  81  is increasing in the state where the detection temperature T is lower than the first target temperature TT 1 , and before the detection temperature T becomes equal to or higher than the switching temperature TS 1 , the control value of the halogen lamp  83  may be set by on/off control instead of the proportional control. For example, the power supply to the halogen lamp  83  may be controlled in a state where the duty ratio is fixed to 10%. 
     When the temperature of the heating roller  81  is decreasing after the target temperature TT is updated from the first target temperature TT 1  to the second target temperature TT 2 , and before the detection temperature T becomes equal to or lower than the switching temperature TS 2 , the control value of the halogen lamp  83  may be set by the on/off control instead of the proportional control. For example, the power supply to the halogen lamp  83  may be controlled by setting the control value of the halogen lamp  83  to zero. Specifically, the halogen lamp  83  may be turned off. 
     In the above embodiment, the thermistor  84  as the temperature sensor outputs a signal depending on a temperature of the heating roller  81 . The present teaching, however, is not limited thereto. For example, the temperature sensor may be disposed to face the pressure roller and may output a signal depending on a temperature of the pressure roller to which heat is transmitted from the heating roller. This allows the temperature sensor to indirectly detect the temperature of the heating roller via the pressure roller. The temperature sensor may be any other sensor than the thermistor. The temperature sensor may be a non-contact type temperature sensor or a contact-type temperature sensor. 
     The temperature sensor may be disposed to directly face the heater, provided that the temperature sensor outputs a signal depending on a temperature of the heater. Namely, the temperature sensor may directly detect the temperature of the heater that is a control object of the controller. When the temperature sensor directly detects the temperature of the heater, control value of the heater may be set as follows. Namely, a target temperature of the heater is set, and then the control value of the heater is set so that the detection temperature of the temperature sensor follows the target temperature. 
     In the above embodiment, the heating roller  81  is an exemplary heating member. The present teaching, however, is not limited thereto. For example, the heating member may be an endless fixing belt provided in a fixing device of a belt-fixing type. In the above embodiment, the fixing device  8  includes the pressure roller  82 , namely, the roller-like pressure member. The present teaching, however, is not limited thereto. For example, the fixing device  8  may include a belt-like pressure member. 
     In the above embodiment, the halogen lamp  83  using radiation heat is an exemplary heater. The present teaching, however, is not limited thereto. The heater may be a ceramic heater or a carbon heater using heat of a resistor. Or, the heater may be, for example, an IH heater that inductively heats the heating member. The heater may be disposed outside the heating member instead of being disposed inside the heating member. 
     In the above embodiment, the laser printer  1  that forms a monochrome image on the sheet S is an exemplary image formation apparatus. The present teaching, however, is not limited thereto. The image formation apparatus may be, for example, a printer configured to form a color image on a sheet. The image formation apparatus is not limited to the printers. For example, the image formation apparatus may be, for example, a copy machine or a multifunction peripheral provided with a document reader, such as a flatbed scanner. 
     The present teaching may be achieved not only as the image formation apparatus, but also as a program that causes the image formation apparatus to execute processing. The program may be provided by being recorded in a non-transitory recording medium. The non-transitory recording medium may include, for example, a CD-ROM, DVD-ROM, and a storage part placed in a server that is connectable to the image formation apparatus via a communication network. The program stored in the storage part of the server may be distributed as information or a signal indicating the program through the communication network such as the internet. 
     The elements described in the embodiment and the modified embodiments may be used in any combination.