Patent Publication Number: US-10766252-B2

Title: Heater and inkjet printer

Description:
The entire disclosure of Japanese patent Application No. 2018-142650, filed on Jul. 30, 2018, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present invention relates to a heater and an inkjet printer. More particularly, the present invention relates to a heater and an inkjet printer having high accuracy in detecting abnormality of a temperature sensor. 
     Description of the Related art 
     An inkjet printer is a printer that ejects small ink droplets from fine nozzles and causes them to fly and to attach to a recording medium, thereby performing printing. The inkjet printer has an advantage of being capable of printing high resolution, high quality images at high speed in a relatively inexpensive manner. 
     Some inkjet printers use a UV ink (ultraviolet curable ink) as ink. An inkjet printer using the UV ink conveys the UV ink stored in an ink tank to an inkjet head through an ink carriage, and ejects it from the inkjet head. 
     In general, while the UV ink is gelled and has high viscosity at normal temperature (about 25° C.), it is subject to solation and its viscosity is significantly reduced when heated to about 85° C. Accordingly, at the time of passing through the ink carriage, the UV ink is heated to about 85° C. to enter the state with low viscosity. In order to obtain high image quality, it is necessary to control the ejection amount of the UV ink from the inkjet head with high accuracy. In order to control the ejection amount of the UV ink with high accuracy, the temperature of the UV ink is highly accurately controlled in the ink carriage, thereby stabilizing the viscosity of the UV ink. 
     As a configuration for heating the UV ink, the ink carriage is attached with a planar heat generator such as a rubber heater. The rubber heater heats the UV ink by conducting heat to the ink through the ink carriage made of metal or the like. 
     The rubber heater includes a rubber sheet made of silicone or the like, and a heat generator (conductor) made of a nichrome wire or the like provided in the rubber sheet. The heat generator generates heat when power is supplied. The power density of the rubber heater for heating ink (about 1 W/cm 2 ) is higher than the power density of a general rubber heater (about 0.6 W/cm 2 ). Accordingly, the rubber heater for heating ink may have a risk that it becomes high temperature to the temperature at which the rubber heater emits smoke and takes fire. In order to avoid such a situation, in the rubber heater for heating ink, the surface temperature of the rubber heater is measured by a thermistor, and power supplied to the rubber heater is controlled using a thyristor such that the temperature measured by the thermistor becomes a target temperature (about 85° C.). 
     Note that a conventional technique related to abnormality detection of the thermistor is disclosed in, for example, JP 2002-117958 A. JP 2002-117958 A discloses a technique of providing two thermistors to a sheet heater of a motorcycle and stopping the heater when a difference between temperatures detected by the two thermistors has reached a predetermined threshold value. 
     The rubber heater is provided with various kinds of safety protection so as not to emit smoke or take fire when abnormality occurs. Examples of the safety protection of the rubber heater include abnormality detection of the thermistor. The abnormality detection of the thermistor indicates detection of a situation in which a temperature to be detected has entered a state that cannot be normally detected due to an error in attachment of the thermistor, adhesion of foreign matter such as paper powder and dust, a failure of the thermistor itself, or the like. Accordingly, as a result of the control of the power to be supplied to the rubber heater on the basis of the erroneous temperature detected by the thermistor, occurrence of emitting smoke or taking fire can be avoided. 
     Conventionally, as a specific method of detecting abnormality of the thermistor, there has been adopted a method of providing two or more thermistors for one rubber heater and monitoring a difference between temperatures measured by each of two thermistors among the thermistors. In this method, when abnormality occurs in one of the two thermistors, the difference between the temperatures measured by each of the two thermistors increases. Accordingly, abnormal is detected when the difference between the temperatures measured by each of the two thermistors becomes larger than an abnormality threshold value. 
     In the conventional techniques, there has been a problem that the accuracy in detecting abnormality of a temperature sensor, such as a thermistor, is low. 
     In general, a planar heat generator such as a rubber heater includes an insulator, and a heat generator disposed on the insulator. The heat generator includes each of a plurality of linear parts extending in parallel with each other at a predetermined interval. Accordingly, in the rubber heater, there is unevenness in temperature on the sheet plane. In the sheet plane of the rubber heater, while the temperature is high at a position near the heat generator, the temperature is low at a position between the linear parts. As a result, in the case where the temperature of the position at which each of the two thermistors is provided is different from each other, even if the two thermistors are normal, the difference in temperature measured by each of the two thermistors increases, whereby abnormality of the thermistor has been erroneously detected at times. 
     In order to improve the accuracy in detecting abnormality of the thermistor in the rubber heater, a method of stabilizing a positional relationship between the thermistor and the heat generator in the rubber sheet is also conceivable. However, since the position of the heat generator in the rubber heater varies among products, it has been difficult to stabilize the positional relationship between the thermistor and the heat generator. 
     Furthermore, in order to suppress erroneous detection of abnormality of the thermistor, a method of increasing a value of the abnormality threshold value is also conceivable. However, when the abnormality threshold value is unnecessarily increased, detection of abnormality is delayed in the case where abnormality actually occurs in the thermistor, which may result in a situation where the rubber heater is maintained at an abnormally high temperature. In particular, in the case where the rubber heater is for heating ink, image abnormality and deterioration of ink may occur if the temperature of the rubber heater continues to be abnormally high. In particular, UV ink generally deteriorates at about 100° C. 
     Note that the problem that the accuracy in detecting abnormality of the temperature sensor is low has not been a problem unique to only a rubber heater or an inkjet printer, but has been a problem common to all heaters including a planar heat generator and a plurality of temperature sensors provided on the planar heat generator to measure a temperature. 
     SUMMARY 
     The present invention is intended to solve the problems described above, and an object thereof is to provide a heater and an inkjet printer having high accuracy in detecting abnormality of a temperature sensor. 
     To achieve the abovementioned object, according to an aspect of the present invention, a heater reflecting one aspect of the present invention comprises: a planar heat generator; a power supply circuit that controls supply of power to the planar heat generator; a plurality of temperature sensors that is provided on the planar heat generator and measures a temperature; and a hardware processor that detects abnormality of the temperature sensor in a case where a difference in temperature measured by each of two of the temperature sensors out of the plurality of temperature sensors exceeds an abnormality threshold value after a predetermined waiting time has elapsed since the supply of power to the planar heat generator is stopped. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention: 
         FIG. 1  is a cross-sectional view illustrating a configuration of an inkjet recording apparatus according to an embodiment of the present invention; 
         FIGS. 2A and 2B  are diagrams illustrating a configuration of a head unit; 
         FIG. 3  is a perspective view illustrating a configuration of an ink heater, which is a perspective view viewed from one direction; 
         FIG. 4  is a perspective view illustrating the configuration of the ink heater, which is a perspective view viewed from another direction; 
         FIG. 5  is a plan view illustrating a configuration of a sheet heater and a thermistor; 
         FIG. 6  is a diagram illustrating a control circuit of the sheet heater in the inkjet recording apparatus; 
         FIGS. 7A and 7B  are diagrams illustrating a positional relationship between thermistors THa and THb and a heat generator according to an embodiment of the present invention; 
         FIGS. 8A and 8B  are graphs schematically illustrating a temporal change of a difference in temperature between a measured temperature TP 1  of the thermistor THa and a measured temperature TP 2  of the thermistor THb in a case C 1 ; 
         FIGS. 9A and 9B  are graphs schematically illustrating a temporal change of a difference in temperature between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb in a case C 2 ; 
         FIG. 10  is a table illustrating a result of abnormality detection of a thermistor in a conventional manner; 
         FIG. 11  is a flowchart illustrating operation of an inkjet recording apparatus according to an embodiment of the present invention; and 
         FIG. 12  is a table illustrating a result of abnormality detection of a thermistor according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     In the following embodiment, a case where an inkjet printer includes a heater will be described. The heater may be included in an apparatus other than the inkjet printer. 
     Configuration of Inkjet Recording Apparatus  1   
     First, a configuration of an inkjet recording apparatus  1  will be described. 
       FIG. 1  is a cross-sectional view illustrating the configuration of the inkjet recording apparatus  1  according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the inkjet recording apparatus  1  (example of the heater and the inkjet printer) according to the present embodiment includes a sheet feeder  10 , an image former  20 , a sheet ejector  30 , and a controller  40  (example of first and second abnormality detection units, first and second stop units, a stop threshold value update unit, and a flow rate acquisition unit). The inkjet recording apparatus  1  conveys a recording medium M from the sheet feeder  10  to the image former  20  under the control of the controller  40 , forms an image on the conveyed recording medium M using the image former  20 , and ejects the recording medium M bearing the formed image to the sheet ejector  30 . 
     The sheet feeder  10  holds the recording medium M on which an image is to be formed, and supplies it to the image former  20  before the image is formed. The sheet feeder  10  includes a sheet feed tray  11 , and a conveyer  12 . 
     The sheet feed tray  11  is tabular, and is capable of placing one or more recording media M thereon. The sheet feed tray  11  moves up and down according to the placed amount of the recording medium M. The sheet feed tray  11  is held at the position at which the uppermost recording medium M is conveyed by the conveyer  12 . 
     The conveyer  12  includes a plurality of (two, in this case) rollers  121  and  122 , and an annular belt  123 . The belt  123  is rotationally driven by the plurality of rollers  121  and  122 . The conveyer  12  includes a conveyance mechanism for conveying the recording medium M on the belt  123 , and a supply unit for delivering the uppermost recording medium M placed on the sheet feed tray  11  to the belt  123 . The conveyer  12  conveys, as the belt  123  rotates, the recording medium M delivered to the belt  123  by the supply unit. 
     The image former  20  ejects ink including the UV ink or the like onto the recording medium M to form an image on the recording medium M. The image former  20  includes an image forming drum  21 , a handover unit  22 , a sheet heating unit  23 , a plurality of head units  24 , an irradiator  25 , and a delivery unit  26 . 
     The image forming drum  21  supports the recording medium M along the cylindrical outer peripheral surface, and conveys the recording medium M as it rotates. The conveyance surface of the image forming drum  21  faces the sheet heating unit  23 , the plurality of head units  24 , and the irradiator  25 . The image forming drum  21  performs, on the recording medium M to be conveyed, processing related to image formation. 
     The handover unit  22  is provided between the conveyer  12  of the sheet feeder  10  and the image forming drum  21 . The handover unit  22  delivers the recording medium M conveyed by the conveyer  12  to the image forming drum  21 . The handover unit  22  includes a swing arm  221 , a cylindrical delivery drum  222 , and the like. The swing arm  221  supports one end of the recording medium M conveyed by the conveyer  12 . The delivery drum  222  delivers the recording medium M supported by the swing arm  221  to the image forming drum  21 . The handover unit  22  picks up the recording medium M on the conveyer  12  using the swing arm  221  to deliver it to the delivery drum  222 , whereby the recording medium M is guided along the outer peripheral surface of the image forming drum  21  and is delivered to the image forming drum  21 . 
     The sheet heating unit  23  heats the recording medium M supported by the image forming drum  21 . The sheet heating unit  23  includes, for example, an infrared heater, and generates heat in response to energization. The sheet heating unit  23  is provided in the vicinity of the outer peripheral surface of the image forming drum  21 , which is on the upstream side of the head units  24  along the conveyance direction of the recording medium M based on the rotation of the image forming drum  21 . The heat generation of the sheet heating unit  23  is controlled by the controller  40  such that, the recording medium M supported by the image forming drum  21 , which passes near the sheet heating unit  23 , is made to have a predetermined temperature. 
     The plurality of head units  24  ejects ink of each color of cyan (C), magenta (M), yellow (Y), and black (K) onto the recording medium M supported by the image forming drum  21 , thereby forming an image on the recording medium M. The head unit  24  is individually provided for each of the colors C, M, Y, and K. In  FIG. 1 , the head units  24  corresponding to the respective colors Y, M, C, and K are provided in that order along the conveyance direction of the recording medium M, which is conveyed as the image forming drum  21  rotates. 
     Note that the head unit  24  according to the present embodiment has a length (width) that covers the entire recording medium M in a direction (width direction) perpendicular to the conveyance direction of the recording medium M. In other words, the inkjet recording apparatus  1  is a line-head inkjet recording apparatus of a one-pass system. The head unit  24  is capable of forming a line head by arranging a plurality of inkjet heads  241  ( FIGS. 2A and 2B ). An internal configuration of the head unit  24  will be described later. 
     After the ink used in the inkjet recording apparatus  1  according to the present embodiment is ejected onto the recording medium M, the irradiator  25  emits an energy ray for curing the ink. The irradiator  25  includes a fluorescent tube such as a low pressure mercury lamp, for example, and emits an energy ray such as an ultraviolet ray by causing the fluorescent tube to emit light. The irradiator  25  is provided in the vicinity of the outer peripheral surface of the image forming drum  21 , which is on the downstream side of the head units  24  with respect to the conveyance direction of the recording medium M based on the rotation of the image forming drum  21 . The irradiator  25  irradiates, with an energy ray, the recording medium M supported by the image forming drum  21  and on which the ink is ejected, thereby curing the ink ejected onto the recording medium M on the basis of the action of the energy ray. 
     Examples of the fluorescent tube include, in addition to the low pressure mercury lamp, a mercury lamp having an operation pressure of about several hundred Pa to 1 MPa, a light source that can be used as a germicidal lamp, a cold-cathode tube, an ultraviolet laser light source, a metal halide lamp, and a light-emitting diode. It is more preferable to employ a light source capable of emitting ultraviolet rays with higher illuminance and consuming less power (e.g., light-emitting diode) among them. Further, the energy ray is not limited to the ultraviolet ray, and may be any energy ray having the property of curing ink according to the property of the ink, and the light source may be replaced depending on the wavelength of the energy ray or the like. 
     The delivery unit  26  conveys the recording medium M irradiated with the energy ray by the irradiator  25  from the image forming drum  21  to the sheet ejector  30 . The delivery unit  26  includes a plurality of (two, in this case) rollers  261  and  262 , an annular belt  263 , and the like. The belt  263  is rotationally driven by the plurality of rollers  261  and  262 . The delivery unit  26  includes a conveyance mechanism for conveying the recording medium M on the belt  263 , and a cylindrical delivery drum  264  for delivering the recording medium M from the image forming drum  21  to the conveyance mechanism. The delivery unit  26  conveys, using the belt  263 , the recording medium M delivered to the belt  263  by the delivery drum  264 , and delivers it to the sheet ejector  30 . 
     The sheet ejector  30  stores the recording medium M delivered from the image former  20  by the delivery unit  26 . The sheet ejector  30  includes a tabular sheet ejection tray  31  and the like, and places the recording medium M having been subject to the image formation on the sheet ejection tray  31 . 
     The controller  40  controls operation of each unit of the inkjet recording apparatus  1 , and performs centralized control on the entire operation. The controller  40  includes a central processing unit (CPU)  41  ( FIG. 6 ), a read-only memory (ROM), a random access memory (RAM), and the like. The controller  40  reads out various processing programs, such as a system program, stored in the ROM, loads them in the RAM, and causes the CPU  41  to execute the programs loaded in the RAM. 
     The ink used in the inkjet recording apparatus  1  includes, for example, a UV ink. The UV ink undergoes, in the state of not being irradiated with the UV, a change of phase between the gel state and the liquid (sol) state depending on the temperature. The UV ink has a phase change temperature of, for example, about 100° C., and is uniformly liquefied (subject to solation) when heated to a temperature equal to or higher than the phase change temperature. Meanwhile, this ink gelates at a temperature equal to or lower than the phase change temperature including normal ambient temperature (0° C. to 30° C.). 
     Next, a configuration of one head unit  24  out of the plurality of head units  24  will be described. 
       FIGS. 2A and 2B  are diagrams illustrating the configuration of the head unit  24 .  FIG. 2A  is a front view, and  FIG. 2B  is a bottom view. Note that, in the drawing, the longitudinal direction of the head unit  24  is regarded as an X direction, the direction along the ink ejection direction of the head unit  24  provided with the ink heater  80  is regarded as a Z direction, and the direction orthogonal to the X direction and the Z direction is regarded as a Y direction. 
     Referring to  FIGS. 2A and 2B , the head unit  24  includes the plurality of inkjet heads  241 , and the ink heater  80 . Here, one head unit  24  includes  16  inkjet heads  241 . The  16  inkjet heads  241  constitutes eight inkjet modules  242  with each two inkjet heads  241  being paired. 
     Referring to  FIG. 2B , each of the inkjet heads  241  includes a plurality of nozzles  2411 . When one inkjet head  241  is focused, the plurality of nozzles  2411  is exposed on the lower surface side of the head unit  24 , and is configured by two rows extending in the X direction. The inkjet head  241  ejects ink from the plurality of nozzles  2411  to form an image on the recording medium M supported by the image forming drum  21 . 
     As illustrated in  FIG. 2B , the eight inkjet modules  242  are configured in two rows extending in the X direction. Each of the eight inkjet modules  242  is disposed zigzag in the two rows with respect to the direction orthogonal to the X direction. 
     As described above, in order to stabilize the fluidity of the ink in an ink tank  50  and the ink ejection amount in the head, the ink heater  80  heats the ink so that the ink in the gel state at about the ambient temperature enters the liquid (sol) state, and supplies the heated ink to each of the plurality of inkjet heads  241 . 
       FIGS. 3 and 4  are perspective views illustrating a configuration of the ink heater  80 .  FIG. 3  is a perspective view viewed from one direction, and  FIG. 4  is a perspective view viewed from another direction different from one direction. 
     Referring to  FIGS. 3 and 4 , the ink heater  80  includes the ink tank  50  (exemplary ink holder), and an ink tank heater  60 . The ink tank  50  is formed in such a manner that a plurality of sub tanks for storing ink is arranged in the longitudinal direction and is integrally molded. The ink tank heater  60  is provided on the outer surface of the ink tank  50 . The ink tank heater  60  heats the ink tank  50 . 
     The ink tank  50  stores ink supplied from an ink storage unit (not illustrated), and supplies the stored ink to the inkjet head  241 . Further, the ink tank  50  collects and stores the ink that has not been ejected from the inkjet head  241 . The ink tank  50  is formed to be long in the X direction, and includes a first sub tank  51  and four second sub tanks  52  integrally molded. The first sub tank  51  and the second sub tank  52  are arranged along the longitudinal direction (X direction) of the ink tank  50 . 
     The first sub tank  51  is provided in a recessed manner at the center of the ink tank  50  in the longitudinal direction (X direction). The first sub tank  51  stores the ink supplied from an ink supply unit (not illustrated), and also stores the ink collected from the inkjet head  241 . 
     The first sub tank  51  includes a flow path  511 , an inflow portion  512 , and a reservoir  513 . The flow path  511  is for causing the supplied ink to flow. The inflow portion  512  is provided at one end of the flow path  511 . The inflow portion  512  is a portion into which the ink supplied or collected from the ink supply unit or the inkjet head  241  flows. The reservoir  513  is provided at the other end of the flow path  511 . The reservoir  513  is a portion that stores the ink having passed through the flow path  511  and supplies the ink to the second sub tank  52 . 
     In other words, the ink supplied from the inflow portion  512  passes through the flow path  511 , and is stored in the reservoir  513 . The ink that has reached the reservoir  513  is delivered to the plurality of second sub tanks  52  by a plurality of pumps (not illustrated). 
     The second sub tanks  52  are provided two by two on both ends of the ink tank  50  in the longitudinal direction (X direction) in a recessed manner. The second sub tank  52  stores the ink supplied from the first sub tank  51 . The ink stored in each of the second sub tanks  52  is supplied to each of the eight inkjet modules  242  provided in the head unit  24 . 
     The ink tank heater  60  covers the entire one side surface of the ink tank  50 . The ink tank heater  60  includes a sheet heater H (exemplary planar heat generator), an elastic member  62 , a metallic plate  63 , a fixing screw  64 , and a thermistor  65 . The sheet heater H is provided on the outer surface of the ink tank  50 , and heats the ink tank  50 . The elastic member  62  is sandwiched between the sheet heater H and the metallic plate  63 . The metallic plate  63  is tabular, and is provided on the surface of the sheet heater H on the side opposite to the side facing the ink tank  50 . The fixing screw  64  presses and fixes the metallic plate  63  to the side of the ink tank  50 . The thermistor  65  is in contact with the sheet heater H. 
     The sheet heater H, the elastic member  62 , the metallic plate  63 , the fixing screw  64 , and the thermistor  65  are separately provided at three portions of the center and the both ends of the ink tank  50  in the longitudinal direction. The sheet heater H and the like separately provided in the three portions are disposed at positions corresponding to the first sub tank  51  provided at the center of the ink tank  50  in the longitudinal direction (X direction) and the second sub tanks  52  provided at the both ends of the ink tank  50  in the longitudinal direction (X direction), respectively. 
     Note that the inkjet recording apparatus  1  includes the head units  24  corresponding to the colors of Y, M, C, and K, and each of the plurality of head units  24  includes the ink tank  50 . Therefore, the inkjet recording apparatus  1  includes a plurality of ink tanks  50  for holding each of the inks of a plurality of different colors (each of the colors Y, M, C, and K). 
       FIG. 5  is a plan view illustrating a configuration of the sheet heater H and the thermistor  65 . Note that only a part of a heat generator  612  is illustrated in  FIG. 5 . 
     Referring to  FIG. 5 , the sheet heater H includes an insulator  611  (exemplary insulator), and the heat generator  612  (exemplary heat generator). The insulator  611  includes a rubber sheet made of silicone or the like. The insulator  611  has an arbitrary planar shape, and in this case, it has a planar shape that is substantially triangular. 
     The heat generator  612  is provided in the insulator  611 , and is embedded in the entire insulator  611 . The heat generator  612  is disposed in the insulator  611  in a corrugated manner, and has a meandering planar shape. The heat generator  612  includes a nichrome wire or stainless steel (SUS thin film formed by etching), or the like. The heat generator  612  includes a plurality of linear parts  612   a  (exemplary linear part), and a plurality of connection end parts  612   b.  Each of the plurality of linear parts  612   a  extends in parallel with each other at a predetermined interval P to obtain target power density per unit area. Each of the plurality of connection end parts  612   b  has an arc shape, and connects two adjacent linear parts  612   a  at the end of the linear part  612   a.    
     The thermistor  65  includes thermistors THa and THb (exemplary temperature sensor). The thermistors THa and THb are of contact-type, and are provided at predetermined positions on the sheet heater H. The thermistors THa and THb function as a plurality of temperature sensors that measures a surface temperature of the sheet heater H. Note that a thermocouple may be used as a temperature sensor instead of the thermistor. 
     The controller  40  performs thermostatic control on the sheet heater H such that the temperature measured by the thermistor  65  becomes a target value Ti in a predetermined condition (in this case, condition in which the sheet heater H shifts to an idling mode). 
     The thermistors THa and THb are provided close to each other in pairs for one sheet heater H. One of the thermistors THa and THb is a thermistor for thermostatic control. That is, in a predetermined condition, the controller  40  maintains the temperature measured by the thermistor for thermostatic control at a predetermined target value Ti. 
     The other one of the thermistors THa and THb is a thermistor for abnormality detection (for safety protection in an emergency). That is, the controller  40  monitors the difference between the temperature measured by the thermistor for abnormality detection and the temperature measured by the thermistor for thermostatic control, and determines that the thermistor is abnormal when the difference in temperature becomes equal to or more than a threshold value. The abnormality of the thermistor assumed in this case is an error in attachment of the thermistor, a mixture of foreign matter between the heater and the contact, a manufacturing failure of the thermistor itself, or the like. 
       FIG. 6  is a diagram illustrating a control circuit of the sheet heater H in the inkjet recording apparatus  1 . 
     Referring to  FIG. 6 , the inkjet recording apparatus  1  further includes a plurality of thyristors SSR (exemplary power supply circuit). Each of the plurality of thyristors SSR is connected between an alternating-current power supply AC and each of the plurality of sheet heaters H, and controls the supply of power to each of the plurality of sheet heaters H. 
     Here, the inkjet recording apparatus  1  includes N (N is a natural number) sheet heaters H. Each of the N sheet heaters H is denoted by a sheet heater H( 1 ), a sheet heater H( 2 ), a sheet heater H( 3 ), and so on, and a sheet heater H(N). In addition, the thermistors THa and THb corresponding to each of the sheet heaters H( 1 ) to H(N) are denoted by thermistors THa( 1 ) and THb( 1 ), thermistors THa( 2 ) and THb( 2 ), thermistors THa( 3 ) and THb( 3 ), and so on, and thermistors THa(N) and THb(N), respectively. Furthermore, the thyristors SSR corresponding to the respective sheet heaters H( 1 ) to H(N) are denoted by a thyristor SSR( 1 ), a thyristor SSR( 2 ), a thyristor SSR( 3 ), and so on, and a thyristor SSR (N), respectively. 
     Here, one sheet heater H( 1 ) is focused. The CPU  41  of the controller  40  controls, in a predetermined condition, on/off of the thyristor SSR( 1 ) using a heater remote signal, thereby controlling energization of the heat generator  612  of the sheet heater H( 1 ) using the thyristor SSR( 1 ). As a result, the CPU  41  maintains the temperature measured by the thermistor for thermostatic control among the thermistors THa( 1 ) and THb( 1 ) at the target value Ti. 
     Note that, as illustrated in  FIG. 1 , the inkjet recording apparatus  1  according to the present embodiment includes four head units  24  corresponding to the respective colors Y, M, C, and K. As illustrated in  FIG. 2B , one head unit  24  includes one ink tank  50 . As illustrated in  FIGS. 3 and 4 , one ink tank  50  includes three sheet heaters H. One sheet heater H includes one thyristor SSR, and two thermistors THa and THb. Therefore, the value of N in the present embodiment is 12(=4×1×3). The value of N may be one, or two or more. 
     Referring to  FIG. 5 , the thermistors THa and THb are attached to the sheet heater H having been complete at predetermined positions determined on the basis of dimensions of the sheet heater H. However, the position of the heat generator  612  varies among the products of the sheet heater H. 
     Moreover, since the heat generator  612  is provided inside the insulator  611  at the time of attaching the thermistors THa and THb, it is difficult to visually confirm the position of the heat generator  612  from the surface (surface appearance) of the sheet heater H. For that reason, the positions at which the thermistors THa and THb are attached vary among the products of the sheet heater H. As a result, there are variations among the products of the sheet heater H in the relationship between the positions of the thermistors THa and THb and the position of the heat generator  612 . 
     Positional Relationship Between Two Thermistors and Heat Generator 
       FIGS. 7A and 7B  are diagrams illustrating a positional relationship between thermistors THa and THb and the heat generator  612  according to an embodiment of the present invention, which are enlarged views of a portion Y in  FIG. 5 .  FIG. 7A  is an example of a case C 1 , and  FIG. 7B  is an example of a case C 2 . 
     Referring to  FIG. 7A , as described above, there are variations among the products of the sheet heater in the relationship between the positions of the two thermistors and the position of the heat generator. Even if the sheet heater H has the same specifications, both of the two thermistors THa and THb may be disposed on the linear part  612   a  of the heat generator  612  as in the case C 1  illustrated in  FIG. 7A , or the thermistor THa, which is one of the two thermistors THa and THb, may be disposed on the linear part  612   a  and the other thermistor THb may be disposed between the two linear parts  612   a  as in the case C 2  illustrated in  FIG. 7B . Moreover, although illustration is omitted, both of the two thermistors THa and THb may be disposed between the two linear parts  612   a.    
     The present inventors conducted the following experiments to confirm the problems of abnormality detection of the thermistor in the conventional manner. 
       FIGS. 8A and 8B  are graphs schematically illustrating a temporal change of a difference in temperature between a measured temperature TP 1  of the thermistor THa and a measured temperature TP 2  of the thermistor THb in the case C 1 .  FIG. 8A  illustrates a case where the thermistors THa and THb are normal, and  FIG. 8B  illustrates a case where the thermistor THa is normal and the thermistor THb is abnormal. Note that, in  FIGS. 8A and 8B  and  FIGS. 9A and 9B , the thermistor THa is set to be the thermistor for thermostatic control, and supply of power to the sheet heater H is controlled such that the temperature measured by the thermistor THa becomes about 85° C. (=target value Ti). 
     Referring to  FIGS. 8A and 8B , in the case C 1  ( FIG. 7A ), when the thermistors THa and THb were normal ( FIG. 8A ), there was substantially no difference between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb. When the thermistor THa was normal and the thermistor THb was abnormal ( FIG. 8B ), a difference in temperature of about 20° C. was generated between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb. 
       FIGS. 9A and 9B  are graphs schematically illustrating a temporal change of the difference in temperature between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb in the case C 2 .  FIG. 9A  illustrates a case where the thermistors THa and THb are normal, and  FIG. 9B  illustrates a case where the thermistor THa is normal and the thermistor THb is abnormal. 
     Referring to  FIGS. 9A and 9B , in the case C 2  ( FIG. 7B ), when the thermistors THa and THb were normal ( FIG. 9A ), a difference in temperature of about 5° C. was generated between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb. When the thermistor THa was normal and the thermistor THb was abnormal ( FIG. 9B ), a difference in temperature of about 25° C. was generated between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb. 
       FIG. 10  is a table illustrating a result of abnormality detection of the thermistor in the conventional manner. 
     Referring to  FIG. 10 , conventionally, the thermistor is determined to be abnormal in the case where an absolute value ΔT of the difference in temperature between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb (=|TP 1 −TP 2 |) exceeds a predetermined threshold value (in this case, 5° C.). Particularly when the state of the UV ink continues to exceed 100° C., the ink deteriorates to generate precipitations, thereby causing a problem such as clogging of an ink flow path. In view of the above, a condition for abnormality determination of the thermistor is strictly set such that the period of time during which the temperature of the ink exceeds 100° C. is minimized (threshold value is set to be a small value) even when abnormality occurs in the thermistor. 
     As illustrated in  FIGS. 8A and 8B , in the case C 1 , there is substantially no difference between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb when the thermistors THa and THb are normal. As a result, the absolute value ΔT of the difference in temperature is less than the predetermined threshold value, whereby the thermistor is determined to be normal. Further, when one of the thermistors (in this case, thermistor THb) is abnormal, the difference in temperature of about 20° C. is generated between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb. As a result, the absolute value ΔT of the difference in temperature is larger than the predetermined threshold value, whereby the thermistor is determined to be abnormal. 
     As illustrated in  FIGS. 9A and 9B , in the case C 2 , the difference in temperature of about 5° C. is generated between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb even when the thermistors THa and THb are normal. This difference in temperature is caused by the difference in temperature between the position at which the thermistor THa is provided and the position at which the thermistor THb is provided. As a result, the absolute value ΔT of the difference in temperature becomes larger than the predetermined threshold value, whereby the thermistor is erroneously determined to be abnormal. Furthermore, when one of the thermistors (in this case, thermistor THb) is abnormal, the difference in temperature of about 25° C. is generated between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb. As a result, the absolute value ΔT of the difference in temperature is larger than the predetermined threshold value, whereby the thermistor is determined to be abnormal. 
     Flowchart 
     In order to enhance the accuracy in detecting abnormality of the thermistor, in the present embodiment, the controller  40  detects abnormality in the case where the difference in temperature measured by each of the thermistors THa and THb exceeds an abnormality threshold value T 1  after a predetermined waiting time WT has elapsed since the stop of power supply to the sheet heater H. This operation will be described using the following flowchart. 
       FIG. 11  is a flowchart illustrating operation of the inkjet recording apparatus  1  according to an embodiment of the present invention. 
     Note that a process indicated by this flowchart is performed in parallel for each of the N sheet heaters H( 1 ) to H(N). In this flowchart, any optional sheet heater H among the N sheet heaters H( 1 ) to H(N) is denoted by a sheet heater H(k) (k is any natural number of 1 to N). In addition, the thermistors THa and THb for measuring the temperature of the sheet heater H(k) are denoted by thermistors THa(k) and THb(k), respectively. The measured temperatures TP 1  and TP 2  of the thermistors THa(k) and THb(k) are denoted by TP 1 (k) and TP 2 (k), respectively. The thyristor SSR that controls energization of the sheet heater H(k) is denoted by a thyristor SSR(k). The target value Ti of the measured temperature of the thermistor for thermostatic control of the sheet heater H(k) is denoted by a value Ti(k). The abnormality threshold value T 1  and a stop threshold value T 2  of the sheet heater H(k) are denoted by an abnormality threshold value T 1 (k) and a stop threshold value T 2 (k), respectively. The waiting time WT set for the sheet heater H(k) is denoted by a waiting time WT(k). Moreover, in this flowchart, the thermistor THa is assumed to be the thermistor for thermostatic control, and the thermistor THb is assumed to be the thermistor for abnormality detection. The target value Ti and the waiting time WT to be described later may be different values for each of the N sheet heaters H, or may be the same value. 
     Referring to  FIG. 11 , when the power source of the inkjet recording apparatus  1  is turned on, the controller  40  sets each of the abnormality threshold value T 1 (k) and the stop threshold value T 2 (k) to a default value (Si). The default value of each of the abnormality threshold value T 1 (k) and the stop threshold value T 2 (k) is, for example, 5° C. Next, the controller  40  determines whether an energization start request of the sheet heater H(k) is received (S 3 ). The controller  40  receives the energization start request when it is necessary to heat the sheet heater H(k), such as at the time of starting printing. The controller  40  repeats the processing of step S 3  until it determines that the energization start request of the sheet heater H(k) is received. 
     When the energization start request of the sheet heater H(k) is received in step S 3  (YES in S 3 ), the controller  40  shifts to a warm-up mode, starts energization control of the sheet heater H(k), and starts acquisition of the measured temperature TP 1 (k) of the thermistor THa(k) and the measured temperature TP 2 (k) of the thermistor THb (S 5 ). Next, the controller  40  determines whether the measured temperature TP 1 (k) of the thermistor THa(k) exceeds the target value Ti(k) (S 7 ). 
     When it is determined that the measured temperature TP 1 (k) of the thermistor THa(k) exceeds the target value Ti in step S 7  (YES in S 7 ), the controller  40  determines that the sheet heater H(k) has reached the target value and shifts to the idling mode. In this case, the controller  40  turns off the thyristor SSR(k) to stop the energization of the sheet heater H(k) (S 13 ). 
     When it is determined that the measured temperature TP 1 (k) of the thermistor THa(k) does not exceed the target value Ti in step S 7  (NO in S 7 ), the controller  40  turns on the thyristor SSR(k) to start (or continue) the energization of the sheet heater H(k) (supply of power to the sheet heater H(k)) (S 9 ). Next, the controller  40  determines whether a difference in temperature Δton is equal to or higher than the stop threshold value T 2 (k) (S 11 ). The difference in temperature ΔTon corresponds to the absolute value ΔT (=|TP 1 (k)−TP 2 (k)|) of the difference in temperature between the measured temperature TP 1 (k) of the thermistor THa(k) and the measured temperature TP 2 (k) of the thermistor THb(k) at the time when the sheet heater H(k) is energized. 
     When it is determined that the difference in temperature ΔTon is not equal to or higher than the stop threshold value T 2 (k) in step S 11  (NO in S 11 ), the controller  40  determines that there is no suspected abnormality of the thermistor, and proceeds to the processing of step S 7 . 
     When it is determined that the difference in temperature ΔTon is equal to or higher than the stop threshold value T 2 (k) in step S 11  (YES in S 11 ), the controller  40  determines that there is suspected abnormality of the thermistor. In this case, the controller  40  proceeds to the processing of step S 13 , and stops the energization of the sheet heater H(k) (S 13 ). 
     Subsequent to step S 13 , the controller  40  determines whether the waiting time WT(k) has elapsed since the stop of the energization of the sheet heater H(k) (S 15 ). The controller  40  repeats the processing of step S 15  until it determines that the waiting time WT(k) has elapsed since the stop of the energization of the sheet heater H(k). 
     When it is determined that the waiting time WT(k) has elapsed since the stop of the energization of the sheet heater H(k) in step S 15  (YES in S 15 ), the controller  40  obtains the measured temperatures TP 1 (k) and TP 2 (k) of the thermistors THa(k) and THb(k), and determines whether a difference in temperature ΔToff is equal to or higher than the abnormality threshold value T 1 (k) (S 17 ). The difference in temperature ΔToff corresponds to the absolute value ΔT (=|TP 1 (k)−TP 2 (k)|) of the difference in temperature between the measured temperature TP 1 (k) of the thermistor THa(k) and the measured temperature TP 2 (k) of the thermistor THb(k) at the time when the sheet heater H(k) is de-energized. 
     Here, there are the following reasons for performing the determination processing of step S 15  on the basis of the difference in temperature ΔToff after the waiting time WT(k) has elapsed. In general, in a planar heat generator such as a sheet heater, while a temperature is high at a position near a heat generator when it is energized, a temperature is low at a position between linear parts, which results in large unevenness in temperature in the surface. Accordingly, in the case where the difference in temperature between the positions at which the two thermistors THa and THb are disposed is originally large as in the case C 2  ( FIG. 7B ), even if the thermistors THa and THb are normal, the difference in temperature ΔTon between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb at the time of energization may become large, and may exceed the stop threshold value T 2 . 
     On the other hand, when the energization of the planar heat generator stops, the temperature in the vicinity of the heat generator decreases as time elapses, and the unevenness in temperature in the surface decreases (temperature of the planar heat generator is equalized). Accordingly, even in the case C 2 , when the thermistors THa and THb are normal, the difference in temperature ΔToff at the time of energization between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb after the waiting time WT has elapsed decreases, and falls below the abnormality threshold value T 1 . 
     Note that the waiting time WT may be set to a fixed value determined on the basis of the maximum flow rate of the ink flowing through the ink tank  50  provided with the sheet heater H. Moreover, the controller  40  may calculate (obtain) the flow rate of the ink on the basis of the content (printing rate, etc.) of a print job executed by the inkjet recording apparatus  1 , and may set the waiting time WT according to the calculated flow rate of the ink. In either case, the waiting time WT is preferably set to be shorter as the flow rate of the ink inside the ink tank  50  provided with the sheet heater H is higher. This is because, when the flow rate of the ink is high, a larger amount of heat is taken from the sheet heater H so that the period of time required to equalize the temperature of the sheet heater H is shortened. The waiting time WT is, for example, about 20 (s). 
     When it is determined that the difference in temperature ΔToff is not equal to or higher than the abnormality threshold value T 1  in step S 17  (NO in S 17 ), the controller  40  does not detect abnormality of the thermistor, and updates each of the abnormality threshold value T 1 (k) and the stop threshold value T 2 (k) from the default value (S 19 ). Specifically, the controller  40  updates the abnormality threshold value T 1 (k) to be a value higher than the difference in temperature ΔToff (difference in temperature ΔToff calculated in step S 17 ) after the waiting time WT(k) has elapsed (e.g., value of (ΔToff+3° C.)). The controller  40  updates the stop threshold value T 2 (k) to be a value higher than the difference in temperature ΔTon (difference in temperature Δton calculated in step S 11 ) at the time of energization (e.g., value of (ΔTon+3° C.)). Thereafter, the updated stop threshold value T 2 (k) is used when the processing of step S 11  is performed, and the updated abnormality threshold value T 1 (k) is used when the processing of step S 17  is performed. 
     Next, the controller  40  determines whether an energization stop request of the sheet heater H(k) is received (S 21 ). The controller  40  receives the energization stop request when there is no need to heat the sheet heater H(k), such as at the time of ending printing. 
     When it is determined that the energization stop request of the sheet heater H(k) is not received in step S 21  (NO in S 21 ), the controller  40  proceeds to the processing of step S 7 . 
     When it is determined that the energization stop request of the sheet heater H(k) is received in step S 21  (YES in S 21 ), the controller  40  determines whether the power source of the inkjet recording apparatus  1  is turned off (S 23 ). 
     When it is determined that the power source of the inkjet recording apparatus  1  is turned off in step S 23  (YES in S 23 ), the controller  40  terminates the process. 
     When it is determined that the power source of the inkjet recording apparatus  1  is not turned off in step S 23  (NO in S 23 ), the controller  40  proceeds to the processing of step S 3 . 
     When it is determined that the difference in temperature ΔToff is equal to or higher than the stop threshold value T 2 (k) in step S 17  (YES in S 17 ), the controller  40  detects abnormality of the thermistor. The controller  40  notifies a user of the abnormality of the thermistor, and stops the energization control of the sheet heater H(k) (S 25 ). Thereafter, the controller  40  terminates the process. 
     Note that the default value of the abnormality threshold value T 1 (k) and the stop threshold value T 2 (k) used in step Si is preferably lower as the interval P between the linear parts  612   a  of the heat generator  612  in the sheet heater H(k) is smaller, and is preferably lower as the target value Ti(k) in the idling mode is higher. This is because, since the heating speed of the ink by the sheet heater H is faster as the interval P between the linear parts  612   a  of the heat generator  612  is smaller and the margin of the difference in temperature between the target value Ti and the temperature at which the ink deteriorates is smaller as the target value Ti is higher, it is necessary to detect abnormality or suspected abnormality of the sheet heater H at a stage in which the temperature of the sheet heater H is lower. 
     Effect of Embodiment 
     According to the embodiment described above, abnormality of the thermistor is detected in the case where the difference in temperature ΔToff measured by each of the thermistors THa and THb exceeds the abnormality threshold value T 1  after the waiting time WT has elapsed since the stop of the supply of power to the sheet heater H (since the stop of energization of the sheet heater H), whereby abnormality of the thermistor can be detected in the state where the difference in temperature caused by the positions at which the thermistors THa and THb are disposed is excluded. As a result, the accuracy in detecting abnormality of the thermistor can be enhanced without the need of unnecessarily increasing the abnormality threshold value. 
     Further, the abnormality threshold value T 1  is updated to a value higher than the difference in temperature ΔToff when the difference in temperature ΔToff is smaller than the abnormality threshold value T 1 , and then abnormality of the thermistor is detected when the difference in temperature ΔToff exceeds the updated abnormality threshold value T 1 , whereby the abnormality threshold value with higher accuracy can be set according to the positions of the thermistors THa and THb. 
     Furthermore, the stop threshold value T 2  is updated to a value higher than the difference in temperature Δton when the difference in temperature ΔT off is smaller than the abnormality threshold value T 1 , and then energization of the sheet heater H stops when the difference in temperature ΔTon exceeds the updated stop threshold value T 2 , whereby it becomes possible to avoid a situation in which the unnecessary abnormality detection process (steps S 13  to S 17 ) is performed despite the fact that the thermistors THa and THb are normal. 
     Moreover, the abnormality threshold value T 1 , the stop threshold value T 2 , and the waiting time WT can be set to optimum values for each of the N sheet heaters H( 1 ) to H(N). This makes it possible to enhance the accuracy in detecting abnormality of the thermistor in each of the N sheet heaters H( 1 ) to H(N). 
     The present inventors conducted the following experiments to confirm the effects described above. 
       FIG. 12  is a table illustrating a result of the abnormality detection of the thermistor according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 12 , for each of exemplary embodiments of the present invention 1 to 5 in which the combination of the position of the thermistor and existence/non-existence of abnormality of the thermistor is different from each other, the energization of the sheet heater H was controlled in accordance with the flowchart illustrated in  FIG. 11 , and the result of the abnormality detection of the thermistor was obtained. The thermistor THa was set to be the thermistor for thermostatic control, and the thermistor THb was set to be the thermistor for abnormality detection. 
     In the exemplary embodiments of the present invention 1 and 2, as in the case C 1  illustrated in  FIG. 7A , both of the two thermistors THa and THb were disposed on the linear part  612   a  of the heat generator  612 . In the exemplary embodiment of the present invention 1, thermistors that operate normally were used as the thermistors THa and THb. In the exemplary embodiment of the present invention 2, a thermistor that operates normally was used as the thermistor THa, and a thermistor that does not operate normally was used as the thermistor THb. 
     As a result, in the exemplary embodiment of the present invention 1, there was substantially no difference between the measured temperature TP 1  of the thermistor THa and the measured temperature TP 2  of the thermistor THb, and the difference in temperature ΔToff was less than the abnormality threshold value T 1 . As a result, no abnormality of the thermistor was detected, and a correct detection result was obtained. After the determination, each of the abnormality threshold value T 1  and the stop threshold value T 2  was updated to 3° C. In the exemplary embodiment of the present invention 2, the difference in temperature ΔTon exceeded the stop threshold value T 2 , and the difference in temperature ΔToff exceeded the abnormality threshold value T 1 . As a result, abnormality of the thermistor was detected, and a correct detection result was obtained. 
     In the exemplary embodiments of the present invention 3 to 5, as in the case C 2  illustrated in  FIG. 7B , one of the two thermistors THa and THb, which is the thermistor THa, was disposed on the linear part  612   a,  and the other thermistor THb was disposed between the two linear parts  612   a.  In the exemplary embodiment of the present invention 3, thermistors that operate normally were used as the thermistors THa and THb. In the exemplary embodiment of the present invention 4, a thermistor that operates normally was used as the thermistor THb, and a thermistor that does not operate normally was used as the thermistor THa. In the exemplary embodiment of the present invention 5, a thermistor that operates normally was used as the thermistor THa, and a thermistor that does not operate normally was used as the thermistor THb. 
     As a result, in the exemplary embodiment of the present invention 3, while the difference in temperature ΔTon was equal to or higher than the stop threshold value T 2 , the difference in temperature ΔToff was less than the abnormality threshold value T 1 . As a result, no abnormality of the thermistor was detected, and a correct detection result was obtained. After the determination, the abnormality threshold value T 1  was updated to 4° C., and the stop threshold value T 2  was updated to 8° C. In the exemplary embodiments of the present invention 4 and 5, the difference in temperature ΔTon exceeded the stop threshold value T 2 , and the difference in temperature ΔToff exceeded the abnormality threshold value T 1 . As a result, abnormality of the thermistor was detected, and a correct detection result was obtained. 
     In addition, although not illustrated in  FIG. 12 , when both of the two thermistors THa and THb were disposed between the two linear parts  612   a  and thermistors that operate normally were used as the thermistors THa and THb (exemplary embodiment of the present invention 6), the difference in temperature ΔTon was about 2.5° C., which was less than the stop threshold value T 2 . Further, the difference in temperature ΔToff was about 0.5° C., which was less than the abnormality threshold value T 1 . As a result, no abnormality of the thermistor was detected, and a correct detection result was obtained. 
     Others 
     One planar heat generator may be provided with three or more temperature sensors. In that case, abnormality of the temperature sensor is detected in the case where a difference in temperature measured by each of two temperature sensors out of the three or more temperature sensors exceeds the abnormality threshold value after a predetermined waiting time has elapsed since the stop of power supply to the planar heat generator. 
     The process in the embodiment described above may be performed by software, or may be performed using a hardware circuit. Further, a program for executing the process in the embodiment described above may be provided, or the program may be recorded in recording medium, such as a CD-ROM, a flexible disk, a hard disk, a ROM, a RAM, and a memory card, which is to be provided to a user. The program is executed by a computer such as a CPU. Furthermore, the program may be downloaded to an apparatus via a communication line such as the Internet. 
     Although embodiments of the present invention have been described and illustrated in detail, it should be considered that the disclosed embodiments are made for purposes of illustration and example only and not limitation in every respect. The scope of the present invention should be interpreted not by the descriptions above but by terms of the appended claims, and it is intended to include all modifications in the meanings equivalent to and within the scope of the claims.