Patent Publication Number: US-8991961-B2

Title: Liquid discharge apparatus, method and storage medium for computer-readably storing program therein

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2011-080773, filed on Mar. 31, 2011, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a liquid discharge apparatus including a liquid discharge head which receives an input of a voltage waveform for discharging liquid from nozzles, a method for the discharging a liquid and a storage medium for computer-readably storing program for the liquid discharge apparatus. 
     BACKGROUND 
     In a printing device including a liquid discharge head driven by a voltage waveform input thereto, a change in head temperature due to, for example, the drive history of the liquid discharge head causes a change in the amount of discharged liquid and fluctuation of the print density, even if the same voltage waveform is input. It is therefore desirable that the voltage value of the voltage waveform to be input (i.e., drive voltage) is appropriately adjusted in accordance with the change of the head temperature. If the drive voltage is adjusted during printing on a recording medium, however, the print density changes during the printing, and thus the image quality is deteriorated. In view of this, an image forming apparatus has been known which constantly detects the temperature of a recording head, and changes the drive voltage of the recording head on the basis of the detected head temperature while a recording area of the recording head is facing a medium gap between recording media. 
     In the above-described image forming apparatus, the temperature of the recording head is constantly detected, and the drive voltage to be input to the head is adjusted on the basis of the latest one of the detected head temperatures during a short time in which the recording area of the recording head is facing the medium gap between recording media. 
     SUMMARY OF THE INVENTION 
     However, image forming may occur during high speed imaging. In this case, if the image forming speed is fast, the time for detecting temperature is further shortened. Consequently, one problem during high speed imaging is that the adjustment of the drive voltage of the head may fail to be completed within the time. If the drive voltage is not adjusted, the print density changes from the print density corresponding to a recording demand (print data command), and thus the image quality is deteriorated. 
     The present invention has been made to address the above-described issue, and an object thereof is to provide to a liquid discharge apparatus and a storage medium for computer-readably storing program therefor capable of appropriately adjusting the voltage waveform to be input to the liquid discharge head, even if the printing speed is increased. 
     To address the above-described issue, a liquid discharge apparatus according to an aspect of the present invention includes a liquid discharge head having a nozzle surface including nozzles for discharging liquid, the liquid discharge head being configured to receive a voltage signal having a waveform for discharging the liquid from the nozzles, a recording medium conveying unit being configured to successively convey a plurality of recording media in a conveying direction, the plurality of recording media being conveyed with a plurality of gaps between each recording medium, a temperature sensor configured to output temperature information of an actual temperature of the liquid discharge head while the nozzle surface is facing a second gap of the plurality of gaps located downstream of a first gap of the plurality of gaps in the conveying direction, a controller. The controller configured to determine, based on the temperature information of the actual temperature received from the temperature sensor and a discharge history relating to the liquid discharged from the liquid discharge head to the recording medium between the first gap and the second gap, an estimated temperature of the liquid discharge head before the nozzle surface faces the first gap and determine the waveform based on the estimated temperature, while the nozzle surface is facing the first gap. 
     A storage device for computer-readably storing a computer-executable program executable by a processor of a liquid discharge apparatus including a liquid discharge head having a nozzle surface including nozzles for discharging liquid, the liquid discharge head being configured to receive a signal having a waveform for discharging the liquid from the nozzles, recording medium conveying unit being configured to successively convey a plurality of recording media in a conveying direction, the plurality of recording media being conveyed with a plurality of gaps between each recording medium, a temperature sensor configured to output temperature information of a actual temperature of the liquid discharge head while the nozzle surface is facing a second gap of the plurality of gaps located downstream of a first gap of the plurality of gaps in the conveying direction. The program causing the processor to execute functions comprising determining, based on the temperature information of the actual temperature received from the temperature sensor and a discharge history relating to the liquid discharged from the liquid discharge head to the recording medium between the first gap and the second gap, a estimated temperature of the liquid discharge head before the nozzle surface faces the first gap and determining the waveform based on the estimated temperature, while the nozzle surface is facing the first gap. 
     A method for discharging a liquid from a liquid discharge apparatus including a liquid discharge head having a nozzle surface including nozzles for discharging liquid, the liquid discharge head being configured to receive a signal having a waveform for discharging the liquid from the nozzles, recording medium conveying unit being configured to successively convey a plurality of recording media in a conveying direction, the plurality of recording media being conveyed with a plurality of gaps between each recording medium, a temperature sensor configured to output temperature information of a actual temperature of the liquid discharge head while the nozzle surface is facing a second gap of the plurality of gaps located downstream of a first gap of the plurality of gaps in the conveying direction. The method comprising the steps of determining, based on the temperature information of the actual temperature received from the temperature sensor and a discharge history relating to the liquid discharged from the liquid discharge head to the recording medium between the first gap and the second gap, a estimated temperature of the liquid discharge head before the nozzle surface faces the first gap and determining the waveform based on the estimated temperature, while the nozzle surface is facing the first gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating a configuration of an inkjet printer (liquid discharge apparatus) according to a first embodiment; 
         FIG. 2  is a plan view illustrating a head body of an ink discharge head (liquid discharge head) used in the inkjet printer; 
         FIG. 3  is an enlarged partial cross-sectional view illustrating the head body of the ink discharge head; 
         FIG. 4  is a block diagram illustrating a configuration of a controlling unit (head input setting changing unit) used in the inkjet printer; 
         FIG. 5  is a front view schematically illustrating a positional relationship between the ink discharge head and gaps generated between a plurality of sheets (recording media); 
         FIG. 6  is a flowchart illustrating a controlling operation of the controlling unit (computer); and 
         FIG. 7  is a flowchart illustrating a controlling operation of a controlling unit (computer) of an inkjet printer (liquid discharge apparatus) according to a second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a liquid discharge apparatus according to an embodiment of the present invention will be described below with reference to the drawings. In the following embodiments, a “liquid discharge apparatus” according to an embodiment of the present invention is applied to an inkjet printer, and ink and an ink discharge head are used as “liquid” and a “liquid discharge head,” respectively. Further, a sheet and a sheet conveying mechanism are used as a “recording medium” and “recording medium conveying unit,” respectively. 
     First Embodiment 
     As illustrated in  FIG. 1 , the inkjet printer  10  includes a substantially rectangular parallelepiped-like housing  12 , four ink discharge heads  14   a  to  14   d  which discharge inks of four colors (magenta, cyan, yellow, and black), respectively, and four ink tanks  16   a  to  16   d  which separately contain the inks of four colors, respectively. The inkjet printer  10  further includes a sheet cassette  18  which stores sheets P, a sheet conveying mechanism  22  which conveys the sheets P, and a controlling unit  24  (controller) which executes a variety of controlling operations. 
     As illustrated in  FIG. 1 , the interior of the housing  12  has a space S which stores a variety of devices, and an upper surface of the housing  12  is provided with a sheet discharge unit  12   a  which receives the sheets P discharged outside the housing  12 . Further, the ink tanks  16   a  to  16   d  are attachably and detachably disposed in a bottom portion of the space S, and the sheet cassette  18  is attachably and detachably disposed above the ink tanks  16   a  to  16   d  in the bottom portion of the space S. Further, the ink discharge heads  14   a  to  14   d  and the controlling unit  24  are disposed in an upper portion of the space S, and the sheet conveying mechanism  22  is disposed in a vertically central portion and an upper portion of the space S. Further, an ambient temperature sensor  25  which measures an ambient temperature T Z  is disposed near the ink discharge heads  14   a  to  14   d  in the upper portion of the space S. 
     Each of the ink discharge heads  14   a  to  14   d  has a nozzle surface  20   a  provided with a plurality of nozzles  20  ( FIG. 3 ) which discharge the ink. Areas facing the plurality of nozzles  20  ( FIG. 3 ) of the respective ink discharge heads  14   a  to  14   d , i.e., areas facing the respective nozzle surfaces  20   a  form discharge areas Q 1  to Q 4  in which the respective inks are discharged to the sheets P. In the present embodiment, the four discharge areas Q 1  to Q 4  are disposed in juxtaposition in the horizontal direction, and the sheet conveying mechanism  22  is configured to successively convey a plurality of sheets P in a conveying direction to pass the sheets P through the discharge areas Q 1  to Q 4 . 
     Configuration of Sheet Conveying Mechanism: As illustrated in  FIG. 1 , the sheet conveying mechanism  22  includes a conveying unit  28 , a sheet feeding unit  30 , a sheet discharging unit  32 , and sheet sensors  33   a  to  33   d . The conveying unit  28  conveys the sheets P to pass the sheets P through the discharge areas Q 1  to Q 4 . The sheet feeding unit  30  is provided upstream of the conveying unit  28  in the conveying direction, and supplies the conveying unit  28  with the sheets P stored in the sheet cassette  18 . The sheet discharging unit  32  is provided downstream of the conveying unit  28  in the conveying direction, and discharges to the sheet discharge unit  12   a  the sheets P having passed the discharge areas Q 1  to Q 4 . The sheet sensors  33   a  to  33   d  are disposed at or near respective upstream end edges of the discharge areas Q 1  to Q 4  to be located near the ink discharge heads  14   a  to  14   d , respectively, and detect the sheets P. 
     The conveying unit  28  includes a pair of belt rollers  34  and  36 , a circular conveying belt  38  stretched between the belt rollers  34  and  36 , a tension roller  40  pressed against the conveying belt  38 , and a platen  42  which horizontally supports a portion of the conveying belt  38  located on the upper side. Further, a rotary shaft  34   a  of the belt roller  34  on one side is connected to a rotary shaft  46   a  of a motor  46  via a gear unit  44 . 
     The sheet feeding unit  30  includes a guide  48 , a sheet feeding roller  50 , a pair of feed rollers  52   a  and  52   b , and a nip roller  54 . The guide  48  forms a sheet feed path R 1  for the sheets P. The sheet feeding roller  50  is provided near an upstream end portion of the guide  48 , and feeds the sheets P stored in the sheet cassette  18  to the sheet feed path R 1 . The feed rollers  52   a  and  52   b  are provided on the sheet feed path R 1 . The nip roller  54  is provided near a downstream end portion of the guide  48 , and presses the sheets P against a surface  38   a  of the conveying belt  38 . Further, a rotary shaft  50   a  of the sheet feeding roller  50  is connected to a rotary shaft (illustration omitted) of a motor  55 . 
     The sheet discharging unit  32  includes a guide  56 , a separating plate  58 , a pair of feed rollers  60   a  and  60   b , and a pair of sheet discharging rollers  62   a  and  62   b . The guide  56  forms a sheet discharge path R 2 . The separating plate  58  is provided near an upstream end portion of the sheet discharge path R 2 , and separates the sheets P from the surface  38   a  of the conveying belt  38 . The feed rollers  60   a  and  60   b  are provided on the sheet discharge path R 2 . The sheet discharging rollers  62   a  and  62   b  are provided near a downstream end portion of the guide  56 , and discharge the sheets P from the guide  56 . 
     The motor  46  of the conveying unit  28  ( FIG. 1 ) and the motor  55  of the sheet feeding unit  30  ( FIG. 1 ) are, for example, stepper motors or servomotors capable of performing highly accurate position control. As illustrated in  FIG. 4 , the motors  46  and  55  are electrically connected to the controlling unit  24 . It is therefore possible to appropriately change a conveying speed V ( FIG. 5 ) of the sheets P by causing the controlling unit  24  to control the motor  46  of the conveying unit  28 , and to appropriately change intervals W 1  to W 3  ( FIG. 5 ) between the sheets P by causing the controlling unit  24  to control the motor  55  of the sheet feeding unit  30 . 
     Each of the sheet sensors  33   a  to  33   d  is a sensor which detects the sheets P in a non-contact manner, and is electrically connected to the controlling unit  24 , as illustrated in  FIG. 4 . It is therefore possible to accurately measure the intervals W 1  to W 3  ( FIG. 5 ) between the sheets P on the basis of the outputs from the sheet sensors  33   a  to  33   d , the conveying speed V ( FIG. 5 ) of the sheets P, and the time measured by a timer (illustration omitted), while the nozzle surface  20   a  is facing the gaps G 1  to G 3  ( FIG. 5 ). The “gap” refers to an area between the sheets P, in which printing is not performed. In the present embodiment, the “gap” corresponds to an area between the sheets P, in which the sheets P are absent. 
     Configuration of Ink Discharge Head: As illustrated in  FIG. 1 , the ink discharge heads  14   a  to  14   d  discharge the respective inks, at the discharge areas Q 1  to Q 4 , respectively, to the sheets P conveyed by the sheet conveying mechanism  22 . Each of the ink discharge heads  14   a  to  14   d  includes a substantially rectangular parallelepiped-like head holder  70  and a head body  72  (shown in  FIG. 2 ). The head holder  70  has longer sides extending in a direction perpendicular to the conveying direction (hereinafter referred to as the “sub-scanning direction”) of the sheets P (hereinafter referred to as the “main scanning direction”). The head body  72  is attached to a lower surface of the head holder  70 . That is, the inkjet printer  10  of the present embodiment is a line-type printer. In the present embodiment, all of the ink discharge heads  14   a  to  14   d  are similarly configured. In the following, therefore, description will be made only of the ink discharge head  14   a , and description of the other ink discharge heads  14   b  to  14   d  will be omitted. 
     As illustrated in  FIG. 3 , the head body  72  of the ink discharge head  14   a  includes a flow channel unit  74  and a plurality (eight in the present embodiment) of actuator units  76  joined to an upper surface of the flow channel unit  74 . The flow channel unit  74  is a laminated body formed by a plurality of metal plates. A lower surface of a nozzle plate  74   a  forming the lowermost layer serves as the nozzle surface  20   a  provided with the plurality of nozzles  20 . Further, as illustrated in  FIG. 3 , manifolds  80  ( FIG. 2 ), sub-manifolds  82  communicating with the manifolds  80 , and a plurality of separate ink flow channels  88  extending from the sub-manifolds  82  to the nozzles  20  through apertures  84  and pressure chambers  86  are formed inside the flow channel unit  74 . As illustrated in  FIG. 2 , an upper surface  74   b  of the flow channel unit  74  is formed with a plurality of ink supply ports  80   a  communicating with the manifolds  80 . 
     Further, although not illustrated, a reservoir unit for reserving the ink is disposed above the head body  72  ( FIG. 1 ) in the head holder  70  ( FIG. 1 ). The reservoir unit is connected to the ink tank  16   a  ( FIG. 1 ) via a tube and a pump  89   a  ( FIG. 4 ). As illustrated in  FIG. 4 , the pump  89   a  is electrically connected to the controlling unit  24 . With the pump  89   a  controlled by the controlling unit  24 , the ink reserved in the ink tank  16   a  ( FIG. 1 ) is supplied to the reservoir unit of the ink discharge head  14   a  with predetermined timing. Other pumps  89   b  to  89   d  illustrated in  FIG. 4  correspond to the ink discharge heads  14   b  to  14   d , respectively. 
     As illustrated in  FIG. 2 , each of the plurality (eight in the present embodiment) of actuator units  76  is formed to have a substantially trapezoidal shape in a plan view. Mutually adjacent ones of the actuator units  76  are disposed in juxtaposition in the main scanning direction such that the respective upper or lower sides of the adjacent actuator units  76  are located on the mutually opposite sides. Further, respective portions located near or included in the plurality of actuator units  76  (the upper surface  74   b  of the flow channel unit  74  in the present embodiment) are provided with temperature sensors  90  each functioning as a “temperature sensor” for detecting the temperature of the corresponding actuator unit  76 . The temperature sensors  90  are electrically connected to the controlling unit  24 . The controlling unit  24  is therefore capable of grasping the temperature of the ink discharge head  14   a  for each of the actuator units  76  on the basis of the outputs from the temperature sensors  90 . The actuator units  76  and the temperature sensors  90  are not necessarily required to correspond to each other in a one-to-one fashion, and a single common temperature sensor  90  may cover the plurality of actuator units  76 . 
     As illustrated in  FIG. 3 , the plurality of actuator units  76  include a plurality of actuators  77  (indicated by grid lines in  FIG. 3 ) corresponding to the pressure chambers  86  of the plurality of separate ink flow channels  88 . Each of the actuators  77  includes a piezoelectric layer  77   a  and electrodes  77   b  and  77   c  disposed to sandwich the piezoelectric layer  77   a . Further, one end portion of a flexible printed circuit (FPC) mounted with driver integrated circuits (ICs) is electrically connected to the respective electrodes  77   b  and  77   c  of the plurality of actuators  77 . The other end portion of the FPC is electrically connected to the controlling unit  24  ( FIG. 1 ). In the present embodiment, the actuators  77  of all of the actuator units  76  are electrically connected to the controlling unit  24  ( FIG. 1 ) via a common FPC. 
     In the driver ICs (illustration omitted), a voltage waveform having a predetermined voltage value and a predetermined waveform is generated on the basis of a signal supplied by the controlling unit  24  ( FIG. 1 ). In the actuator units  76 , the actuators  77  are driven on the basis of the voltage waveform to discharge the ink from the nozzles  20 . In some cases, therefore, a change in temperature occurs in the actuator units  76  owing to transmission of heat generated in the driver ICs (illustration omitted) or physical deformation of the actuators  77 , and causes fluctuation of a discharge characteristic of the ink discharged from the nozzles  20  ( FIG. 3 ). For example, if the ink discharge amount discharged to the sheets P is increased, the frequency of deformations of the actuators  77  is increased, and the heat generation amount of the driver ICs is increased. In some cases, therefore, the temperature of the actuator units  76  rises, and the ink discharge amount is unnecessarily increased. Meanwhile, if the ink discharge amount discharged to the sheets P is reduced (including a zero amount), the frequency of deformations of the actuators  77  is reduced (including zero times), and the heat generation amount of the driver ICs is reduced. If the ambient temperature T Z  is lower than the temperature of the ink discharge head  14   a , therefore, the temperature of the actuator units  76  falls and the ink discharge amount is unnecessarily reduced in some cases. In addition, the viscosity of ink in the nozzles  20  will decrease if the temperature of the actuator units  76  becomes high, therefore the temperature of the actuator units  76  rise and the ink discharge amount is unnecessarily increased, even if the energy inputted into the actuator units  76  is the same. 
     In view of the above, to suppress unnecessary fluctuation of the ink discharge amount, the present embodiment causes the controlling unit  24  to change at least one of the voltage value and the waveform (pulse width, for example) of the voltage waveform to be input to the ink discharge heads  14   a  to  14   d.    
     Configuration of Controlling Unit: The controlling unit  24  ( FIG. 1 ) is a computer including a not-illustrated central processing unit (CPU) (including a timer), a nonvolatile memory which rewritably stores a control program executed by the CPU and a variety of data, and a random access memory (RAM) which temporarily stores data in the execution of a program. Further, as illustrated in  FIG. 4 , the control program executed by the CPU and the nonvolatile memory or the RAM realize an image data storing unit  92 , a head controlling unit  94 , a conveyance controlling unit  96 , a liquid transport controlling unit  98 , a discharge history storing unit  100 , a temperature information processing unit  102 , a first estimated temperature calculating unit  104 , a head input setting changing unit  106 , a second estimated temperature calculating unit  108 , and an estimated temperature correcting unit  110 . In addition, the controlling unit  24  may be constituted by ASIC (Application Specific Integrated Circuit) or a FPGA (Field-Programmable Gate Array). 
     The image data storing unit  92  stores image data transmitted from, for example, a personal computer. In general, image data has density values of colors corresponding to respective pixels arranged in a matrix corresponding to a print area of a sheet P. Further, after being stored in the image data storing unit  92 , the image data is converted into data corresponding to the ink discharge heads  14   a  to  14   d . Specifically, the image data is converted, for each of the pixels, into discharge amount data which indicates the amount of the ink to be discharged from the nozzles  20  ( FIG. 3 ) in four levels of zero, small, medium, and large droplet amounts. 
     The head controlling unit  94  controls the voltage value and the waveform of the voltage waveform to be input to the ink discharge heads  14   a  to  14   d  ( FIG. 1 ) such that a predetermined amount of ink according to the discharge amount data is discharged to positions in the sheet P corresponding to the respective pixels. For example, the head controlling unit  94  controls the voltage value such that the voltage value is increased in accordance with the increase of the ink discharge amount (zero, small, medium, or large droplet amount) read from the discharge amount data. Alternatively, on the basis of a control signal transmitted from the head controlling unit  94 , the driver ICs provided in the ink discharge heads  14   a  to  14   d  generate a voltage waveform corresponding to the ink discharge amount (zero, small, medium, or large droplet amount) read from the discharge amount data. The head controlling unit  94  may simultaneously control both of the voltage value and the waveform of the voltage waveform, not just controlling one of the voltage value and the waveform. Herein, the control of the voltage waveform (the voltage value and the waveform) is a commonly used control required for a normal printing operation. 
     The conveyance controlling unit  96  controls the motor  46  of the conveying unit  28  ( FIG. 1 ) and the motor  55  of the sheet feeding unit  30  ( FIG. 1 ) to convey the sheets P to the discharge areas Q 1  to Q 4  ( FIG. 1 ) with predetermined timing and appropriately change the conveying speed V ( FIG. 5 ) of the sheets P and the intervals W 1  to W 3  ( FIG. 5 ) between the sheets P. The liquid transport controlling unit  98  controls the operation of the pumps  89   a  to  89   d  to supply the respective inks to the respective reservoir units of the ink discharge heads  14   a  to  14   d . The discharge history storing unit  100  stores a “discharge history” of the ink discharge heads  14   a  to  14   d . Herein, the “discharge history” refers to a history relating to ink discharge conditions of the ink discharge heads  14   a  to  14   d . Specifically, the respective ink discharge amounts discharged from the ink discharge heads  14   a  to  14   d  and the dot counts of the respective inks in the ink discharge heads  14   a  to  14   d  form the “discharge history.” The “discharge history” is generated on the basis of the image data (discharge amount data) stored in the image data storing unit  92 . 
     The temperature information processing unit  102  acquires temperature information from respective signals output from the ambient temperature sensor  25  and the plurality of temperature sensors  90 . The first estimated temperature calculating unit  104  functions for calculating a first estimated temperature t 1  (estimated temperature) of each of the ink discharge heads  14   a  to  14   d  corresponding to the time in which the nozzle surface  20   a  is facing the first gap G 1 . On the basis of a first actual temperature T 1  (actual temperature) of each of the ink discharge heads  14   a  to  14   d  acquired by the temperature information processing unit  102  while the nozzle surface  20   a  is facing the second gap G 2  ( FIG. 5 ) and the discharge history (ink discharge amount, for example) of the ink discharged to the sheet P located between the first gap G 1  and the second gap G 2 , the first estimated temperature calculating unit  104  calculates the first estimated temperature t 1  of each of the ink discharge heads  14   a  to  14   d  corresponding to the time in which the nozzle surface  20   a  is facing the first gap G 1 . The first estimated temperature t 1  is calculated by a look-up table which contains a combination of the first actual temperature T 1  and the discharge history. In addition, a calculation formula may be used instead of a look-up table, and the first estimated temperature t 1  may be calculated. 
     For example, as illustrated in  FIG. 5 , as to one of the actuator units  76  of the ink discharge head  14   a , if the ink discharge amount discharged to the sheet P located between the first gap G 1  and the second gap G 2  is used as the “discharge history,” the value of the first estimated temperature t 1  of the ink discharge head  14   a  corresponding to the time in which the nozzle surface  20   a  is facing the first gap G 1  is increased to be higher than the first actual temperature T 1  in accordance with the increase of the ink discharge amount. Therefore, the first estimated temperature calculating unit  104  calculates the first estimated temperature t 1  to be higher than the first actual temperature T 1  such that the difference in temperature between the first estimated temperature t 1  and the first actual temperature T 1  is increased in accordance with the increase of the ink discharge amount. Further, it is considered that a temperature rise amount between the second gap G 2  and the first gap G 1  is increased in accordance with the increase of a temperature rise amount between the second gap G 2  and the third gap G 3  located downstream of the second gap G 2  in the conveying direction. Therefore, the first estimated temperature calculating unit  104  calculates the first estimated temperature t 1  such that the first estimated temperature t 1  is increased in accordance with the increase of the temperature rise amount between the third gap G 3  and the second gap G 2 . It is thereby possible to make the first estimated temperature t 1  approach an actual temperature T 0  (temperature) of the ink discharge head  14   a  measured while the nozzle surface  20   a  is facing the first gap G 1 . 
     Further, the first actual temperature T 1  of the ink discharge heads  14   a  to  14   d  is also affected by the ambient temperature T Z . Therefore, the first estimated temperature calculating unit  104  corrects the value such that the first estimated temperature t 1  is increased if the ambient temperature T Z  is higher than the first actual temperature T 1  and the difference between the two temperatures is increased. Conversely, the first estimated temperature calculating unit  104  corrects the value such that the first estimated temperature t 1  is reduced if the ambient temperature T Z  is lower than the first actual temperature T 1  and the difference between the two temperatures is increased. The combinations of the first actual temperature T 1 , the ink discharge amount, and the ambient temperature T Z , and the values of the first estimated temperature t 1  are previously stored in the nonvolatile memory as the look-up table or the calculation formula, and are used to calculate the first estimated temperature t 1 . 
     The head input setting changing unit  106  functions as “head input setting changing unit” for changing, on the basis of the first estimated temperature t 1 , the setting of at least one of the voltage value and the waveform of the voltage waveform to be input to the ink discharge heads  14   a  to  14   d , while the nozzle surface  20   a  is facing the first gap G 1  of the plurality of gaps G 1  to G 3  generated between the plurality of sheets P conveyed by the sheet conveying mechanism  22  functioning as “recording medium conveying unit.” It is considered that the ink discharge amount is unnecessarily increased in the ink discharge heads  14   a  to  14   d  in accordance with the increase of the first estimated temperature t 1 . Further, it is considered that, if the ambient temperature T Z  is lower than the temperature of the ink discharge heads  14   a  to  14   d , the ink discharge amount is unnecessarily reduced in accordance with the reduction of the first estimated temperature t 1 . Therefore, the head input setting changing unit  106  controls the voltage value of the voltage waveform such that the voltage value is reduced in accordance with the increase of the first estimated temperature t 1 , and controls the voltage value of the voltage waveform such that the voltage value is increased in accordance with the reduction of the first estimated temperature t 1 . Alternatively, the head input setting changing unit  106  changes the setting to generate a voltage waveform which reduces the ink discharge amount in accordance with the increase of the first estimated temperature t 1 , or changes the setting to select, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which reduces the ink discharge amount in accordance with the increase of the first estimated temperature t 1 . Further, the head input setting changing unit  106  changes the setting to generate a voltage waveform which increases the ink discharge amount in accordance with the reduction of the first estimated temperature t 1 , or changes the setting to select, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which increases the ink discharge amount in accordance with the reduction of the first estimated temperature t 1 . The voltage waveform which reduces or increases the ink discharge amount refers to a voltage waveform which reduces or increases the ink discharge amount as compared with the voltage waveform at the first actual temperature T 1  (reference temperature). With this configuration, the actually discharged ink discharge amount is kept substantially constant regardless of the temperature. It is therefore possible to suppress excessive fluctuation of the ink discharge amount and thereby suppress fluctuation of the print density. As to which one of the voltage value and the waveform should be controlled, the setting may be changed as appropriate. Only one of the voltage value and the waveform may be controlled, or both thereof may be controlled. 
     The head input setting changing unit  106  also functions as “facing time detecting unit” for detecting (calculating) the duration of a state in which the nozzle surface  20   a  is facing the first gap G 1  (hereinafter referred to as the “facing time”). The intervals W 1  to W 3  of the sheets P are determined as the image data stored in the image data storing unit  92  or initial values of the inkjet printer  10 , and are normally set to a constant value. The head input setting changing unit  106  detects (calculates) the facing time on the basis of the image data stored in the image data storing unit  92  or the initial values of the inkjet printer  10  and the conveying speed V ( FIG. 5 ) of the sheets P. 
     In the present embodiment, the temperature sensor  90  functioning as “first head temperature measuring unit” is provided for each of the plurality of actuator units  76 . Therefore, the first estimated temperature calculating unit  104  calculates the first estimated temperature t 1  for each of the plurality of actuator units  76 , and the head input setting changing unit  106  controls the setting of at least one of the voltage value and the waveform of the voltage waveform for each of the plurality of actuator units  76 . 
     In a normal printing operation, the first estimated temperature t 1  is calculated in the first estimated temperature calculating unit  104 , as described above. To more appropriately adjust the voltage value and the waveform, however, it is desirable to correct the first estimated temperature t 1  in accordance with the difference between the first estimated temperature t 1  and the actual temperature T 0  such that the first estimated temperature t 1  approaches the actual temperature T 0 . Therefore, the estimated temperature correcting unit  110  functioning as “estimated temperature correcting unit” corrects the first estimated temperature t 1  such that the first estimated temperature t 1  approaches the actual temperature T 0 . That is, the temperature sensor  90  measures a second actual temperature T 2  (previous actual temperature) of the ink discharge head  14   a  while the nozzle surface  20   a  is facing a fourth gap G 4  (the same as the third gap G 3  in the present embodiment) located downstream of the second gap G 2  in the conveying direction. Further, the second estimated temperature calculating unit  108  ( FIG. 4 ) calculates, on the basis of the second actual temperature T 2  and a previous discharge history (ink discharge amount, for example) of the ink discharged to the sheet P located between the second gap G 2  and the fourth gap G 4 , a second estimated temperature t 2  (previous estimated temperature) of the ink discharge head  14   a  corresponding to the time in which the nozzle surface  20   a  is facing the second gap G 2 . Further, the estimated temperature correcting unit  110  functioning as the “estimated temperature correcting unit” corrects, on the basis of the difference between the second estimated temperature t 2  and the second actual temperature T 2 , the first estimated temperature t 1  calculated by the first estimated temperature calculating unit  104 . For example, if the second estimated temperature t 2  is higher than the second actual temperature T 2 , the first estimated temperature t 1  is also considered to be higher than the first actual temperature T 1  by a similar degree. Therefore, the estimated temperature correcting unit  110  functioning as the “estimated temperature correcting unit” corrects the first estimated temperature t 1  such that the first estimated temperature t 1  is higher than the value calculated by the first estimated temperature calculating unit  104 . If the ink discharge amount substantially changes, the estimated temperature correcting unit  110  corrects the first estimated temperature t 1  by also taking the fluctuation of the ink discharge amount into account. 
     Controlling Operation of Controlling Unit: A controlling operation of the controlling unit (computer)  24  on an ink discharge head  14   a  illustrated in  FIG. 5  will be described below in accordance with the flowchart of  FIG. 6 . As illustrated in  FIG. 6 , it is determined at Step S 1  whether or not the facing time detected by the head input setting changing unit  106  functioning as the “facing time detecting unit” is less than the time required to measure the actual temperature T 0  of the ink discharge head  14   a  and change the voltage waveform (at least one of the voltage value and the waveform) (hereinafter referred to as the “necessary time”). If the facing time is less than the necessary time, a “YES” determination is made. If the facing time is equal to or more than the necessary time, a “NO” determination is made. 
     Then, if a “YES” determination is made at Step S 1 , the first actual temperature T 1  of the ink discharge head  14   a  corresponding to the time in which the nozzle surface  20   a  is facing the second gap G 2  is measured at Step S 3 , and the first estimated temperature t 1  of the ink discharge head  14   a  corresponding to the time in which the nozzle surface  20   a  is facing the first gap G 1  is calculated at Step S 5 . In addition, Step S 5  is completed even before the nozzle surface  20   a  is facing the first gap G 1 . Thereafter, it is determined at Step S 7  whether or not the difference between the first actual temperature T 1  and the first estimated temperature t 1  is equal to or greater than a first predetermined value. If a “YES” determination is made, the setting of at least one of the voltage value and the waveform of the voltage waveform is changed at Step S 9 . In addition, Step S 9  is performed while the nozzle surface  20   a  is facing the first gap G 1 . Thereafter, the procedure proceeds to Step S 11 . Meanwhile, if a “NO” determination is made, the procedure directly proceeds to Step S 11 . At Step S 11 , whether or not to complete the controlling operation is determined. If it is determined to continuously perform the printing operation on the sheet P, a “NO” determination is made, and the procedure returns to Step S 1 . If it is determined to complete the printing operation, a “YES” determination is made, and the controlling operation is completed. The first predetermined value is a value beforehand set up from an experiment. Two or more first predetermined values are set up, each corresponding to a different first actual temperature T 1 , and these values are stored in the nonvolatile memory. The first predetermined value is defined as a temperature difference where there is not an unacceptable deterioration in the image. If the difference between the actual temperature and the estimated temperature is less than the first predetermined value, then the setting of the voltage waveform is not changed by the head input setting changing unit. If the difference is equal to or greater than the first predetermined value, the head input setting changing unit changes the setting of the voltage waveform. 
     If a “NO” determination is made at Step S 1 , the first actual temperature T 1  is measured at Step S 13 , and the actual temperature T 0  is measured at Step S 15 . The actual temperature T 0  is the actual temperature of the ink discharge head  14   a  measured while the nozzle surface  20   a  is facing the first gap G 1 . The actual temperature T 0  is directly detected by the temperature sensors  90 . Thereafter, it is determined at Step S 17  whether or not the difference between the first actual temperature T 1  and the actual temperature T 0  is equal to or greater than a second predetermined value. If a “YES” determination is made, the procedure proceeds to Step S 9 . If a “NO” determination is made, the procedure proceeds to Step S 11 . The second predetermined value as well as the first predetermined value is beforehand set up from an experiment, and is stored in the nonvolatile memory. In this embodiment, the second predetermined value is the same value as the first predetermined value. 
     As described above, the head input setting changing unit  106  functioning as the “head input setting changing unit” changes the setting of at least one of the voltage value and the waveform of the voltage waveform, when the difference between the first actual temperature T 1  and the first estimated temperature t 1  is equal to or greater than the first predetermined value (Step S 9 ). It is therefore possible to prevent frequent changes of the setting of the voltage value and the waveform and thereby reduce the power consumption. Further, the head input setting changing unit  106  functioning as the “head input setting changing unit” changes the setting of at least one of the voltage value and the waveform by using the actual temperature T 0  in place of the first estimate temperature t 1 , when the facing time is equal to or more than the necessary time (Steps S 13  to S 17  and Step S 9 ). It is therefore possible to more appropriately change the setting of at least one of the voltage value and the waveform of the voltage waveform on the basis of the actual temperature T 0  of the ink discharge head  14   a , when the facing time is equal to or more than the necessary time. 
     At Step S 9 , if the ink discharge amount discharged to the sheet P located between the first gap G 1  and the second gap G 2  is equal to or greater than a third predetermined value, the head input setting changing unit  106  functioning as the “head input setting changing unit” may control the voltage waveform to reduce the voltage value, generate a voltage waveform which reduces the ink discharge amount, or select, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which reduces the ink discharge amount. Further, if the ambient temperature T Z  is equal to or lower than a fourth predetermined value, the head input setting changing unit  106  functioning as the “head input setting changing unit” may increase the voltage value, change the setting to generate a voltage waveform which increases the ink discharge amount, or change the setting to select, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which increases the ink discharge amount. Further, if the ambient temperature T Z  is equal to or higher than a fifth predetermined value, the head input setting changing unit  106  functioning as the “head input setting changing unit” may reduce the voltage value, change the setting to generate a voltage waveform which reduces the ink discharge amount, or change the setting to select, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which reduces the ink discharge amount. 
     For example, if the ink discharge amount is equal to or greater than the third predetermined value, the temperature of the ink discharge head  14   a  rises to a predetermined temperature or higher, and the ink discharge amount is unnecessarily increased. Therefore, the ink discharge amount is suppressed by the reduction of the voltage value, the generation of a voltage waveform which reduces the ink discharge amount, or the selection, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which reduces the ink discharge amount. It is thereby possible to adjust the ink discharge amount to an appropriate value. The third predetermined value is a value beforehand set up from an experiment. Two or more third predetermined values are set up, each corresponding to a different first actual temperature T 1 , and these values are stored in the nonvolatile memory. The third predetermined value is set as the value changed to the difference in temperature by which the temperature of the ink discharge head  14   a  is equivalent to the first predetermined value with the heat which arises by ink discharge. Further, for example, if the ambient temperature T Z  is equal to or lower than the fourth predetermined value, the temperature of the ink discharge head  14   a  falls to a predetermined temperature or lower, and the ink discharge amount is unnecessarily reduced. Therefore, the ink discharge amount is increased by the increase of the voltage value, the generation of a voltage waveform which increases the ink discharge amount, or the selection, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which increases the ink discharge amount. It is thereby possible to adjust the ink discharge amount to an appropriate value. The fourth predetermined value is a value beforehand set up from an experiment. Two or more forth predetermined values are set up, each corresponding to a different first actual temperature T 1 , and these values are stored in the nonvolatile memory. Further, for example, if the ambient temperature T Z  is equal to or higher than the fifth predetermined value, the temperature of the ink discharge head  14   a  rises to a predetermined temperature or higher, and the ink discharge amount is unnecessarily increased. Therefore, the ink discharge amount is suppressed by the reduction of the voltage value, the generation of a voltage waveform which reduces the ink discharge amount, or the selection, from the voltage waveforms stored in the nonvolatile memory, a voltage waveform which reduces the ink discharge amount. It is thereby possible to adjust the ink discharge amount to an appropriate value. The fifth predetermined value is a value beforehand set up from an experiment. Two or more fifth predetermined values are set up, each corresponding to a different first actual temperature T 1 , and these values are stored in the nonvolatile memory. 
     Second Embodiment 
       FIG. 7  is a flowchart illustrating a controlling operation of a controlling unit (computer) in an inkjet printer according to a second embodiment. The controlling operation of the controlling unit on an ink discharge head  14   a  illustrated in  FIG. 5  will be described below in accordance with the flowchart of  FIG. 7 . As illustrated in  FIG. 7 , in the second embodiment, the first actual temperature T 1  is first measured at Step S 21 , and the first estimated temperature t 1  is calculated at Step S 23 . Thereafter, it is determined at Step S 25  whether or not the difference between the first actual temperature T 1  and the first estimated temperature t 1  is equal to or greater than a sixth predetermined value. The sixth predetermined value is the same value as the first predetermined value. 
     If a “YES” determination is made at Step S 25 , the procedure proceeds to Step S 29 . At Step S 29 , it is determined whether or not the facing time detected by the head input setting changing unit  106  functioning as the “facing time detecting unit” is less than the time required to change the setting of the voltage waveform (at least one of the voltage value and the waveform) (hereinafter referred to as the “changing time”). In addition, Step S 29  is completed even before the nozzle surface  20   a  is facing the first gap G 1 . Then, if a “YES” determination is made, a process of extending the facing time is performed at Step S 33 , and thereafter the procedure proceeds to Step S 30 . If a “NO” determination is made, the procedure directly proceeds to Step S 30 . Then, the setting of at least one of the voltage value and the waveform of the voltage waveform is changed at Step S 30 . In addition, Step S 30  is performed while the nozzle surface  20   a  is facing the first gap G 1 . Thereafter, the procedure proceeds to Step S 31 . 
     In the process of extending the facing time, the head input setting changing unit  106  functioning as the “facing time detecting unit” detects the facing time in which the nozzle surface  20   a  is facing the first gap G 1 . Further, if the facing time is less than the time required to change the setting of at least one of the voltage value and the waveform of the voltage waveform, the conveyance controlling unit  96  functioning as “conveying speed controlling unit” temporarily reduces the conveying speed V to thereby extend the facing time. Alternatively, if the facing time is less than the time required to change the setting of at least one of the voltage value and the waveform of the voltage waveform, the conveyance controlling unit  96  functioning as “conveyance interval controlling unit” temporarily increases the conveyance interval W 1  ( FIG. 5 ) to thereby extend the facing time. 
     Meanwhile, if a “NO” determination is made at Step S 25 , the procedure directly proceeds to Step S 31 . At Step S 31 , whether or not to complete the controlling operation is determined. If it is determined to continuously perform the printing operation on the sheet P, a “NO” determination is made, and the procedure returns to Step S 21 . If it is determined to complete the printing operation, a “YES” determination is made, and the controlling operation is completed. 
     In the second embodiment, the process of extending the facing time is performed to temporarily extend the facing time, only when it is difficult to change the setting of the voltage waveform (at least one of the voltage value and the waveform) within the facing time. Even if the printing speed is increased, therefore, it is possible to appropriately perform the setting of at least one of the voltage value and the waveform of the voltage waveform, while maintaining a high printing speed. 
     Other Embodiments 
     In the above-described embodiments, the “liquid discharge apparatus” according to an embodiment of the present invention is applied to the inkjet printer which discharges ink. In another embodiment, the “liquid discharge apparatus” according to an embodiment of the present invention may be applied to a processing liquid discharge apparatus which discharges a processing liquid or a liquid discharge apparatus which discharges another liquid. Further, as to the liquid discharging method, the method using actuators may be replaced by a method of discharging a liquid by using pressure generated when the volume of the liquid is expanded by a heat generating element. Further, the “liquid discharge apparatus” according to an embodiment of the present invention may be applied to a serial printer in place of the above-described line printer. 
     In the above-described embodiments, the first actual temperature T 1  is measured while the nozzle surface  20   a  is facing the first gap G 1 . This is based on consideration that, if an ink discharge head  14  is performing the discharging operation, noise may be generated in a circuit of the inkjet printer  10  owing to the driving of the ink discharge head  14  and prevent accurate measurement of the first actual temperature T 1 . In the above-described embodiments, therefore, the actual temperature is measured while the nozzle surface  20   a  is facing a gap between the sheets P, in which the sheets P are absent. However, the actual temperature may be measured while the nozzle surface  20   a  is facing a sheet P, unless the ink discharge head  14  is performing the discharging operation. In this case, the “gap” includes not only the area between the sheets P, in which the sheets P are absent, but also an area in an end portion of the sheet P, in which printing is not performed.