Patent Publication Number: US-11020965-B2

Title: Printing apparatus and print head heating method

Description:
BACKGROUND 
     Field 
     The present disclosure relates to a printing apparatus that drives a print head using power in a power storage unit and relates to a print head heating method. 
     Description of the Related Art 
     Since a motor in printing apparatuses frequently switches between driven and stopped states, printing apparatuses use current having the maximum value larger than the maximum value of current with which electronic devices that consume the same level of power. US2017/0334226 discloses an inkjet printing apparatus that utilizes a power storage element so that the apparatus operates even when power supplied from a power supply unit is small. After execution of a sequence operation, the inkjet printing apparatus stores power needed for executing the next sequence operation in the power storage element, and then starts the next sequence operation. Time needed for raising voltage across the power storage element is secured, whereby shortage of power supplied from an external power supply can be also solved during operation to be performed thereafter. 
     In addition, it is known that ink discharge performance of the inkjet-type printing apparatus can be maintained by heating a print head. Japanese Patent Application Laid-Open No. 2000-108328 discloses heating a print head by supplying a print head with a driving pulse having a small pulse width to the extent no bubbles are generated in ink. 
     SUMMARY 
     When a heating element is used for heating in the same manner as in Japanese Patent Application Laid-Open No. 2000-108328 in printing apparatuses that include a power storage unit such as the one disclosed in US2017/0334226, there is the following concern. That is, when power supplied to the power storage unit is small, there is a concern that it takes time to store power needed for the next operation in the power storage unit after heating a print head, and the temperature of the print head warmed by the heating decreases to a temperature that is no longer suitable for the next operation. 
     In consideration of the foregoing, the present disclosure has been made to solve the above inconvenience and features a technique for utilizing stored power to heat a print head and causing the print head to have a temperature suitable for operation when the operation is started after the heating. 
     According to an aspect of the present disclosure, a printing apparatus includes a print head including an ink discharge port and a heating element for heating the print head to heat ink contained in the print head, a power storage unit configured to store therein electric charge supplied from an external power supply, a power detection unit configured to detect power supplied from the external power supply, a temperature detection unit configured to detect a temperature of the print head, a heating control unit configured to control, as heating control, heating of the heating element to heat the print head by driving the heating element using electric charge stored in the power storage unit, based on a detection result by the temperature detection unit, an execution unit configured to execute a predetermined operation using the print head and using electric charge stored in the power storage unit after the heating control is completed, and a temperature determination unit configured to determine a target temperature, the target temperature being a temperature to which the heating element is driven to heat the print head in the heating control to bring a temperature of the print head to a set predetermined temperature or higher when the predetermined operation starts, in accordance with the supplied power detected by the power detection unit and with the set temperature. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an apparatus configuration of a printing apparatus according to a first exemplary embodiment. 
         FIGS. 2A, 2B, and 2C  are schematic diagrams illustrating a configuration of a print head according to the first exemplary embodiment. 
         FIG. 3  is a block diagram illustrating a power supply control configuration of the printing apparatus according to the first exemplary embodiment. 
         FIG. 4  is a block diagram illustrating an entire control configuration of the printing apparatus according to the first exemplary embodiment. 
         FIG. 5  is a block diagram illustrating processing procedure in a head temperature control circuit according to the first exemplary embodiment. 
         FIG. 6  is a flowchart illustrating a print head heating process according to the first exemplary embodiment. 
         FIG. 7  is a diagram illustrating a relation between an elapsed time and a head temperature in a case where a temperature of the print head is decreased from a predetermined temperature and the relation thereof with control parameters according to the first exemplary embodiment. 
         FIG. 8  is a flowchart illustrating a print head heating process according to a second exemplary embodiment. 
         FIG. 9  is a flowchart illustrating a print head heating process according to a third exemplary embodiment. 
         FIGS. 10A and 10B  are diagrams each illustrating changes in temperature and in stored-power amount according to the first to third exemplary embodiments. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     &lt;Entire Configuration&gt; 
       FIG. 1  is a schematic perspective view of an inkjet printing apparatus  300  (hereinafter printing apparatus  300 ) in a first exemplary embodiment. In  FIG. 1 , inkjet print heads  107  and  108  each have a print head and an ink tank in an integrated manner. While a print head of tank-integrated type is used in the present exemplary embodiment, a print head that is detachable from an ink tank may be used instead. The first print head  107  includes ink tanks of cyan, magenta, and yellow ink, and the second print head  108  includes an ink tank of black ink. Each of the print heads  107  and  108  includes a recording chip  202  having ink discharge ports arrayed in the Y direction to perform printing by discharging the ink from the individual discharge ports. A sheet feed roller  105  rotates to feed a printing medium P and also functions to hold the printing medium P. A conveyance roller  103  rotates while pressing the printing medium P in cooperation with an auxiliary roller  104  and intermittently conveys the printing medium P in the positive Y direction. 
     A platen  101  supports the back surface of the printing medium P in a printing position. A carriage  106  supports the first print head  107  and the second print head  108  and moves in the X directions. The carriage  106  reciprocates in a printing area in the X directions by a carriage belt  102  which is driven by a carriage motor (not illustrated) when printing is executed on a printing medium. The position and the speed of the carriage  106  are detected by an encoder sensor (not illustrated) mounted on the carriage  106  and an encoder scale (not illustrated) stretched across the printing apparatus. The movement of the carriage  106  is controlled based on these position and speed. The print heads  107  and  108  discharge ink while the carriage  106  moves, to execute printing on a printing medium. 
     The carriage  106  is on standby at a home position h when printing is not being executed or when operation such as recovery operation for the print head is performed. A recovery unit  109  (not illustrated) is provided at the home position h. The recovery unit  109  includes a wiping mechanism that wipes out ink droplets adhering to the front surfaces (discharge port surfaces) of the discharge ports in the print heads  107  and  108  to recover the normal state of the surfaces of the discharge ports. The recovery unit  109  further includes a capping mechanism to cover the discharge ports and a suction mechanism to suction ink from the discharge ports via the capping mechanism. 
     &lt;Print Head Configuration&gt; 
       FIGS. 2A, 2B and 2C  are schematic diagrams illustrating a configuration of the first print head  107  according to the present exemplary embodiment.  FIG. 2A  is a perspective view illustrating the first print head  107 .  FIG. 2B  is a partially transparent schematic view illustrating the first print head  107  as viewed in the Z direction. The first print head  107  receives a print signal from the printing apparatus body via a contact pad  201 , and power to drive the print head  107  is supplied thereto. The recording chip  202  includes a substrate provided with ink discharge heaters that are energy-generating elements for generating energy for discharging ink. This substrate is formed of, for example, silicon. The recording chip  202  further has thereon a diode sensor  203  to detect the temperature of the substrate and a discharge port formation member for forming a discharge port array  204  to discharge cyan ink, a discharge port array  205  to discharge magenta ink, and a discharge port array  206  to discharge yellow ink. The recording chip  202  further has thereon a sub-heater  207  for heating ink, which is a heating element disposed in a shape extensively surrounding the discharge port arrays  204 ,  205 , and  206 . This sub-heater  207  heats the substrate in the print head  107  by having voltage applied thereto, so that the substrate thus heated heats the ink. The sub-heater  207  is formed of a single metal such as aluminum or another metal or an alloy of aluminum or another metal, the resistance value of which changes depending on the temperature thereof. The sub-heater  207  may be formed of a single layer or may be formed of a plurality of layers. The sub-heater  207  does not necessarily need to surround the discharge port arrays  204 ,  205 , and  206  in the form of a single continuous member and is formed to be able to substantially uniformly heat the entirety of the discharge port arrays  204 ,  205 , and  206 . 
       FIG. 2C  is an enlarged view of the discharge port array  204  for cyan ink in the print head  107 . Discharge ports  209  to discharge 5 pl of ink and discharge ports  211  to discharge 2 pl of ink are disposed on opposite sides of an ink chamber  208  in  FIG. 2C . Immediately beneath the respective discharge ports (in the positive Z direction), 5-pl ink discharge heaters  210  and 2-pl ink discharge heaters  212  are disposed as corresponding heating elements. With voltage applied to the ink discharge heaters  210  and  212 , thermal energy is generated, so that ink is discharged from the discharge ports  209  and  211 . The number of the discharge ports  209  to discharge 5 pl of ink and the number of discharge ports  211  to discharge 2 pl of ink are 160. Each adjacent two of the discharge ports  209  and  211  in the Y direction have an interval of 1/600 inches therebetween, thus being configured to provide a printed pixel density of 600 dpi. Ink can be heated when drive pulses set to levels that can keep ink from being discharged are applied to the ink discharge heaters  210  and  212 . Hereinafter, such heating control is referred to as short pulse heating control. In addition, the sub-heater  207  is capable of heating ink by transmitting heat to the ink via a member in the substrate in the neighborhood of the sub-heater  207 . 
     The printing apparatus  300  according to the present exemplary embodiment adjusts the temperature of the print head substrate and the temperature of ink by performing the short pulse heating control and controlling the sub-heater  207 . According to the present exemplary embodiment, heating is carried out to increase the temperature of ink near each of the discharge ports. However, the diode sensor  203  is attached to the substrate and measures the temperature of the substrate, thus not being configured to directly measure the temperature of ink. When ink is heated, the substrate is also heated, ink in the print head  107  and the substrate are brought to temperatures of substantially the same value. Therefore, in the present exemplary embodiment, the temperature of the substrate serves as a head temperature. Between the short pulse heating control and sub-heater heating control in the present exemplary embodiment, the amount of thermal energy generated per unit of time is larger in the short pulse heating control. Therefore, the short pulse heating control increases the temperature of the print head  107  faster. Meanwhile, while printing is being executed, the ink discharge heaters  210  and  212  are being used for discharging ink and are not used for short pulse heating control. Given this point, according to the present exemplary embodiment, the sub-heater heating control is executed when the temperature of ink is heated to a target temperature during printing, and the short pulse heating control is executed when the temperature of ink is heated to a target temperature not during printing. 
     The head temperature is adjusted through the sub-heater heating control and the short pulse heating control in such a manner that feedback control is performed by switching the print head substrate state between heated and not-heated so that a temperature based on a detection value acquired from the diode sensor  203  described later can be closer to a target temperature. The same is applied to the second print head  108 , which is not illustrated. 
     &lt;Power-Feed Configuration for Power Supply&gt; 
       FIG. 3  is a block diagram illustrating a power-feed configuration for a power supply of the printing apparatus  300  according to the present exemplary embodiment. An external power supply  301  according to the present exemplary embodiment is, for example, a personal computer (PC) provided with a (universal serial bus) USB port. The external power supply  301  may be a PC that corresponds to USB 2.0 and USB 3.0. Alternatively, the external power supply  301  may be a PC or a capacitor that corresponds to a power storage standard for USBs such as the Battery Charging Specification or to a large power feeding capability such as USB Power Delivery. Further alternatively, the external power supply  301  may be a device, such as an AC adapter, that is not provided with a USB interface. 
     An external power input unit  302  is a connector for providing connection to the external power supply  301 . 
     A supplied-power detection unit  303  detects power supplied from the external power supply  301  to the external power input unit  302 . Power that can be supplied from the external power input unit  302  is thus detected. Desirably, this detection of the power that can be supplied is automatically performed upon connection to the external power supply  301 . For example, the external power input unit  302  that has a shape corresponding to a USB standard can determine the standard by using a USB communication cable. Alternatively, a dedicated connector may be utilized for the external power input unit  302 , so that the determination is made through a communication or the like that has been uniquely arranged with the external power supply  301 . Because a voltage drop occurs due to a resistance component such as a connector or a cable that connects together the external power supply  301  and the external power input unit  302 , it is more desirable to measure power that can be actually supplied, than to determine power that can be logically supplied. Power actually supplied can be measured by measuring current or voltage. Thus, the external power supply  301  can be prevented from being excessively burdened by being caused to supply power that is larger than actually supplied from the external power input unit  302 . According to the present exemplary embodiment, power actually supplied is detected by measuring voltage. The supplied-power detection unit  303  thus configured enables charging power to be appropriately set by a power charging control unit  308  described later in relation to various kinds of power that can be supplied that are defined by a plurality of standards. 
     Power acquired from the external power input unit  302  is supplied to a voltage conversion unit  304  and the power charging control unit  308 . The power is converted by the voltage conversion unit  304  to have voltage with which to drive a system-related load  305  and then consumed by the system-related load  305 . The system-related load  305  includes a system control unit  306  and a necessary-power amount prediction unit  307 . The system control unit  306  includes a central processing unit (CPU) to perform system control of the inkjet printing apparatus  300  and a memory. The necessary-power amount prediction unit  307  is a device configured to predict the amount of power needed during execution of operation such as image printing. According to the present exemplary embodiment, the amount of power predicted by the necessary-power amount prediction unit  307  is used by the system control unit  306  to set power storage target voltage for the power storage unit  309  and to control the power storage unit  309 . 
     The power charging control unit  308  utilizes power input from the external power input unit  302  to store power in the power storage unit  309 . During this storing, power storage current with which the power charging control unit  308  stores electric charge in the power storage unit  309  is controlled so that the sum of the power storage current and the current to be consumed in the voltage conversion unit  304  can be kept from exceeding assumed tolerable current of the external power supply  301 . The maximum power storage current is thus controlled. In a configuration where the supplied-power detection unit  303  refers to the communication or the standard when detecting power that can be supplied, charging power is desirably set smaller than power that can be supplied theoretically. An electric double layer capacitor is desirably used as the power storage unit  309  in consideration of its capability to speedily store and discharge power and being less prone to degradation from repeated power charging and discharging. Note that a power storage current value is determined subject to the condition that the value does not exceed current that can be supplied by the external power supply  301  described above and in consideration of other factors. Those factors include the power storage capability of the power charging control unit  308  itself and the maximum power storage current that is allowed to flow through the power storage unit  309  to provide electric charge to the power storage unit  309 . 
     The stored-power amount detection unit  310  detects the amount of stored power in the power storage unit  309 . A method for the detection is selected in accordance with the type of the power storage unit  309 . For example, the method may include estimating the amount of stored electric charge by measuring the voltage across the terminals of the power storage unit  309  or may include setting up a coulomb counter by observing current input to and output from the power storage unit  309 . The present exemplary embodiment is assumed to employ a method that includes detecting the voltage across the terminals of the power storage unit  309  to estimate the amount of the stored power. 
     The stored-power amount detection unit  310  is connected to the system control unit  306  and utilized as information to be used for performing control according to the present exemplary embodiment. 
     The voltage conversion unit  311  converts voltage from the power storage unit  309  into voltage necessary for the drive-related load  312 . In a case where an electric double layer capacitor is used as the power storage unit  309 , discharging power therefrom results in a large drop in voltage across the terminals thereof because the amount of stored electric charge and the voltage across the terminal are proportional to each other. The voltage conversion unit  311  is desirably compatible with a relatively wide range of input voltage to be able to tolerate such a voltage drop caused when the power storage unit  309  discharges power. The drive-related load  312  refers to driving of any member or members in the printing apparatus  300  from those illustrated in  FIG. 1  such as the carriage belt  102 , the conveyance roller  103 , and the print heads  107  and  108 , and the recovery unit  109 . According to the present exemplary embodiment, power from the external power supply  301  is supplied to the drive-related load  312  via the power storage unit  309 . However, an alternative configuration may be employed in which the drive-related load  312  is connected directly to both the power storage unit  309  and the external power supply  301 , and power can be supplied to the drive-related load  312  directly from the external power supply  301 . In such a case, when the external power supply  301  is one that supplies relatively small power, power is supplied to the drive-related load  312  after being temporarily stored power storage unit  309 . When the external power supply  301  is one that supplies relatively large power, power supply is switched so that the external power supply  301  can directly supplies power to the drive-related load  312 . 
     Regarding the drive-related load  312 , it is assumed that whether to apply current to each of the print heads  107  and  108  and whether to cause each motor to operate or stop are controlled based on determination of the system control unit  306 . 
     Operation to be performed by the printing apparatus  300  thus configured is described next. 
     Upon connection of the external power supply  301  to the external power input unit  302 , power acquired from the external power input unit  302  is converted into voltage for the system-related load  305  by the voltage conversion unit  304  and then supplied to the system-related load  305 . At the same time, the power other than current for the system load is stored in the power storage unit  309  by the power charging control unit  308 . The stored-power amount in the power storage unit  309  is monitored by the stored-power amount detection unit  310 , and the power charging control unit  308  stops power from being stored in the power storage unit  309  when the stored power reaches a predetermined value. Power stored in the power storage unit  309  is supplied to the drive-related load  312  via the voltage conversion unit  311 . When the amount of stored power in the power storage unit  309  decreases to below a predetermined value as a result of operation by the drive-related load  312 , power is stored by the power charging control unit  308 . 
     &lt;Entire Control Configuration&gt; 
       FIG. 4  is a block diagram illustrating the entire control configuration of the printing apparatus  300  according to the present exemplary embodiment. Constituent elements of the present control configuration are basically categorized into software-based control units and hardware-based processing units. The software-based control units correspond to the part of the system-related load  305  in  FIG. 3 , include processing units that individually access a main bus line  405  in  FIG. 4  such as an image input unit  403 , an image signal processing unit  404  that responds to the image input unit  403 , and a central control unit CPU  400 . The hardware-based processing units correspond to the drive-related load  312  in  FIG. 3 . The drive-related load  312  includes processing units illustrated in  FIG. 4  such as an operation unit  408 , a recovery operation control circuit  409 , a head temperature control circuit  414 , a head drive control circuit  416 , a carriage drive control circuit  406 , and a conveyance control circuit  407 . The CPU  400  typically includes the ROM  401  and the RAM  402 , provides appropriate printing conditions to input information, and executes printing while driving the ink discharge heaters  210  and  212  in the print heads  107  and  108 . The CPU  400  controls the power charging control unit  308  based on information on the amount of stored power in the power storage unit  309  detected by the stored-power amount detection unit  310 . The CPU  400  also controls the head temperature control circuit  414  (described later) based on information on the amount of stored power in the power storage unit  309  detected by the stored-power amount detection unit  310 . 
     The ROM  401  has a computer program for executing recovery operation on a print head previously stored therein and provides recovery conditions such as a preliminary discharge condition to the recovery operation control circuit  409  and the print heads  107  and  108 . A recovery motor  410  drives the print heads  107  and  108  and members that carry out recovery operation on the print heads  107  and  108 , which are a wiping blade  411 , a cap  412 , and a suction pump  413 . Based on a detection result from the diode sensor  203  that detects head temperatures, the head temperature control circuit  414  determines driving conditions to be applied to driving of the sub-heaters  207  on the print heads  107  and  108 . The head drive control circuit  416  then drives the sub-heaters  207  based on the determined driving conditions. 
     The head drive control circuit  416  also drives the ink discharge heaters  210  and  212  on the print heads  107  and  108 . This driving of these heaters  210  and  212  causes the print heads  107  and  108  to perform ink temperature adjustment for ink discharge, preliminary discharge, and temperature adjustment control. A computer program for executing the temperature adjustment control has been stored in, for example, the ROM  401  and causes operation, such as detection of the head temperatures and driving of the sub-heaters  207 , to be executed via circuits such as the head temperature control circuit  414  and head drive control circuit  416 . Note that the head drive control circuit  416  drives ink discharge heaters  210  and  212  by using drive signals each composed of a pre-pulse and a main pulse, and ink is discharged. 
     &lt;Head Temperature Acquisition Control&gt; 
     Print head temperature acquisition control in the present exemplary embodiment is described next.  FIG. 5  is a block diagram illustrating the flow of processing in the head temperature control circuit  414  and processing to be performed on software via a read-only memory (ROM)  401  and a random access memory (RAM)  402 . When voltage based on the print head temperatures is input to the head temperature control circuit  414  from the diode sensors  203  provided on the print heads  107  and  108 , the amplifier  501  amplifies the values of the voltage. The amplified voltage values are then digitalized by an analog-digital (AD) converter  502 . Diode sensor voltage values ADdi obtained through the digitalization are converted into diode temperatures, which are referred to as head temperatures Th herein, by use of an ADdi-temperature conversion formula  503  stored in the ROM  401 . In parallel, when voltage based on the environment temperature surrounding the printing apparatus  300  is input from a thermistor  415  to the head temperature control circuit  414 , the AD converter  505  digitalizes the voltage. A thermistor voltage value ADtm obtained through the digitalization is converted into a thermistor temperature Tenv by use of an ADtm-temperature conversion table  506  stored in the ROM  401 . The head temperature Th and thermistor temperature Tenv thus obtained are input to the head temperature detector  504  to be used for control described later according to the present exemplary embodiment. 
     The flow of the print head heating process in the printing apparatus  300  configured as described above is described next. If the head temperatures Th are low when the print heads  107  and  108  are used to print an image or to perform ink discharge (preliminary discharge) that has no effect on image printing, discharging a desired amount of ink or even discharging any ink may fail. Therefore, the head temperatures are raised by heating the print heads  107  and  108  before discharge is started. The print heads  107  and  108  are heated so that the head temperatures Th when ink discharge is started can become a set temperature T 1  or higher. According to the present exemplary embodiment, if the amount of stored power in the power storage unit  309  is less than power needed for ink discharge after the heating process is performed, power is stored in the power storage unit  309 . Because the heating is not provided while power is being stored, the head temperatures Th decrease over the period from when the heating operation is ended to when ink discharge is started. In consideration of this point, the heating process provides heating in which a target temperature Tn that is the set temperature T 1  or higher is set so that the head temperatures Th at the start of discharge can be the set temperature T 1  or higher even if the head temperatures Th have decreased. The following describes heating the print heads  107  and  108  by short pulse heating. Alternatively, the head temperatures Th may be raised by heating provided by the sub-heaters  207 . Heating is provided so that the head temperatures Th can reach the target temperature Tn, and the heating process is ended when the head temperature detector  504  detects that the head temperatures Th are the target temperature Tn or higher. 
       FIG. 6  is a flowchart illustrating processing procedure of the print head heating process in the printing apparatus  300  according to the first exemplary embodiment. The heating process in step S 600  and steps subsequent thereto is a process to be performed when the CPU  400  causes the head temperature control circuit  414  and the print heads  107  and  108  to operate by executing a computer program stored in the ROM  401 . 
     In step S 600 , the heating process is started when the CPU  400  acknowledges a preliminary discharge instruction or a printing instruction. 
     Subsequently, in step S 601 , the supplied-power detection unit  303  detects the supplied power P 1  that is being supplied from the external power supply  301  connected to the external power input unit  302 . 
     Subsequently, in step S 602 , a target temperature correction value ΔT is set based on the supplied power P 1  using the set temperature T 1  for the print heads  107  and  108 . The set temperature T 1  has been set in advance and stored in the ROM  401 , and is read out from the ROM  401 . The target temperature correction value ΔT is set so that, even if the head temperatures Th decreases while the power charging control unit  308  stores power in the power storage unit  309  after the print heads  107  and  108  are heated, the head temperatures Th at the start discharge may be the set temperature T 1  or higher. A calculation method for the target correction temperature ΔT is detailed later. 
     Subsequently, in step S 603 , the target temperature for the head temperatures is set to (T 1 +ΔT) and determines the temperature thus set to be the target temperature Tn in the heating process. 
     Subsequently, in step S 604 , the target temperature Tn is compared with a maximum set temperature Tmax. The maximum set temperature Tmax is the upper limit of a range of temperature that does not affect stable discharge. If the target temperature Tn is the maximum set temperature Tmax or lower, the processing proceeds to step S 606 . If the target temperature Tn is higher than the maximum set temperature Tmax, the value of the target temperature Tn is replaced by the value of the maximum set temperature Tmax from (T 1 +ΔT) in step S 605 , and the processing proceeds to step S 606 . Through Steps S 604  and S 605 , the target temperature Tn that enables the print head  107  or  108  to be heated to as high a temperature as possible can be set even when the target temperature Tn set in step S 603  is higher than a range of temperature that enables ink to be stably discharged. 
     In subsequent step S 606 , the head temperature detector  504  detects the head temperatures Th, and the stored-power amount detection unit  310  detects the power storage voltage Ve. 
     Subsequently, in step S 607 , the head temperatures Th are compared with the target temperature Tn. If the head temperatures Th are the target temperature Tn or higher, the heating is ended because the target temperature Tn or higher has been reached through the heating. If any of the head temperatures Th is lower than the target temperature Tn, the processing proceeds to step S 608 . 
     In step S 608 , the power storage voltage Ve is compared with the minimum power storage voltage Vmin. The minimum power storage voltage Vmin is voltage that prevents voltage from falling below operation ensuring voltage Vth, which is the lower limit of a range of voltage that does not affect stable heating when operation in subsequent step S 609  is performed. If the power storage voltage Ve is less than the minimum power storage voltage Vmin, the processing returns to step S 606  without heating. If the power storage voltage Ve is the minimum power storage voltage Vmin or more, the print heads  107  and  108  are heated for t 1  milliseconds in step S 609 . The print heads  107  and  108  are heated with drive signals sent from the head drive control circuit  416  to the respective ink discharge heaters  210  and  212  of the print heads  107  and  108 . The drive signals provide pulses that are short to the extent that no bubbles are generated in ink. In this manner, when the print heads  107  and  108  are heated in step S 609 , voltage across the power storage unit  309  is prevented from dropping to the lower limit (hereinafter referred to as operation ensuring voltage) of a range of voltage that can drive the print heads  107  and  108  or that does not affect stable operation of the entire printing apparatus  300 . 
     After the heating in step S 609 , the processing proceeds to step S 606 , so that the heating may be repeated until the head temperatures Th become the target temperature Tn or higher. 
     After the completion of the heating process, power is stored until voltage across the power storage unit  309  becomes ink-discharge voltage V 1 , which is voltage needed for discharging ink. Ink then starts to be discharged. When the heating is ended while the power storage voltage Ve is less than the minimum power storage voltage Vmin in S 608 , the target temperature Tn or higher has not been reached through the heating. However, ink starts to be discharged when the ink-discharge voltage V 1  or higher is reached after the completion of the heating process. The target temperature Tn is set so that the set temperature T 1  may be reached in a power storage time tc. Therefore, ink discharge may be started the power storage time tc later than the completion of the heating process so that ink discharge may be started after the head temperatures reach the set temperature T 1 . 
     Next, a control method and a method for setting parameters used in steps S 602 , S 604 , S 605 , and S 608  are described. 
     A target temperature correction value ΔT in step S 602  is described. From the supplied power P 1  detected by the supplied-power detection unit  303 , the power storage time tc is predicted, which is required for the power charging control unit  308  to store necessary stored power amount in the power storage unit  309  for ink discharge after heating the print heads  107  and  108 . Subsequently, a temperature decrease in the head temperature Th that is expected to occur in the next power storage time tc, and this temperature decrease is set as the target temperature correction value ΔT. The set temperature T 1  herein is set to temperature at which the print heads  107  and  108  suitably discharge ink, which is 50° C. according to the present exemplary embodiment. 
     The power storage time tc is set to maximum possible power storage time in the present exemplary embodiment. The power storage time tc is calculated as time needed for the power storage unit  309  to store power until the ink-discharge voltage V 1  needed for the ink-discharge operation after the heating is reached, by using the operation ensuring voltage Vth as the starting point. The ink-discharge voltage V 1  herein is obtained by the system control unit  306  after the necessary-power amount prediction unit  307  predicts a power consumption amount needed for operation to be performed after the heating. The power storage time tc is independent of the power storage voltage Ve and is found by a formula tc=(V 1 −Vth)/Q 1  on the assumption that the supplied power P 1  is stored at substantially constant power storage speed Q 1 . For the power storage speed Q 1 , the power storage speeds Q 1  that correspond to various values of the supplied power P 1  have been previously stored in the ROM  401 . In the above-described manner, the power storage time tc that corresponds to a particular value of the supplied power P 1  can be obtained. 
     The target temperature correction value ΔT can be obtained using the power storage time tc and a temperature decrease curve based on measured head temperatures. The relation between the time and the head temperature Th in the temperature decrease curve has been stored in the ROM  401  in the form of an approximation formula or a table.  FIG. 7  illustrates a graph of a temperature decrease curve. The graph depicts the relation between the elapsed time and the head temperature Th and the relation thereof with control parameters according to the present exemplary embodiment in a case where the temperature of the print head  107  or  108  is decreased from a certain temperature. As illustrated in  FIG. 7 , the target temperature correction value ΔT is obtained by finding the difference (Tx−T 1 ) of the set temperature T 1  with a temperature Tx at a time point tb that is at least the power storage time tc earlier than a time point ta at which the set temperature T 1  is reached. 
     An alternative method for setting the target temperature correction value ΔT may be employed in which, while a table or the like that prescribes the target temperature correction value ΔT in association with the supplied power P 1  and the set temperature T 1  has been stored in advance in the ROM  401 , the target temperature correction value ΔT is read out onto the RAM  402  as appropriate to be set. 
     In steps S 604  and S 605 , the maximum set temperature Tmax is desirably set to a value (Tth−Ta) obtained by subtracting a temperature Tth from a temperature increase Ta that is expected to occur to the print head  107  or  108  through the heating in step S 609 . The temperature Tth is the upper limit of a range of temperature that can ensure that the print head  107  or  108  can operate. Thus, the head temperature Th can be prevented from exceeding Tth even when the print head  107  or  108  has been heated in step S 609 . 
     In step S 608 , the minimum power storage voltage Vmin is desirably set to a value (Vth+Va) obtained by adding a voltage drop Va to the operation ensuring voltage Vth. The voltage drop Va is a voltage drop expected to occur to the power storage unit  309  through the heating of the print head  107  or  108  in step S 609 . Thus, the power storage voltage Ve can be prevented from falling below the operation ensuring voltage Vth even when the print head  107  or  108  has been heated in step S 609 . 
     Upon completion of the heating when the heating process is ended, the power storage unit  309  has stored therein power needed for the ink-discharge operation, and the ink-discharge operation is started. 
     In a case where the target temperature Tn is set to the maximum set temperature Tmax in step S 605 , the head temperature Th is lower than the set temperature T 1  at the start of the ink-discharge operation. Although the ink-discharge operation is started even if the head temperature Th is lower than the set temperature T 1  at the start of the ink-discharge operation in the present exemplary embodiment, the ink-discharge operation may be started after the print head  107  or  108  is heated again to the set temperature T 1  before the start of the ink-discharge operation. 
     Alternatively, the target temperature correction value ΔT may be calculated with consideration given to the environment temperature. For example, the relation between the time and the temperature in the temperature decrease curve for the print head  107  or  108  has been stored in the ROM  401  in the form of an approximation formula or a table with respect to each value of the environment temperature Tenv. The target temperature correction value ΔT that corresponds to the environment temperature Tenv can be obtained using, in step S 602 , the approximation formula or the table that corresponds to the environment temperature Tenv after the head temperature detector  504  detects the environment temperature Tenv in step S 601 . Thus, the target temperature correction value ΔT can be obtained with higher accuracy. 
     In the first exemplary embodiment, the target temperature Tn for the print heads  107  and  108  is corrected assuming that voltage at the start of power storage when the power charging control unit  308  stores power in the power storage unit  309  after the print head heating is the operation ensuring voltage Vth that is a fixed value. In a second exemplary embodiment, the target temperature Tn is corrected further based on the result of measurement of voltage at the start of the power storage.  FIG. 8  illustrates a flowchart for a heating process in the second exemplary embodiment. Elements different from those in the first exemplary embodiment are mainly described, and descriptions of the identical elements are omitted. 
     In step S 800 , the heating process is started when the CPU  400  receives the preliminary discharge instruction or the printing instruction in the same manner as in step S 600 . 
     Subsequently, in step S 801 , the supplied-power detection unit  303  detects the supplied power P 1  that is being supplied from the external power supply  301  connected to the external power input unit  302 . In addition, the head temperature detector  504  detects the head temperatures Th, and the stored-power amount detection unit  310  detects the power storage voltage Ve of the power storage unit  309 . 
     Subsequently, in step S 802 , a tentative target temperature T 3  is set, and the number n of times that temperature calculation is attempted is set to 1. The tentative target temperature T 3  is the set temperature T 1  or higher and has been previously set to a certain desirable value. 
     Subsequently, in step S 803 , post-heating power storage voltage V 2 , which is power storage voltage after the print head  107  or  108  is heated from the head temperature Th to the target temperature T 3 , is calculated using the supplied power P 1 , the head temperature Th, the power storage voltage Ve, and the tentative target temperature T 3 . A method for obtaining the post-heating power storage voltage V 2  is described later. 
     If the post-heating power storage voltage V 2  is the minimum power storage voltage Vmin or more in step S 804  subsequently, the processing proceeds to step S 805 . If the post-heating power storage voltage V 2  is less than the minimum power storage voltage Vmin, the processing proceeds to step S 812 . The processing in step S 812  and steps subsequent thereto is described later. 
     In step S 805 , based on the supplied power P 1 , the power storage time tc needed for the power charging control unit  308  to store power while causing the voltage across the power storage unit  309  to reach V 1  from V 2  after the head temperature Th is heated to T 3  is found using the post-heating power storage voltage V 2  and the ink-discharge voltage V 1 . The power storage time tc is found using the formula tc=(V 1 −Vth)/Q 1  with the post-heating power storage voltage V 2  used in place of the operation ensuring voltage Vth used in step S 602  in the first exemplary embodiment. 
     Subsequently, in step S 806 , the target temperature correction value ΔT is found using the set temperature T 1 , the power storage time tc, and the approximation formula or the table in the same manner as in step S 602  in the first exemplary embodiment. First of all, to bring a temperature at the start of ink discharge to the set temperature T 1  when the power storage time tc is needed, the temperature Tx needed when the heating process is ended is found. Subsequently, the target temperature correction value ΔT is found using the formula ΔT=Tx−T 1 , and the target temperature Tn is set to (T 1 +ΔT) in step S 807 . 
     Subsequently, in step S 808 , the target temperature Tn is compared with a maximum set temperature Tmax in the same manner as in step S 604 . If the target temperature Tn is higher than the maximum set temperature Tmax, the value of the target temperature Tn is replaced by the value of the maximum set temperature Tmax from (T 1 +ΔT) in step S 809 , and the processing proceeds to step S 814 . If the target temperature Tn is the maximum set temperature Tmax or lower, the processing proceeds to step S 810 . Through the above processing, the target temperature Tn can be set to a temperature that enables the print head  107  or  108  to be heated to as high a temperature as possible that enables stable discharge of the print head  107  or  108  even when the target temperature Tn set in step S 807  is higher than a range of temperature that enables ink to be stably discharged. 
     If the processing has proceeded to step S 810 , the tentative target temperature T 3  is compared with the target temperature Tn in step S 810 . If the tentative target temperature T 3  is the target temperature Tn or higher, the head temperature can be maintained at the set temperature T 1  or higher even after the power charging control unit  308  stores power in the power storage unit  309  after the heating. The heating is then started in step S 814 . In contrast, if the tentative target temperature T 3  is lower than the target temperature Tn, the processing proceeds to step S 811 . In step S 811 , a temperature step Ts is added to the tentative target temperature T 3 , and the tentative target temperature is updated to (T 3 +Ts), which is followed by increment of n by 1. The processing then returns to step S 803 . The temperature step STs is an interval of temperature at which a temperature desired to be detected is measured and is set to a predetermined value in advance. 
     If the processing has proceeded to step S 814 , the head temperature Th is compared with the target temperature Tn. If the head temperature Th is the target temperature Tn or higher, it means that sufficient heating has been provided, and the heating process is therefore ended. If the head temperature Th is lower than the target temperature Tn, the processing proceeds to step S 815 . In step S 815 , the print heads  107  and  108  are heated for t 1  milliseconds in the same manner as in step S 609  in the first exemplary embodiment. Thereafter, the head temperature detector  504  detects the head temperature Th in step S 816 , and the processing returns to S 814 . Steps S 814  to S 816  are repeated until the head temperature Th reaches the target temperature Tn or higher. 
     After the completion of the heating process, power is stored until voltage across the power storage unit  309  becomes the ink-discharge voltage V 1 , which is voltage needed for discharging ink. Ink then starts to be discharged. 
     In step S 804 , if the post-heating power storage voltage V 2  is less than the minimum power storage voltage Vmin, it is determined in step S 812  whether n is 1. If n is 1, it means that heating to the tentative target temperature T 3  that has been set for the first time after the start of the heating process is impossible with the power storage voltage Ve of the power storage unit  309  detected in step S 803 . The heating process is therefore ended. If n is greater than 1, the processing proceeds to S 813 . For example, if n is 3, heating to the tentative target temperature T 3  that has been set with n=3 is impossible because the post-heating power storage voltage V 2  exceeds the minimum power storage voltage Vmin. However, heating to the tentative target temperature T 3  that has been set with n=2 is possible without having the post-heating power storage voltage V 2  exceeding the minimum power storage voltage Vmin. Therefore, in step S 813 , the target temperature Tn is set to the tentative target temperature (T 3 −Ts) with n=3, that is, the tentative target temperature T 3  with n=2, and the heating is started in step S 814 . The processing in step S 804  can prevent the voltage across the power storage unit  309  from falling below the operation ensuring voltage Vth when the print heads  107  and  108  are heated after step S 814 . 
     An example of the method for obtaining the post-heating power storage voltage V 2  in step S 803  is described. First of all, time th needed for heating the print heads  107  and  108  from the head temperature Th to the tentative target temperature T 3  is found while power needed for heating the print heads  107  and  108  is denoted as P 2 . It can be simply considered that the time th needed for the heating is proportional to the difference between the temperatures and inversely proportional to the power. Based on this consideration, the time th needed for the heating can be found using the formula th=A×(T 3 −Th)/P 2 . The term “A” here is a constant, the value of which can be experimentally obtained. Subsequently, a voltage drop ΔV in the power storage unit  309  as a result of heating of the print heads  107  and  108  for the time th with the power P 2  is found using the supplied power P 1 , the power P 2 , and the time th. It can be simply considered that the voltage drop ΔV is proportional to the product of consumed power and the time th. Based on this consideration, the voltage drop ΔV can be found using the formula ΔV=B×(P 2 −P 1 )×th. The term “B” here is a constant, the value of which can be experimentally obtained. The post-heating power storage voltage V 2  can be obtained using the formula V 2 =Ve−ΔV with the power P 2 , the constant A, and the constant B stored in the ROM  401  or the RAM  402  and used as appropriate. 
     The first and second exemplary embodiments illustrate methods in which the target temperature Tn is corrected and set prior to heating the print heads  107  and  108 . A third exemplary embodiment illustrates processing in which, while the print heads are heated, the target temperature Tn is successively corrected and set in accordance with voltage of the corresponding time point across the power storage unit  309 .  FIG. 9  is a flowchart illustrating a heating process according to the third exemplary embodiment. Elements different from those of the first and second exemplary embodiments are mainly described, and descriptions of the identical elements are omitted. 
     In step S 900 , the heating process is started when the CPU  400  receives the preliminary discharge instruction or the printing instruction in the same manner as in step S 600  in the first exemplary embodiment. 
     Subsequently, in step S 901 , the supplied-power detection unit  303  detects the supplied power P 1  that is being supplied from the external power supply  301  connected to the external power input unit  302 . 
     Subsequently, in step S 902 , the head temperature detector  504  detects the head temperatures Th, and the stored-power amount detection unit  310  detects the power storage voltage Ve of the power storage unit  309 . 
     In step S 903 , the power storage time tc needed for the power charging control unit  308  to store power while increasing the voltage across the power storage unit  309  from Ve to V 1  is found using the supplied power P 1 , the power storage voltage Ve, and the ink-discharge voltage V 1 . The power storage time tc is found using the formula tc=(V 1 −Vth)/Q 1  with the power storage voltage Ve used in place of the operation ensuring voltage Vth used in step S 602  in the first exemplary embodiment. 
     Subsequently, the same processing as is performed in steps S 602  and S 603  in the first exemplary embodiment is performed in steps S 904  and S 905 . The same processing as is performed in step S 607  in the first exemplary embodiment is performed in subsequent step S 906 . 
     Subsequently, in step S 907 , the head temperatures Th are compared with the maximum set temperature Tmax. If the head temperature Th is the maximum set temperature Tmax or higher, the heating process is ended. If the head temperature Th is lower than the maximum set temperature Tmax, the processing proceeds to step S 908 . 
     Subsequently, the same processing as is performed in steps S 608  and S 609  in the first exemplary embodiment is performed in steps S 908  and S 909 . The power storage voltage Ve is compared with the minimum power storage voltage Vmin in step S 908 . If the power storage voltage Ve is less than the minimum power storage voltage Vmin, the heating is ended. If the power storage voltage Ve is the minimum power storage voltage Vmin or more, the print heads  107  and  108  are heated for t 1  milliseconds in step S 909 . After the print heads  107  and  108  are heated in S 909 , the processing returns to step S 902 . 
     After the completion of the heating process, power is stored until voltage across the power storage unit  309  becomes the ink-discharge voltage V 1 , which is voltage needed for discharging ink. Ink then starts to be discharged. 
       FIGS. 10A and 10B  are schematic diagrams each illustrating the head temperature and the voltage across the power storage unit  309  in the first to third exemplary embodiments until the head temperature reaches the set temperature T 1  after the heating process is performed.  FIG. 10A  illustrates a case where the external power supply  301  is an alternating current (AC) adapter or the like and the supplied power P 1  is relatively large.  FIG. 10B  illustrates a case where the external power supply  301  is a USB 2.0 capable device and the supplied power P 1  is relatively small. In  FIGS. 10A and 10B , the target temperature Tn is the maximum set temperature Tmin or lower, and the post-heating power storage voltage V 2  is Vmin or less. In the case of  FIG. 10A , the supplied power P 1  is large. Therefore, a voltage drop in the power storage unit  309  when the print head is heated is small and the power charging control unit  308  stores power in the power storage unit  309  at a high speed after the heating. As a result, the power storage time tc is short and the target temperature correction value ΔT is small. In contrast, in the case of  FIG. 10B , the supplied power P 1  is small. Therefore, a voltage drop in the power storage unit  309  when the print head is heated is large and the power charging control unit  308  stores power in the power storage unit  309  at a low speed after the heating. As a result, the power storage time tc is long and the target temperature correction value ΔT is large. Accordingly, the target temperature Tn is higher than in a case where the supplied power P 1  is larger. With the target temperature Tn thus set in accordance with the supplied power P 1 , provided that Tn is Tmax or less and that V 2  is Vmin or more, the head temperature Th can be heated to a temperature of T 1  or higher, which is suitable for ink discharge, when ink-discharge operation is started. This heating is achievable without print head  107  or  108  heated again and regardless of how large or small the supplied power P 1 . 
     In the first to third exemplary embodiments, the operation to be performed after the heating process is ink discharge operation. However, the present exemplary embodiments are not limited to the configuration. For example, since the viscosity of ink decreases as the temperature of the ink is raised, when the discharge port surfaces are wiped using a wiping blade with the ink being in that state, the ink that adheres to the discharge port surfaces returns into the discharge ports or becomes easier to wipe off. 
     OTHER EMBODIMENTS 
     Embodiment(s) of the present disclosure can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     According to exemplary embodiments of the present disclosure, a target temperature based on supplied power is set, and heating is performed. Thus, at the start of operation to be performed after a print head is heated, ink has a temperature that is suitable for the operation. 
     While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2018-184615, filed Sep. 28, 2018, which is hereby incorporated by reference herein in its entirety.