Patent Description:
There is a liquid circulation device in which liquid circulates along a circulation path that includes a liquid discharge head. Such a liquid circulation device is used in an ink jet printer or the like and ejects ink from the liquid discharge head onto a recording medium to form an image thereon. The ink used in the ink jet printer has a temperature range that is considered suitable for ejected ink droplets. Therefore, a heater can be installed along the circulation path and controlled so that temperature of the ink flowing through the circulation path is maintained within the suitable temperature range.

Normally, this type of liquid circulation device performs heating control in a state in which there is a sufficient amount of ink to be circulated in the circulation path. However, heating control may also need to be performed when the ink is not being circulated for some reason. If the heating control is performed while the ink is not being circulated, the temperature of the heater may rise abruptly. If overheating occurs and the temperature exceeds an upper limit, a thermal fuse is likely to be triggered or the heater may be broken. Since the temperature of ink in the vicinity of the heater also rises abruptly, the characteristics of the ink are likely to be altered near the heater.

To this end, there is provided a liquid discharge apparatus according to claim <NUM> and a method for controlling a liquid circulation device comprised in a liquid discharging apparatus according to claim <NUM>.

Documents <CIT>, <CIT>, <CIT> and <CIT> disclose liquid discharge apparatus comprising temperature control means.

Embodiments provide a technique for preventing faults due to overheating in a liquid circulation device in which circulating liquid is heated by a heater or a liquid discharge device using the such a circulation device.

In general, according to one embodiment, a liquid circulation device includes a liquid chamber that stores a liquid to be supplied to a liquid discharge head, a pipeline through which the liquid can be circulated between the liquid chamber and the liquid discharge head, a heater configured to heat the liquid circulating in the pipeline, a first temperature sensor configured to measure a temperature of the heater, a second temperature sensor configured to measure a temperature of the liquid circulating in the pipeline, and a controller. The controller is configured to control the heater based on the temperature of the liquid measured by the second temperature sensor and monitor a change in the temperature of the heater as measured by the first temperature sensor and turn off the heater when the change satisfies a predetermined stop condition.

Hereinafter, a liquid circulation device and a liquid discharge apparatus according to one or more embodiments will be described with reference to the drawings. In this disclosure, an ink jet printer that ejects ink droplets from a circulation type liquid discharge head to form an image on a recording medium is described as an example of a liquid circulation device and a liquid discharge apparatus.

First, a first embodiment will be described with reference to <FIG>. <FIG> is a schematic side view of an inkjet printer <NUM>. The inkjet printer <NUM> includes a plurality of liquid discharge apparatuses <NUM>, a head support mechanism <NUM>, a head drive mechanism <NUM>, a medium support mechanism <NUM>, and a printer controller <NUM>.

The head support mechanism <NUM> supports a plurality of liquid discharge apparatuses <NUM> in along the direction of arrow A. The head drive mechanism <NUM> reciprocates the plurality of liquid discharge apparatuses <NUM> in the directions of arrow A moving the head support mechanism <NUM> in the direction of arrow A. The medium support mechanism <NUM> movably supports a recording medium S in a direction orthogonal to the direction of the arrow A at a position facing the plurality of liquid discharge apparatuses <NUM>. The printer controller <NUM> controls each component of the ink jet printer <NUM> including the head drive mechanism <NUM>.

Each liquid discharge apparatus <NUM> integrally includes a liquid discharge head <NUM> and a liquid circulation device <NUM>. The liquid discharge apparatus <NUM> ejects, for example, ink I from the liquid discharge head <NUM> while moving in the direction of the arrow A to form an image on the recording medium S that is supported by the medium support mechanism <NUM>.

The liquid discharge apparatuses <NUM> each eject a different color ink, for example, cyan ink, magenta ink, yellow ink, black ink, and white ink, are ejected. Although the inks used by the plurality of liquid discharge apparatuses <NUM> are all different from each other, the plurality of liquid discharge apparatuses <NUM> have the same configuration. The color or other characteristics of the ink I used are not limited to those examples. For example, instead of white ink, transparent glossy ink, special ink that develops color when irradiated with infrared rays or ultraviolet rays, or the like may be ejected.

<FIG> is a schematic diagram of the liquid discharge head <NUM> and the liquid circulation device <NUM> of the liquid discharge apparatus <NUM>. The liquid discharge head <NUM> includes an ink supply port <NUM> and an ink discharge port <NUM>. The liquid discharge head <NUM> forms an ink flow path inside the liquid discharge head <NUM> so that ink supplied from the supply port <NUM> is discharged from the discharge port <NUM>.

In the liquid discharge head <NUM>, a plurality of pressure chambers are formed in the middle part of the ink flow path. The liquid discharge head <NUM> supplies ink to each pressure chamber through the ink flow path. The liquid discharge head <NUM> includes a nozzle and an actuator for each pressure chamber. The nozzle communicates with a corresponding pressure chamber. The actuator displaces a volume of the corresponding pressure chamber. In the liquid discharge head <NUM>, ink I in the pressure chamber is ejected from the nozzle communicating with the pressure chamber by changing the volume of the pressure chamber by action of the actuator. The liquid discharge head <NUM> sends the ink in each pressure chamber that has not been ejected from the nozzle to the discharge port <NUM>.

The liquid circulation device <NUM> is integrally connected to the liquid discharge head <NUM> at the upper part of the liquid discharge head <NUM> by, for example, a metallic connecting component. The liquid circulation device <NUM> includes an upstream tank <NUM>, a downstream tank <NUM>, a replenishment tank <NUM>, a circulation pump <NUM>, and a replenishment pump <NUM>. The upstream tank <NUM>, the downstream tank <NUM>, and the replenishment tank <NUM> store the same type of ink. The circulation pump <NUM> is for sending the ink stored in the downstream tank <NUM> to the upstream tank <NUM>. The replenishment pump <NUM> replenishes the upstream tank <NUM> with the ink stored in the replenishment tank <NUM>.

The liquid circulation device <NUM> includes a first pipeline <NUM>, a second pipeline <NUM>, a third pipeline <NUM>, and a fourth pipeline <NUM>. The first pipeline <NUM> is a pipeline connecting the upstream tank <NUM> and the supply port <NUM> of the liquid discharge head <NUM>. The second pipeline <NUM> is a pipeline connecting the discharge port <NUM> of the liquid discharge head <NUM> and the downstream tank <NUM>. The third pipeline <NUM> is a pipeline connecting the downstream tank <NUM> and the upstream tank <NUM>. The fourth pipeline <NUM> is a pipeline connecting the supply tank <NUM> and the upstream tank <NUM>.

The ink stored in the upstream tank <NUM> is transferred to the supply port <NUM> of the liquid discharge head <NUM> through the first pipeline <NUM>, and passes through the ink flow path inside the liquid discharge head <NUM>. The ink discharged from the discharge port <NUM> is transferred to the downstream tank <NUM> through the second pipeline <NUM> and stored in the downstream tank <NUM>. The ink stored in the downstream tank <NUM> is transferred to the upstream tank <NUM> through the third pipeline <NUM> by action of the circulation pump <NUM>, and returns to the upstream tank <NUM>.

As described above, the first pipeline <NUM>, the second pipeline <NUM>, and the third pipeline <NUM> configure a circulation path <NUM> for circulating ink between the upstream tank <NUM> and the liquid discharge head <NUM>.

The liquid discharge apparatus <NUM> ejects the ink that circulates in the circulation path <NUM> from the liquid discharge head <NUM>. For that reason, the amount of ink stored in the upstream tank <NUM> is gradually reduced. If the amount of ink decreases, ink stored in the replenishment tank <NUM> is transferred to the upstream tank <NUM> through the fourth pipeline <NUM> by action of the replenishment pump <NUM> so that the upstream tank <NUM> is replenished (refilled) with the ink.

In the liquid circulation device <NUM>, liquid level sensors <NUM> and <NUM> are provided inside the upstream tank <NUM> and the downstream tank <NUM>, respectively. The liquid level sensor <NUM> measures the amount of ink in the upstream tank <NUM>. The liquid level sensor <NUM> measures the amount of ink in the downstream tank <NUM>. The printer controller <NUM> appropriately controls the replenishment pump <NUM> based on the measurement results of the liquid level sensors <NUM> and <NUM> to replenish the upstream tank <NUM> with the ink from the replenishment tank <NUM>. The printer controller <NUM> replenishes the upstream tank <NUM> to adjust the amount of ink circulating in the circulation path <NUM> to be an appropriate amount (level).

In the liquid circulation device <NUM>, pressure sensors <NUM> and <NUM> are provided in the upstream tank <NUM> and the downstream tank <NUM>, respectively. The pressure sensor <NUM> measures pressure in the upstream tank <NUM>. The pressure sensor <NUM> measures pressure in the downstream tank <NUM>. The liquid circulation device <NUM> includes a pressure adjustment mechanism <NUM>. The pressure adjustment mechanism <NUM> opens the upstream tank <NUM> and the downstream tank <NUM> to the outside (atmosphere), or pressurizes and depressurizes the downstream tank <NUM>. The printer controller <NUM> appropriately controls the pressure adjustment mechanism <NUM> based on the measurement results of the pressure sensors <NUM> and <NUM> to adjust pressure in the circulation path <NUM>. The printer controller <NUM> adjusts ink pressure of the nozzle in the liquid discharge head <NUM> by adjusting pressure in the circulation path <NUM>.

In the liquid circulation device <NUM>, a heater <NUM> is provided in the middle part of the first pipeline <NUM>. The heater <NUM> heats the ink flowing in the first pipeline <NUM>. In the liquid circulation device <NUM>, a first temperature sensor <NUM> is provided on the heater <NUM>. The first temperature sensor <NUM> measures the temperature of the heater <NUM>. In the liquid circulation device <NUM>, a second temperature sensor <NUM> is provided in the middle part of the second pipeline <NUM>. The second temperature sensor <NUM> measures the temperature of the ink flowing through the second pipeline <NUM>. The printer controller <NUM> controls ON or OFF of energization of the heater <NUM> based on the measurement result of the second temperature sensor <NUM>, and adjusts the temperature of the ink flowing through the circulation path <NUM> to an appropriate temperature. The printer controller <NUM> prevents the heater <NUM> from overheating based on the measurement result of the first temperature sensor <NUM>.

<FIG> is a hardware block diagram of the printer controller <NUM>. The printer controller <NUM> includes a processor <NUM>, a read only memory (ROM) <NUM>, a random access memory (RAM) <NUM>, a timer <NUM>, an operation panel <NUM>, a communication interface <NUM>, a motor <NUM>, and a liquid discharge apparatus interface <NUM>. The processor <NUM>, the ROM <NUM>, the RAM <NUM>, the timer <NUM>, the operation panel <NUM>, the communication interface <NUM>, the motor <NUM>, and the liquid discharge apparatus interface <NUM> are connected to each other by a system bus <NUM>. The system bus <NUM> includes an address bus, a data bus, and the like.

The processor <NUM> controls each component of the ink jet printer <NUM> and performs various functions thereof according to an operating system and one or more application programs.

The ROM <NUM> stores the operating system and application programs described above. The ROM <NUM> may store data required for the processor <NUM> to execute a process for controlling each component.

The RAM <NUM> stores data required for the processor <NUM> to execute a process. The RAM <NUM> is also used as a work area where data is temporarily stored by the processor <NUM>. The work area includes an image memory onto which print data (corresponding to the image to be printed on recording medium S) is loaded.

The timer <NUM> tracks current time or the like. The operation panel <NUM> includes an operation unit and a display unit. On the operation unit, function keys such as a power key and an error release key are disposed. The display unit can display various states of the ink jet printer <NUM>.

The communication interface <NUM> is a network interface circuit configured to receive print data from a client terminal connected via a network such as a local area network (LAN). If an error occurs in the ink jet printer <NUM>, for example, the communication interface <NUM> transmits a signal notifying the client terminal or the like of the error. The motor <NUM> is a drive source for the head support mechanism <NUM> and the medium support mechanism <NUM>.

The liquid discharge apparatus interface <NUM> relays signals transmitted and received between the processor <NUM> and the plurality of liquid discharge apparatuses <NUM>. The signals transmitted from the processor <NUM> to each liquid discharge apparatus <NUM> include: a signal for controlling the liquid discharge head <NUM>, a signal for controlling each of the circulation pump <NUM>, the replenishment pump <NUM>, the pressure adjustment mechanism <NUM>, and the heater <NUM> in the liquid circulation device <NUM>, and the like. The signals transmitted from each liquid discharge apparatus <NUM> to the processor <NUM> include measurement signals from the liquid level sensors <NUM> and <NUM>, the pressure sensors <NUM> and <NUM>, and the first and second temperature sensors <NUM> and <NUM> in the liquid circulation device <NUM>.

In such a configuration, the processor <NUM> performs at least ink heating control processing <NUM> and overheating monitoring control processing <NUM>. These functions are performed for each liquid discharge apparatus <NUM>. These functions are implemented by one or more programs stored in the ROM <NUM>.

<FIG> is a flowchart of the ink heating control processing <NUM> performed on one liquid discharge apparatus <NUM>. The ink heating control processing <NUM> for the other liquid discharge apparatuses <NUM> is similar, and thus the description here will be omitted. The heater <NUM> and the second temperature sensor <NUM> are provided in the liquid circulation device <NUM> of the target liquid discharge apparatus <NUM>.

When the processor <NUM> starts the ink heating control <NUM> processing, the processor <NUM> turns ON energization to the heater <NUM> as ACT <NUM>. Then, the processor <NUM> acquires a liquid temperature LT measured by the second temperature sensor <NUM> as ACT <NUM>. The liquid temperature LT is a temperature of the ink flowing through the second pipeline <NUM>.

The processor <NUM> checks whether the liquid temperature LT is equal to or greater than a first threshold value TSa as ACT <NUM>. The first threshold value TSa is an upper limit temperature in a suitable temperature range for ejecting ink droplets. An optimum value of the first threshold value TSa depends on the type of ink used in the liquid discharge apparatus <NUM>, a flow rate of the ink, and the like, and is stored in the RAM <NUM>.

If the liquid temperature LT is less than the first threshold value TSa, the processor <NUM> determines that the check result in ACT <NUM> is NO and returns to ACT <NUM>. The processor <NUM> repeatedly executes the processes of ACT <NUM> and ACT <NUM> until the liquid temperature LT becomes equal to or greater than the first threshold value TSa.

If the liquid temperature LT becomes equal to or greater than the first threshold value TSa, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> turns OFF energization to the heater <NUM> as ACT <NUM>. Then, the processor <NUM> acquires the liquid temperature LT measured by the second temperature sensor <NUM> as ACT <NUM>.

The processor <NUM> checks whether the liquid temperature LT is less than a second threshold value TSb as ACT <NUM>. The second threshold value TSb is a lower limit temperature in the suitable temperature range for ejecting ink droplets. An optimum value of the second threshold value TSb depends on the type of ink used in the liquid discharge apparatus <NUM>, the flow rate of the ink, and the like, and is stored in the RAM <NUM>.

If the liquid temperature LT is equal to or greater than the second threshold value TSb, the processor <NUM> determines that the check result in ACT <NUM> is NO and returns to ACT <NUM>. The processor <NUM> repeatedly executes the processes of ACT <NUM> and ACT <NUM> until the liquid temperature LT becomes less than the second threshold value TSb.

If the liquid temperature LT becomes less than the second threshold value TSb, the processor <NUM> determines that the check result in ACT <NUM> is YES and returns to ACT <NUM>. The processor <NUM> turns ON energization to the heater <NUM> as ACT <NUM>. After that, the processor <NUM> executes the processes of ACT <NUM> and subsequent actions in the same manner as described above.

As described above, the processor <NUM> controls turning ON or OFF of power to the heater <NUM> so that the temperature of the ink flowing through the circulation path <NUM> falls within the suitable temperature range for ejecting ink droplets. By this control, during normal use of the ink jet printer <NUM>, the temperature of the ink ejected from each liquid discharge apparatus <NUM> is appropriately adjusted to be within the suitable temperature range for ejecting ink droplets. As a result, high quality printing becomes possible.

Here, the processor <NUM> performs the ink heating control processing <NUM> by controlling an output of the heater <NUM> based on the ink temperature measured by the second temperature sensor <NUM>.

In the liquid circulation device <NUM>, ink heating control is usually performed by the processor <NUM> in a state in which a sufficient amount of ink is being circulated in the circulation path <NUM>. However, such ink heating control may be performed without the ink presently being circulating, due to, for example, a failure of the circulation pump <NUM> or an occurrence of an error in the process of controlling the circulation pump <NUM>. If the ink heating control is performed when the ink is not being circulated, the temperature of the heater <NUM> will rise abruptly. If the temperature of the heater <NUM> rises abruptly and overheating that exceeds the upper limit temperature of the heater <NUM> occurs, a thermal fuse is likely to be triggered or the heater <NUM> is likely to be broken. Since the temperature of the ink in the vicinity of the heater <NUM> rises abruptly, the characteristics of the ink are likely to be altered. In order to prevent such issues, the processor <NUM> performs the overheating monitoring control processing <NUM> for monitoring overheating of the heater <NUM>.

<FIG> is a flowchart of the overheating monitoring control processing <NUM> performed for a liquid discharge apparatus <NUM>. The overheating monitoring control processing <NUM> for each liquid discharge apparatus <NUM> is similar, and thus specific description will be omitted. The heater <NUM> and the first temperature sensor <NUM> are provided in the liquid circulation device <NUM> of the target liquid discharge apparatus <NUM>.

The processor <NUM> checks whether energization to the heater <NUM> is turned ON as ACT <NUM>. If energization to the heater <NUM> is not turned ON, it is determined that the check result in ACT <NUM> is NO. The processor <NUM> waits for energization to the heater <NUM> to be turned ON.

In the ink heating control processing <NUM> described with reference to <FIG>, if energization to the heater <NUM> is turned ON, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> acquires a heater temperature HT measured by the first temperature sensor <NUM> as ACT <NUM>. The heater temperature HT is the temperature of the heater <NUM>. The processor <NUM> stores the heater temperature HT in the RAM <NUM> as a previous heater temperature HTa as ACT <NUM>.

The processor <NUM> waits for one second using the function of the timer <NUM> as ACT <NUM>. After one second lapses, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> acquires the heater temperature HT measured by the first temperature sensor <NUM> as ACT <NUM>.

The processor <NUM> calculates a temperature rise HTx per second by subtracting the previous heater temperature HTa stored in the RAM <NUM> from the heater temperature HT as ACT <NUM>. Then, the processor <NUM> checks whether the temperature rise HTx is equal to or greater than a third threshold value TSc as ACT <NUM>.

The third threshold value TSc is a value suitable for recognizing that the temperature rise HTx per second, that is, rate of the temperature change per unit time, has a risk of overheating of the heater <NUM>. For example, if the temperature of the heater <NUM> rises by about <NUM> in one second, the heater <NUM> is considered to be at risk of overheating of the heater <NUM>. Therefore, the third threshold value TSc is set to <NUM>.

In ACT <NUM>, if the temperature rise HTx is less than the third threshold value TSc, the processor <NUM> determines that the check result in ACT <NUM> is NO and returns to ACT <NUM>. The heater temperature HT acquired in the process of ACT <NUM> is written into the RAM <NUM> as the previous heater temperature HTa. After that, the processor <NUM> executes the processes of ACT <NUM> and subsequent actions in the same manner as described above.

In ACT <NUM>, if the temperature rise HTx is equal to or greater than the third threshold value TSc, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> turns OFF energization to the heater <NUM> as ACT <NUM>. Thus, the processor <NUM> ends the control of the procedure illustrated in the flowchart of <FIG>.

As such, the processor <NUM> monitors the heater temperature HT measured by the second temperature sensor <NUM> if energization to the heater <NUM> is turned ON. Then, if the temperature rise HTx per second of the heater temperature HT becomes equal to or greater than the third threshold value TSc, the processor <NUM> turns OFF power to the heater <NUM>. If the temperature rise HTx per second of the heater temperature HT becomes equal to or greater than the third threshold value TSc, there is considered to be a risk of overheating being caused by the heater <NUM>. If overheating occurs, a thermal fuse is likely to be triggered or the heater <NUM> is likely to be broken. Since the temperature of the ink in the vicinity of the heater <NUM> rises abruptly, the characteristics of the ink are likely to be altered.

In this embodiment, if the temperature rise HTx per second of the heater temperature HT becomes equal to or greater than the third threshold value TSc, energization to the heater <NUM> is turned OFF. Accordingly, it is possible to prevent overheating by the heater <NUM>. As a result, the thermal fuse will not be triggered and the heater <NUM> will not break due to overheating. Likewise, the characteristics of the ink near heater <NUM> will not be altered due to overheating.

Here, the processor <NUM> performs the overheating monitoring control processing <NUM> by monitoring the temperature change of the heater <NUM> measured by the first temperature sensor <NUM> and turning off the heater <NUM> if the temperature change satisfies the predetermined condition.

<FIG> is a graph of temperature changes of the heater <NUM> controlled by the ink heating control processing <NUM> and the overheating monitoring control processing <NUM>. In <FIG>, the vertical axis represents the temperature HT(°C) of the heater <NUM>, and the horizontal axis represents the time t (seconds).

In <FIG>, when the time t is zero , energization of the heater <NUM> is started (turned ON) in ACT <NUM> of the ink heating control processing <NUM>. If energization to the heater <NUM> is turned ON, the temperature HT of the heater <NUM> will gradually rise with the lapse of time t as illustrated in a section Wa. However, the rate of the temperature change per unit time in section Wa does not become equal to or greater than the rate set by the third threshold value TSc.

In <FIG>, when the time t equals time ta, energization to the heater <NUM> is turned OFF in ACT <NUM> of the ink heating control processing <NUM>. When energization to the heater <NUM> is turned OFF, the temperature HT of the heater <NUM> begins to decrease with time t as illustrated in a section Wb.

In <FIG>, when the time t is equal to time tb, the upstream tank <NUM> is replenished with the ink from the replenishment tank <NUM> by the replenishment pump <NUM>. By the replenishment, the temperature of the ink circulating in the circulation path <NUM> would usually be reduced, and thus energization to the heater <NUM> is turned ON. As a result, the temperature HT of the heater <NUM> rises, as illustrated in a section Wc. However, the rate of the temperature change per unit time of this section Wc does not become equal to or greater than the rate which is set by the third threshold value TSc.

In <FIG>, when the time t is equal to time tc, energization to the heater <NUM> is turned OFF in ACT <NUM> of the ink heating control processing <NUM>. When energization to the heater <NUM> is turned OFF, the temperature HT of the heater <NUM> decreases as illustrated in a section Wd.

In <FIG>, when the time t is equal to time td, the ink is not circulating in the circulation path <NUM>. If the ink does not circulate in the circulation path <NUM>, the temperature HT of the heater <NUM> rises sharply and substantially as illustrated in a section We. The rate of the temperature change per unit time in this section We is equal to or greater than the rate which is set by the third threshold value TSc. If such a state of rising temperature of the heater <NUM> is left unaddressed, the temperature HT will exceed an upper limit temperature, for example, <NUM>, as illustrated at time te. If the temperature HT of the heater <NUM> exceeds the upper limit temperature, the thermal fuse triggers or the heater <NUM> breaks. The characteristics of the ink are also altered due to overheating.

In this embodiment, if an abrupt rise in the heater temperature HT, such as illustrated in the section We, is detected, energization to the heater <NUM> is turned OFF at an early stage in the section We. Accordingly, it is possible to prevent overheating of the heater <NUM> beyond the upper limit temperature. As a result, there is no concern that the thermal fuse operates or the heater <NUM> breaks. The ink characteristics are less likely to be altered due to overheating when the maximum experienced temperature of the ink is kept lower.

Although the first embodiment of the liquid circulation device <NUM> and the liquid discharge apparatus <NUM> is described above, the present disclosure is not limited to the examples described above.

For example, in the first embodiment, the third threshold value TSc to be compared with the inclination of the temperature change per unit time is set as a fixed value. However, in other examples, the third threshold value TSc may be variable according to the temperature of the surrounding environment in which the liquid circulation device <NUM> is located.

<FIG> is a graph illustrating a correspondence between environmental temperature and the third threshold value TSc. In <FIG>, the vertical axis represents the third threshold value TSc and the horizontal axis represents the environmental temperature. The third threshold value TSc is set to <NUM>, for example, if the environmental temperature is <NUM>. Then, if the environmental temperature becomes less than <NUM>, the third threshold value TSc is reduced. When the environmental temperature becomes lower, the temperature of the heater <NUM> does not rise as easily. Therefore, the third threshold value TSc can be reduced. That is, the inclination of the temperature change per unit time is made gentle. Thus, even if the temperature of the heater <NUM> becomes more difficult to increase, it is possible to effectively prevent overheating such that the temperature of the heater <NUM> does not exceed the upper limit temperature. Accordingly, even if the environmental temperature is low, overheating of the heater <NUM> can be reliably prevented.

If the environmental temperature becomes higher than <NUM>, the third threshold value TSc can be increased. When the environmental temperature becomes high, the initial (pre-energization) temperature of the heater <NUM> tends to rise as well. Therefore, the third threshold value TSc is increased. That is, the inclination of the temperature change per unit time is made steep. Thus, even if the starting temperature of the heater <NUM> increases, it is possible to effectively prevent overheating such that the temperature of the heater <NUM> does not exceed the upper limit temperature. Accordingly, even if the environmental temperature is high, overheating of the heater <NUM> can be reliably prevented.

<FIG> is a graph illustrating a correspondence between the ink flow rate of the circulation path <NUM> and the third threshold value TSc. In <FIG>, the vertical axis represents the third threshold value TSc and the horizontal axis represents the ink flow rate. The third threshold value TSc is set to <NUM>, for example, when the ink flow rate is <NUM>/min. Then, if the ink flow rate is more than <NUM>/min, the third threshold value TSc can be reduced. When the ink flow rate increases, the temperature of the heater <NUM> does not rise as easily. Therefore, the third threshold value TSc can be reduced. That is, the inclination of the temperature change per unit time is made gentle. Thus, even if the ink flow rate increases, it is possible to effectively prevent overheating such that the temperature of the heater <NUM> does not exceed the upper limit temperature. Accordingly, even if the ink flow rate increases, overheating of the heater <NUM> can be reliably prevented.

If the ink flow rate is less than <NUM>/min, the third threshold value TSc can be increased. For example, if the ink flowing through the circulation path <NUM> entrains a large amount of air bubbles, then less heat is taken away by the ink circulation, and thus the temperature of the heater <NUM> rises more abruptly. Therefore, the third threshold value TSc is increased. That is, the inclination of the temperature change per unit time is made steeper. Thus, even if the temperature of the heater <NUM> rises abruptly, it is possible to effectively prevent overheating such that the temperature of the heater <NUM> does not exceed the upper limit temperature. Accordingly, even if the ink flow rate decreases, overheating of the heater <NUM> can be reliably prevented.

In the first embodiment, the unit time for monitoring the inclination of the temperature change is set to one second. The unit time is not limited to one second. The unit time may be greater than one second, or less than one second.

Next, a second embodiment will be described with reference to <FIG>. Differences between the second embodiment and the first embodiment relate to the functioning of the processor <NUM> with respect to the overheating monitoring control processing <NUM>. The processing other than the overheating monitoring control processing <NUM> is common to the first embodiment, and <FIG> are applied to the second embodiment without any change, and thus detailed descriptions thereof will be omitted.

<FIG> depicts data stored in the RAM <NUM>. In the second embodiment, the RAM <NUM> includes an area for a timer counter <NUM>, an area for a first temperature <NUM>, an area for a second temperature <NUM>, an area for a temperature difference <NUM>, an area for a first shift register <NUM>, and an area as a second shift register <NUM>. Those areas are provided for each liquid discharge apparatus <NUM>.

The timer counter <NUM> stores time t in synchronization with the timer <NUM>. The first temperature <NUM> is the temperature of the heater <NUM> measured by the first temperature sensor <NUM>. In the following, the first temperature <NUM> stored in the RAM <NUM> is referred to as a heater temperature HTm. The second temperature <NUM> is the temperature of the heater <NUM> previously stored as the first temperature <NUM>. In the following, the second temperature <NUM> stored in the RAM <NUM> is represented as a heater temperature HTn. The temperature difference <NUM> is a difference value between the heater temperature HTm (the first temperature <NUM>) and the heater temperature HTn (the second temperature <NUM>). In the following, the temperature difference <NUM> is represented as a difference temperature HTd.

The first shift register <NUM> includes sixty data storage areas SRa (<NUM> - <NUM>) referred to as the first to the sixtieth storage areas SRa. The first shift register <NUM> sequentially shifts data stored in each of the first to the fifty-ninth data storage areas SRa (<NUM> - <NUM>) to the second to the sixtieth data storage areas SRa (<NUM> - <NUM>) according to the timing of data shift, and stores new data in the first data storage area SRa which was made vacant by the shift. The second shift register <NUM> includes five data storage areas SRb (<NUM> - <NUM>) referred to as the first to the fifth storage areas SRb. The second shift register <NUM> sequentially shifts data stored in each of first to fourth data storage areas SRb (<NUM> - <NUM>) to second to fifth data storage areas SRa (<NUM> - <NUM>) according to the timing of the data shift, and stores new data in the first data storage area SRb which was made vacant by the shift.

<FIG> and <FIG> are flowcharts of the overheating monitoring control processing <NUM> performed on one liquid discharge apparatus <NUM> in the second embodiment. The overheating monitoring control processing <NUM> for the other liquid discharge apparatuses <NUM> is similar, and thus the description here will be omitted. The heater <NUM>, the first temperature sensor <NUM>, and the second temperature sensor <NUM>, are provided in the liquid circulation device <NUM> of the target liquid discharge apparatus <NUM>. The timer counter <NUM>, the first temperature <NUM>, the second temperature <NUM>, the temperature difference <NUM>, the first shift register <NUM>, and the second shift register <NUM> are stored in the RAM <NUM> for the target liquid discharge apparatus <NUM>.

The processor <NUM> waits for energization to the heater <NUM> to be turned ON as ACT <NUM>. As described above, in ACT <NUM> of <FIG>, energization to the heater <NUM> is turned ON. If the processor <NUM> detects that energization to the heater <NUM> is turned ON, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> resets the time t of the timer counter <NUM> to zero as ACT <NUM>.

Next, the processor <NUM> starts the timer <NUM> as ACT <NUM>. Then, the processor <NUM> waits for one second to elapse using the timer <NUM> as ACT <NUM>. Once one second elapses, the processor <NUM> determines that the result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> increments the time t of the timer counter <NUM> by one as ACT <NUM>.

The processor <NUM> acquires the liquid temperature LT measured by the second temperature sensor <NUM> as ACT <NUM>. The liquid temperature LT is the temperature of the ink flowing through the second pipeline <NUM>. The processor <NUM> checks whether the liquid temperature LT reaches a target temperature TSt as ACT <NUM>. The target temperature TSt is any temperature equal to or greater than the second threshold value TSb, which is the lower limit temperature of the suitable temperature range for ejecting ink droplets, and equal to or lower than the first threshold value TSa, which is the upper limit temperature of the suitable temperature range for ejecting ink droplets. The target temperature TSt is set within the range between the second threshold value TSb or more and the first threshold value TSa or less described above.

Initially, when energization to the heater <NUM> is turned ON, the liquid temperature LT must be lower than the target temperature TSt. The processor <NUM> determines that the check result in ACT <NUM> is NO and returns to ACT <NUM>. The processor <NUM> executes the processes of ACT <NUM> and subsequent actions in the same manner as described above. That is, the processor <NUM> acquires the liquid temperature LT every second and waits for the temperature to rise to the target temperature TSt.

If the liquid temperature LT reaches the target temperature TSt, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> resets the time t of the timer counter <NUM> to zero as ACT <NUM>. The processor <NUM> acquires the heater temperature HT measured by the first temperature sensor <NUM> as ACT <NUM>. The processor <NUM> rewrites the heater temperature HTm stored as the first temperature <NUM> with the heater temperature HT measured by the first temperature sensor <NUM> as ACT <NUM>. After that, the processor <NUM> proceeds to ACT <NUM> of <FIG>.

The processor <NUM> waits for one second to elapse as ACT <NUM>. If one second elapses, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> increments the time t of the timer counter <NUM> as ACT <NUM>.

The processor <NUM> acquires the heater temperature HT measured by the first temperature sensor <NUM> as ACT <NUM>. The processor <NUM> rewrites the temperature HTn stored as the second temperature <NUM> with the temperature HTm stored as the first temperature <NUM> as ACT <NUM>. Then, the processor <NUM> rewrites the temperature HTm stored as the first temperature <NUM> with the heater temperature HT acquired in ACT <NUM>, as ACT <NUM>. The processing order of the processes of ACT <NUM> and ACT <NUM> may be changed. That is, after rewriting the temperature HTn (the second temperature <NUM>) with the temperature HTm (the first temperature <NUM>), the heater temperature HT measured by the first temperature sensor <NUM> may be acquired, and the temperature HTm (the first temperature <NUM>) may be rewritten with the heater temperature HT.

The processor <NUM> calculates a difference value between the temperature HTn stored as the second temperature memory <NUM> and the temperature HTm stored as the first temperature memory <NUM>, and stores the calculated difference temperature HTd in the RAM <NUM>, as ACT <NUM>. That is, the processor <NUM> stores the amount of change in the heater temperature HT in the latest one second as the difference temperature HTd in the RAM <NUM>. Incidentally, the difference temperature HTd becomes a positive value if the heater temperature HT rises in the latest one second, and becomes a negative value if the heater temperature HT falls in the latest one second.

The processor <NUM> sequentially shifts data stored in each of the first to fifty-ninth data storage areas SRa of the first shift register <NUM> to the second to sixtieth data storage areas SRa as ACT <NUM>. Then, the processor <NUM> writes the difference temperature HTd stored as the temperature difference <NUM> to the first data storage area SRa vacated by the data shift as ACT <NUM>.

The processor <NUM> checks whether the time t of the timer counter <NUM> reaches a value of "<NUM>" as ACT <NUM>. If the time t of the timer counter <NUM> has not yet reached "<NUM>", the processor <NUM> determines that the check result in ACT <NUM> is NO and returns to ACT <NUM>. The processor <NUM> executes the processes of ACT <NUM> and subsequent actions in the same manner as described above. That is, the processor <NUM> acquires the present heater temperature HT (temperature HTm) every second, calculates the difference temperature HTd from the heater temperature HT (temperature HTn) acquired one second before, and repeats the process of writing the difference temperature HTd into the first shift register <NUM> while sequentially shifting the difference temperature HTd. Thus, once the time t of the timer counter <NUM> reaches "<NUM>", the difference temperature HTd for the past <NUM> minute, that is, the amount of change in the heater temperature HT every second is stored in time series in all of the first to sixtieth data storage areas SRa of the first shift register <NUM>.

The processor <NUM> calculates a fourth threshold value TSd based on each difference temperature HTd of the first shift register <NUM> as ACT <NUM>. Specifically, the processor <NUM> acquires all the positive values, that is, the difference temperature HTd if the heater temperature HT rises in one second, among the difference temperatures HTd in the past <NUM> minute. Then, the processor <NUM> calculates an average value of the acquired total difference temperatures HTd, and sets a value <NUM> times the average value as the fourth threshold value TSd.

The processor <NUM> sequentially shifts data stored in the first to fourth data storage areas SRb of the second shift register <NUM> to the second to fifth data storage areas SRb as ACT <NUM>. The processor <NUM> checks whether the difference temperature HTd stored as the temperature difference memory <NUM> is equal to or greater than the fourth threshold value TSd as ACT <NUM>.

If the difference temperature HTd is less than the fourth threshold value TSd, it is determined that the inclination of the temperature change per unit time of the heater <NUM> does not cause overheating of the heater <NUM>. However, if the difference temperature HTd becomes equal to or greater than the fourth threshold value TSd, it is determined that there is a risk of overheating of the heater <NUM>.

If the difference temperature HTd is less than the fourth threshold value TSd, the processor <NUM> determines that the check result in ACT <NUM> is NO and proceeds to ACT <NUM>. The processor <NUM> writes data "<NUM>" into the first data storage area SRb of the second shift register <NUM> vacated by the data shift as ACT <NUM>. The data "<NUM>" is data indicating that the difference temperature HTd is less than the fourth threshold value TSd.

On the other hand, if the difference temperature HTd is equal to or greater than the fourth threshold value TSd, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> writes data "<NUM>" into the first data storage area SRb of the second shift register <NUM> vacated by the data shift as ACT <NUM>. The data "<NUM>" is data indicating that the difference temperature HTd is equal to or greater than the fourth threshold value TSd.

After finishing the process of ACT <NUM> or ACT <NUM>, the processor <NUM> proceeds to ACT <NUM>. The processor <NUM> checks whether three or more the data "<NUM>" are stored in the data storage areas SRb of the second shift register <NUM> as ACT <NUM>. If two or less the data "<NUM>" are present, the processor <NUM> determines that the check result in ACT <NUM> is NO and returns to ACT <NUM>. That is, if the number of times of occurrences of the inclination of the temperature change per unit time of the heater <NUM>, which should be recognized as having a risk of causing overheating of the heater <NUM>, is two or less in the past five seconds, the processor <NUM> continues monitoring by the overheating monitoring control processing <NUM>. The processor <NUM> executes the processes of ACT <NUM> and subsequent actions in the same manner as described above.

On the other hand, if three or more the data "<NUM>" are present, the processor <NUM> determines that the check result in ACT <NUM> is YES and proceeds to ACT <NUM>. The processor <NUM> turns OFF energization to the heater <NUM> as ACT <NUM>. That is, if the number of times of occurrences of the inclination of the temperature change per unit time of the heater <NUM>, which should be recognized as having a risk of causing overheating of the heater <NUM>, reaches three in the past five seconds, the processor <NUM> stops the heater <NUM> and stops monitoring by the overheating monitoring control processing <NUM>. Thus, the processor <NUM> ends the control of the procedure illustrated in the flowcharts of <FIG> and <FIG>.

Here, the processor <NUM> functions as a recording unit by executing the processes of ACT <NUM> to ACT <NUM> in <FIG>. That is, the processor <NUM> uses the first shift register <NUM> of the RAM <NUM> to record data of the temperature of the heater <NUM> measured by the first temperature sensor <NUM>. The data of the temperature is the difference temperature HTd, that is, the temperature change amount of the heater <NUM> for each unit time.

The processor <NUM> functions as a determination unit by executing the process of ACT <NUM> of <FIG>. That is, the processor <NUM> determines the fourth threshold value TSd based on data recorded by the recording unit. Specifically, the processor <NUM> determines <NUM> times the average of the positive temperature change amounts as the fourth threshold value TSd.

Furthermore, the processor <NUM> functions as a control unit by executing the processes of ACT <NUM> to ACT <NUM> of <FIG>. That is, the processor <NUM> monitors the temperature change of the heater <NUM> based on data recorded by the recording unit, and turns off the heater <NUM> if the temperature change satisfies the stop condition. Specifically, if the temperature change amount per unit time of the heater <NUM> exceeds the fourth threshold value TSd three times or more in five seconds, the processor <NUM> determines that the temperature change satisfies the stop condition, and stops the output of the heater <NUM>.

Also in such a configuration, if the risk of overheating of the heater <NUM> increases, energization to the heater <NUM> is automatically turned OFF. Accordingly, it is possible to prevent the thermal fuse from operating or the heater from breaking. There is no concern that the temperature in the vicinity of the heater <NUM> will rise abruptly and the ink characteristics will be altered. In the second embodiment, the temperature change per unit time obtained from the measured value of the temperature of the heater <NUM> is recorded in the RAM <NUM>. Then, overheating of the heater <NUM> is predicted from the number of times of occurrences of the inclination of the temperature change per unit time of the heater <NUM> which should be recognized as having a risk of overheating of the heater <NUM>. Accordingly, overheating of the heater <NUM> can be prevented more reliably.

In the second embodiment, the processor <NUM> checks whether the liquid temperature LT reaches the target temperature TSt as ACT <NUM>. Regarding this point, any change may be made so as to check whether the time t of the timer counter <NUM> reaches a set time. The set time is any given time at which the liquid temperature LT is assumed to reach the target temperature TSt. In such a case, the processor <NUM> determines that the check result in ACT <NUM> is NO until the time t of the timer counter <NUM> reaches the set time. If the time t reaches the set time, the processor <NUM> determines that the check result in ACT <NUM> is YES.

In the second embodiment, the processor <NUM> calculates the difference temperature HTd from the heater temperature HT one second before and records the difference temperature HTd in the first shift register <NUM>. Regarding this point, for example, the difference temperature from the heater temperature HT two seconds before may be calculated and the difference temperature may be recorded in the first shift register <NUM>.

The processor <NUM> records, for example, the heater temperature HT measured every second in the first shift register <NUM>. Then, the processor <NUM> obtains the difference temperature HTd from the heater temperature HT recorded in the adjacent data storage area SRa and calculates the fourth threshold value TSd. The processor <NUM> checks whether the difference temperature HTd between the heater temperature HT of the first data storage area SRa and the heater temperature HT of the second data storage area SRa is equal to or greater than the fourth threshold value TSd. That is, the temperature data recorded by the recording unit may be used as the temperature HT of the heater <NUM>.

In the second embodiment, the fourth threshold value TSd is calculated as <NUM> times the average of the positive temperature change amounts. The method for calculating the fourth threshold value TSd is not limited to the method for calculating the fourth threshold value TSd as described above. For example, the fourth threshold value TSd may be calculated by setting a value greater than <NUM> or a value smaller than <NUM>, instead of <NUM> times, as a multiple of the average of positive temperature change amounts. Alternatively, the fourth threshold value TSd may be a fixed value which is randomly set.

In the second embodiment, the processor <NUM> continues monitoring by the overheating monitoring control processing <NUM> if the number of times of occurrences of the inclination of the temperature change per unit time of the heater <NUM>, which should be recognized as having a risk of overheating of the heater <NUM>, is two or less in the past five seconds, and stops the heater <NUM> and monitoring if the number of times of occurrences is three or more. The timing of stopping monitoring is not limited to the timing if the number of times of occurrences of the inclination of the temperature change per unit time of the heater <NUM> is three or more. For example, the processor <NUM> may continue monitoring if the number of times of occurrences is three or less, and may stop monitoring if the number of times of occurrences is four or more. Alternatively, the processor <NUM> may stop monitoring if the number of times of occurrences is two or more.

In the first or second embodiment, the heater <NUM> is disposed in the first pipeline <NUM> connecting the upstream tank <NUM> and the liquid discharge head <NUM>. An arrangement place of the heater <NUM> is not limited to the first pipeline <NUM>. For example, the heater <NUM> may be disposed in the upstream tank <NUM>.

In the first or second embodiment, the second temperature sensor <NUM> is disposed in the second pipeline <NUM> connecting the liquid discharge head <NUM> and the downstream tank <NUM>. However, the arrangement location of the second temperature sensor <NUM> is not limited to the second pipeline <NUM>. For example, the second temperature sensor <NUM> may be disposed in the middle part of the first pipeline <NUM> or the third pipeline <NUM>, or may be disposed inside the upstream tank <NUM> or downstream tank <NUM>. Alternatively, the second temperature sensor <NUM> may be disposed on the ink flow path of the liquid discharge head <NUM>.

In the first or second embodiment, the liquid circulation device <NUM> and the liquid discharge apparatus <NUM> use ink. However, the liquid used in the liquid circulation device <NUM> and the liquid discharge apparatus <NUM> is not limited to ink. The technical concepts of the present disclosure can be applied to a liquid circulation device and a liquid discharge apparatus that use a liquid other than ink by setting the third threshold value TSc to an appropriate value.

Claim 1:
A liquid discharge apparatus, comprising:
a liquid discharge head configured to discharge a liquid; and
a liquid circulation device (<NUM>), wherein the liquid circulation device (<NUM>), comprises:
a liquid chamber (<NUM>) configured to store a liquid to be supplied to a liquid discharge head (<NUM>);
a pipeline through which the liquid is circulated between the liquid chamber and the liquid discharge head;
a heater (<NUM>) configured to heat the liquid in the pipeline;
a first temperature sensor (<NUM>) to measure a temperature of the heater;
a second temperature sensor (<NUM>) to measure a temperature of the liquid in the pipeline; and
a controller configured to:
control the heater based on the temperature of the liquid as measured by the second temperature sensor, and
monitor a change in the temperature of the heater over time as measured by the first temperature sensor and turn off the heater when the change satisfies a predetermined stop condition
characterized in that the controller is further configured to:
monitor a rate of change in the temperature of the heater per unit time, and
turn off the heater when the rate of change exceeds a first threshold value.