1. Field of the Invention
The present invention relates to a liquid-discharge control method and a liquid discharging apparatus. More particularly, the invention relates to a liquid-discharge control method when discharging a liquid using a head according to a liquid discharging method utilizing the generation of a bubble by heat, and to a liquid discharging apparatus. The invention also relates to an ink-discharge control method when performing recording on a recording medium using ink as a liquid, and to an ink-jet recording apparatus.
2. Description of the Related Art
From among conventionally known ink-jet recording methods, a recording method (a bubble-jet recording method) has been widely known in which a bubble is generated by heating ink within one of ink discharging ports and a liquid channel communicating therewith (these two components will be hereinafter termed a xe2x80x9cnozzlexe2x80x9d) by heating means, such as a heater or the like, and a fine ink droplet is discharged from the ink discharging port onto a recording medium by the function of the bubble, in order to form an image by consecutively discharging ink droplets from corresponding ink discharging ports in the same manner. This method has been applied to printers, copiers and the like.
Recording heads which adopt the bubble-jet recording method are suitable for high-speed recording and high-quality image recording, because it is easy to increase the number of nozzles mounted in the head (provision of multiple nozzles) and to provide high-density nozzles. Particularly, attempts to increase the recording speed of a printer, a copier or the like by providing multiple nozzles or increasing the driving frequency for the recording head are actively being tried. There are also attempts to realize high resolution by reducing the amount of droplets (the amount of discharge) of ink, and thereby to improve the quality of a recorded image to the high-quality level of photography.
In the bubble-jet recording, a recording method has been known in which, when performing recording by discharging ink from a discharging port by a bubble generated by heating the ink, the bubble is caused to communicate with the atmospheric air if the internal pressure of the bubble is negative (U.S. Pat. No. 5,218,376). According to this method, it is possible to prevent the generation of ink mist during ink splash or discharge, so that the recording medium or the inside of the apparatus is not stained with the ink. In addition, since ink between the generated bubble and the discharging port can be substantially entirely discharged and the amount of the discharged ink is determined by the shape of the nozzle and the position of the heater, it is possible to perform stable recording in which the amount of the discharged ink droplet is always constant.
In the conventional bubble-jet recording method, when the duty ratio of an image is high or the ambient temperature is high, the temperature of the recording head is raised, so that the discharging direction may slightly change.
FIGS. 11A through 13F are schematic diagrams illustrating manners in each of which an ink droplet is discharged from an ink discharging port of a recording head. The recording head adopts a recording method in which a bubble generated during recording is caused to communicate with the atmospheric air.
FIGS. 11A-11C are enlarged views of a nozzle of the recording head.
In FIGS. 11A-11C, there are shown an electrothermal transducer 31 for heating ink, an ink discharging port 32, an ink supply port 33, and a discharging-port plate 35. FIG. 11A is a side cross-sectional view of the ink discharging nozzle. FIG. 11B is a side cross-sectional view of the ink discharging nozzle shown in FIG. 11A as seen from a direction rotated by 90 degrees from the state shown in FIG. 11A. FIG. 11C is a top plan view of the ink discharging nozzle. Line 11A-11A xe2x80x2 shown in FIG. 1C is a line of cutting plane for providing the side cross section shown in FIG. 11A. Line 11B-11Bxe2x80x2 shown in FIG. 11C is a line of cutting plane for providing the side cross sectional view shown in FIG. 11B.
FIGS. 12A-12C and 12D-12F are diagrams, each illustrating how ink is discharged in the ink discharging state of the present invention in which a bubble generated from the recording head discharges ink by communicating with the atmospheric air in a state of negative pressure, as seen from the direction shown in FIG. 11A.
FIGS. 12A-12C illustrate how the ink is discharged when the temperature of the recording head is high. FIGS. 12D-12F illustrate how the ink is discharged when the same energy (driving signal) as in the case shown in FIGS. 12A-12C is applied to the electrothermal transducer in a state in which the temperature of the recording head is close to the room temperature.
As can be understood from FIGS. 12A-12F, the generated bubble is larger when the temperature is high than when the temperature is close to the room temperature, and the direction of ink discharge slightly changes depending on the difference between the temperatures of the recording head. It is considered that this is because the amount of ink slightly remaining in the vicinity of the ink discharging port differs depending on the size of the generated bubble, and a portion where the rear end of the ink leaves the ink discharging port differs depending on the difference in the amount of ink remaining in the vicinity of the ink discharging port.
That is, in FIG. 12B, consider the amounts of ink remaining at portions n and m, each surrounded by a circle. When the temperature of the recording head is high and therefore the size of the generated bubble is large, the amount of ink remaining at portion m is small, and as shown in FIG. 12C, the direction of the ink droplet discharged from ink remaining at portion n opposite to portion m of the ink discharging port slightly deviates from the center of the ink discharging port (indicated by a broken line in FIG. 12C). On the other hand, when the temperature of the recording head is low and therefore the size of the generated bubble is small, although a bubble tends to be generated slightly toward the ink supply port, this tendency is small, and, as shown in FIG. 12F, ink is discharged substantially rectilinearly along the center line of the ink discharging port.
In the case of the conventional recording head in which a bubble does not communicate with the atmospheric air, when the discharged ink droplet is separated from ink within the nozzle, the influence of the separated ink remaining in the vicinity of the ink discharging port does not cause any particular problem, because the ink returns to the inside of the nozzle. However, in the recording head having the above-described configuration in which a bubble generated by driving the electrothermal transducer is caused to communicate with the atmospheric air, the above-described phenomenon that, when the temperature of the recording head is high, the portion where an ink droplet is separated during ink discharge is in the vicinity of the inner wall of the ink discharging port and the discharging direction deviates, as shown in FIG. 12B, is observed.
FIGS. 13A-13C, and 13D-13F are diagrams, each illustrating how ink is discharged from the same viewpoint as shown in FIG. 11B, in order to illustrate the state of ink discharge shown in FIGS. 12A-12F in further detail.
As in the case of FIGS. 12A-12C, FIGS. 13A-13C illustrate how the ink is discharged when the temperature of the recording head is high. FIGS. 13D-13F illustrate how the ink is discharged when the same energy (driving signal) as in the case shown in FIGS. 13A-13C is applied to the electrothermal transducer in a state in which the temperature of the recording head is close to the room temperature.
As can be understood from FIGS. 13A-13C, when the temperature of the recording head is high, the generated bubble is large, and asymmetrical with respect to the center of the ink discharging port. Accordingly, as shown in FIG. 13A, a difference tends to occur between the velocity vectors P and Q of the ink due to the growth of the bubble, and a difference in the amount of ink remaining in the inner wall of the ink discharging port tends to occur between portions S and T. As a result, deviation in the discharging direction occurs.
On the other hand, when the temperature of the recording head is close to the room temperature, the generated bubble is small, and relatively symmetrical with respect to the center of the ink discharging port. Accordingly, as shown in FIG. 13D, a difference is hardly produced between the velocity vectors P and Q of the ink due to the growth of the bubble, and as shown in FIGS. 13E and 13F, the ink discharging direction is rectilinear along the center line of the ink discharging port (as indicated by a broken line shown in FIG. 13F) on an average.
As studied above, in the conventional head in which a bubble communicates with the atmospheric air in a state of negative pressure, the ink discharging direction tends to deviate when the temperature of the head is raised. As a result, the position where the ink droplet adheres on a recording sheet deviates, thereby causing problems to be solved such that white stripes are produced on the recorded image, or unevenness in density is produced due to overlap of recorded dots.
An undisclosed technique relating to the present invention will now be described.
As a result of further detailed studies about the relationship between the manner of ink discharge and input energy in a recording head in which a bubble communicates with the atmospheric air during ink discharge, the phenomenon that a part of the discharged ink droplet falls onto the electrothermal transducer (see FIG. 14C) is observed. Recent detailed studies have shown that this phenomenon of falling of the ink droplet during ink discharge occurs when the power of bubble generation is smaller than the power of bubble generation during ink discharge in which the bubble communicates with the atmospheric air, as in the above-described cases shown in FIGS. 12D-12F and FIGS. 13D-13F, at the same head temperature.
That is, according to the recent knowledge of the assignee of the present application, even in an ink discharge method in which ink is discharged by causing the bubble to communicate with the atmospheric air when the inner pressure of the bubble is negative, when input energy to the electrothermal transducer is high and the power of bubble generation is high, as can be understood from FIGS. 12D-12F and FIGS. 13D-13F, a discharging state in which the liquid droplet is disconnected from the liquid remaining in the vicinity of the discharging port at a position near the end portion of the discharging port is provided. On the other hand, when input energy to the electrothermal transducer is low and the power of bubble generation is relatively low, a discharging state in which the liquid droplet is disconnected from the liquid falling onto the electrothermal transducer in the vicinity of the center line of the discharging port occurs, as shown in FIGS. 14A-14C.
The term xe2x80x9cfallxe2x80x9d of the liquid or ink used in this specification indicates not only dropping of the liquid or ink in the direction of gravity, but also the movement and adherence of a part of the liquid or ink to be discharged, onto the surface of the substrate where the electrothermal transducer is provided, irrespective of the direction of the head.
Even in the discharging method having such a phenomenon of fall, the discharged ink droplet has an appropriate discharging speed, for example, 10-25 m/sec, which is sufficient as the discharging speed. Hence, the possibility that an ink droplet having an extremely high discharging speed is produced as in the case of ink discharge in which the phenomenon of fall does not occur is not present, and rebound of an ink droplet from the recording medium (the generation of mist) hardly occurs.
Furthermore, since the disconnection of the liquid does not occur in the vicinity of the end portion of the discharging port, deviation of the discharging direction of the droplet, which occurs at random when the phenomenon of fall is not present, hardly occurs.
Even if the phenomenon of fall occurs, this phenomenon does not influence the ink discharging speed. Hence, in ink discharge in which the bubble communicates with the atmospheric air, it is desirable to perform ink discharge by stably producing the phenomenon of fall, from the viewpoint of stabilization of the discharging direction of the droplet and suppression of the generation of mist.
However, even when adjusting the energy (the driving signal) supplied to the electrothermal transducer so that the ink droplet falls during ink discharge at the room temperature, the phenomenon of fall may not occur, in some cases, as shown in FIGS. 13D-13F because the energy of bubble generation increases although the same energy is supplied to the electrothermal transducer, if the head temperature is raised. In such a case, the position where the droplet is disconnected during ink discharge differs as described above, so that the ink discharging direction is not constant. As a result, the position where the ink droplet adheres to the recording medium deviates, thereby causing the generation of white stripes on the recorded image or the generation of unevenness in density due to differences in the overlapped positions of recorded dots. These phenomena may cause degradation in the quality of the recorded image.
FIGS. 15A and 15B are enlarged diagrams, each illustrating a longitudinal line recorded by a recording head having two columns of ink discharging nozzles. When recording a desired image by the recording head having such a configuration, since two columns of nozzles are present, recording is usually controlled so that ink is discharged by providing a time difference between the two columns of nozzles. In order to facilitate the following description, one column of nozzles is termed a column A, and another column of nozzles is termed a column B.
In each of the nozzle columns A and B, nozzles are arranged with a density of 300 dots per inch (300 dpi), and the columns A and B are arranged in a state of being shifted by {fraction (1/600)} inch in the direction of the columns with each other. The interval between the columns A and B is {fraction (1/600)} inchxc3x975 (for 5 pixels of 600 dpi). When driving such a recording head with a driving frequency of 10 kHz, a time difference for recording 5 pixels of 600 dpi may be provided between the discharging timings for the nozzle columns A and B. This value equals 100xc3x975 xcexcsec. Thus, as shown in FIG. 15A, when a one-dot longitudinal line is recorded, respective dots are arranged along the longitudinal line.
FIG. 15B illustrates the positions of discharged dots when the discharging direction has slightly changed due to a change in the temperature of the recording head. Since the nozzle columns A and B are in a symmetrical positional relationship, a change in the ink discharging direction is enhanced. When the ink discharging direction has changed in the above-described manner, the recorded image is disturbed. Particularly when performing high-resolution recording, it is necessary to control the ink discharging position with higher accuracy.
When the positions of adherence of discharged dots on the recording medium deviate as a result of changes in the ink discharging direction as shown in FIG. 15B, a texture may be produced in a pattern representing halftone. Usually, an error diffusion method, a dither method or the like is used as the method for representing pseudo-halftone. It is considered that the above-described texture is produced by synchronism of a specific pattern used in the above-described method with deviation in the positions of recorded dots. The generation of such texture also causes degradation in the picture quality.
The present invention has been made in consideration of the above-described conventional approach and undisclosed technique.
It is an object of the present invention to provide a recording control method and a recording apparatus in which very accurate recording positions can always be obtained irrespective of changes in the temperature of a recording head.
According to one aspect of the present invention, a liquid-discharge control method, in which a bubble is generated by supplying a liquid with thermal energy by driving one of electrothermal transducers, and the liquid is discharged from a corresponding one of nozzles by causing the bubble to communicate with the atmospheric air, includes a detection step of detecting a temperature of a head where the electrothermal transducers and the nozzles are provided, an adjusting step of adjusting a width of a driving signal for driving the electrothermal transducer so that a part of the discharged liquid falls onto a side where the electrothermal transducer is provided, based on the temperature detected in the detection step, and a recording step of discharging the liquid by driving the head using the driving signal having the adjusted width adjusted in the adjusting step.
According to another aspect of the present invention, a liquid discharging apparatus including a head, which includes electrothermal transducers, each for generating a bubble by supplying a liquid with thermal energy, and nozzles for discharging the liquid, for causing the bubble to discharge the liquid by causing the generated bubble to communicate with the atmospheric air, includes detection means for detecting a temperature of the head, adjustment means for adjusting a width of a driving signal for driving one of the electrothermal transducers so that a part of the discharged liquid falls onto a side where the electrothermal transducer is provided, based on the temperature detected by the detection means, and recording means for discharging the liquid by driving the head using the driving signal having the adjusted width adjusted by the adjustment means.
The foregoing and other objects, advantages and features of the present invention will become more apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.