The present invention relates to an ink jet recording apparatus, such as a printer or a plotter, and a method of driving the same, and more particularly, to an ink jet recording apparatus having a recording head including a plurality of nozzle rows.
Some of ink jet recording apparatus (hereinafter simply called xe2x80x9crecording apparatusxe2x80x9d), such as some printers or plotters, have a recording head including a plurality of nozzle rows for effecting color recording, high-speed recording, or the like recording operation.
In the recording head, the amount of ink to be ejected varies when a drive voltage of a drive signal is increased or decreased, whereby the density of an image deviates from designed standard density. For this reason, setting of an optimal drive voltage is important. To this end, the amount of ink has conventionally been determined for each nozzle row, and a drive voltage of a drive signal has conventionally been determined such that an average calculated from the thus-determined amount of ink assumes a target amount.
For instance, in a case where an ink droplet of 8.0 pL (picoliters, the same also applies in the following description), which is a target amount, is ejected through use of a recording head having a total number of seven nozzle rows, the amount of ink a1 is taken for the first nozzle row; the amount of ink a2 is taken for the second nozzle row; . . . , and the amount of ink a7 is taken for the seventh nozzle row. A drive voltage is set such that a value resulting from division of the sum of the amounts of ink a1 to a7 by 7 assumes a value of 8.0 pL.
Since the amount of ink ejected from the recording head tends to vary from one nozzle row to another nozzle row, identifying information indicating a variation in the amount of ink is imparted to respective nozzle rows.
A drive signal of the thus-set drive voltage is supplied to pressure generating elements (e.g., piezoelectric transducers) of the recording head, thereby causing the elements to eject ink droplets. Further, the number of times ink droplets are ejected per unit area is increased or decreased by reference to the identifying information. Thus, an image whose image density and color balance have been adjusted is recorded.
Recently, strong demand has arisen for a recording apparatus of this type which produces a higher image quality. Further, the amount of ink has also reduced considerably to, e.g., 4 to 2 pL. In the case of such ink droplets of smaller amount, a difference in velocity of ink droplets between nozzle rows becomes greater than in the case of a related-art recording apparatus. When the above-stated adjustment method was applied to such a related-art recording apparatus, some ink droplets were found to fly at a velocity lower than the minimum required velocity.
A deficiency in flight velocity often results in deviation of an ink droplet from a regular landing position. This is considered to be attributable to the following. Namely, the recording apparatus is configured to eject ink droplets while a recording head is moved in a main scanning direction, and the trajectory of ink droplets having flown deviates from a normal trajectory for reasons of a deficiency in flight velocity.
It has turned out that a deviation in the landing position induces degradation of image quality, such as appearance of graininess in a recorded image or occurrence of a curvature of a line. Further, ink droplets have also been found to turn into mist before arrival at a print recording medium.
In the case of the small amount of ink, a variation arising in amount of ink between nozzle rows also becomes large. However, if the variation in the amount of ink has exceeded an allowable range, ink ejected from a nozzle row, which eject a smaller amount of ink, is found to fail to fill a portion in a solid painted image, thereby causing a white streak.
The invention has been conceived in light of the circumstances and aims at improving a recorded image quality by use of a small amount of ink.
In order to achieve the above object, according to the present invention, there is provided an ink jet recording apparatus, comprising:
a recording head provided with:
a plurality of nozzles;
a plurality of pressure chambers, each communicated with one nozzle; and
a plurality of pressure generating elements, each associated with one pressure chamber and actuated to eject ink from one associated nozzle;
an ejection controller, which specifies a nozzle from which an ink droplet having the least weight is ejected as a reference nozzle; and
a driving signal generator, which generates a driving signal applied to the respective pressure generating elements, the drive signal having a driving voltage determined such that an ink droplet ejected from the reference nozzle has a predetermined flight velocity or more.
Preferably, the ejection controller specifies a nozzle from which an ink droplet having the lowest flight velocity is ejected as the reference nozzle.
With such configurations, the flight velocities of ink droplets ejected from nozzles which provide the lowest flight velocity become not less than a required velocity. Therefore, even when an extremely small amount of ink is ejected, the ink can be impacted on a predetermined position without fail, thereby preventing transformation of ink into mist. Ink droplets ejected from the remaining nozzles have flight velocities at least equal to or greater than that provided by the reference nozzles, and hence the accuracy of landing position can be ensured, thereby preventing transformation of ink into mist.
Since a high correlation exists between the flight velocities of ink droplets and the amount of ink ejected, the flight velocities of ink drop lets become not less than the required velocity, thereby ensuring a required amount of ink. Consequently, there can be prevented occurrence of a white streak, which would otherwise be caused by a deficiency in the amount of ink.
Alternatively, it is preferable that the ejection controller specifies a nozzle from which an ink droplet having the lowest amount is ejected as the reference nozzle.
In this case, the flight velocities of ink droplets are determined on the basis of the amount of ink ejected. Hence, in addition to the foregoing advantages, the invention can simplify a measurement device and procedures and hence is suitable for mass production.
Preferably, the recording head is provided with a first identifier which indicates an ink amount ejected from each nozzle when the driving signal is applied.
Here, it is preferable that the ejection controller includes an image density corrector which determines a number of ink ejection per a unit area of each nozzle in accordance with the first identifier.
With such configurations, the hue of a recorded image can be made equal to a designed hue while the accuracy of landing position of an ink droplet is ensured. Further, the density of the image can be made equal to designed density.
Here, it is preferable that the ink jet recording apparatus further comprises a mode selector which selects one recording mode among a plurality recording modes, each defined by an ejectable minimum amount of an ink droplet. The recording head is provided with a plurality of first identifiers each indicating an ink amount ejected from each nozzle in an associated recording mode, when the driving signal is applied. The image density corrector determines the ink ejection number based on a first identifier associated with a recording mode selected by the mode selector.
Here, it is preferable that the image density corrector determines the ink ejection number only when a recording mode in which the ejectable minimum ink drop amount is less than a predetermined amount is selected by the mode selector.
With such configurations, an identifier suitable for the subject recording mode can be used, so that the image quality can be further improved.
Preferably, the recording head is provided with a second identifier which indicates a difference between a target ink ejection amount and an ink amount ejected from each nozzle when the driving signal is applied.
Here, it is preferable that the ejection controller includes an image density corrector which determines a number of ink ejection per a unit area of each nozzle in accordance with the first identifier and the second identifier.
With such configurations, the hue of a recorded image can be made equal to a designed hue while the accuracy of landing position of an ink droplet is ensured. Further, the density of the image can be made equal to designed density.
Preferably, the recording head is provided with a Tc rank which is determined with reference to a natural period of ink in the pressure chamber, and referred to determine the driving voltage.
With this configuration, a setting properly reflecting a characteristic of the recording head can be performed in accordance with a flight velocity of an ink droplet which varies in response to the natural period.
Preferably, the ink jet recording apparatus further comprises a mode selector which selects one recording mode among a plurality recording modes, each defined by an ejectable minimum amount of an ink droplet. The recording head is provided with a plurality of first identifiers each indicating an ink amount ejected from each nozzle in an associated recording mode, when the driving signal is applied. The recording head is provided with a plurality of second identifiers each indicating a difference between a target ink ejection amount and an ink amount ejected from each nozzle in an associated recording mode, when the driving signal is applied. The image density corrector determines the ink ejection number based on a first identifier and a second identifier associated with a recording mode selected by the mode selector.
Such a configuration enables use of an identifier suitable for the recording mode, thus improving an image quality to a more extent.
Preferably, the recording head is provided with a plurality of nozzle rows. The pressure generating elements are unitized with respect to each nozzle row.
Preferably, the pressure generating elements are piezoelectric vibrators.
Preferably, the recording head is provided with a plurality of nozzle rows each associated with one color. The ejection controller specifies a nozzle rows from which ink droplets having the least weight are ejected as a reference nozzle rows. The driving voltage is determined such that each ink droplet ejected from the reference nozzle row has the predetermined flight velocity.
According to the present invention, there is also provided a method of driving an ink jet recording apparatus, comprising the steps of:
providing a recording head provided with:
a plurality of nozzles; and
a plurality of pressure generating elements, each associated with one nozzle and actuated to eject ink therefrom;
measuring weights of the respective ink droplets ejected from the recording head;
specifying a nozzle from which an ink droplet having the least weight is ejected as a reference nozzle;
generating a drive signal having a driving voltage determined such an extent that an ink droplet ejected from the reference nozzle has a predetermined flight velocity or more.
Preferably, the reference nozzle is specified as a nozzle from which an ink droplet having the lowest flight velocity is ejected.
Alternatively, it is preferable that the reference nozzle is specified as a nozzle from which an ink droplet having the lowest amount is ejected.
Preferably, the driving method further comprises the step of determining a number of ink ejection per a unit area of each nozzle in accordance with an ink amount ejected from each nozzle when the driving signal is applied.