1. Field of the Invention
The present invention relates to a technology for driving a liquid-drop ejection head that ejects liquid drops of ink or the like.
2. Description of the Related Art
In an image forming apparatus using an inkjet head, at the time of forming an image on a recording medium, images and characters are created by dots formed by liquid drops ejected from the inkjet head, which have landed on the recording medium. Generally, each dot is formed only on a grid of a predetermined size.
In this type of image forming apparatus, when an image is formed with a resolution of, for example, 600 dpi×600 dpi (600 dots×600 dots with respect to a space of 1 inch square), four types of dots of different sizes are used to record one pixel with 5 gray scales including no dot formation by using four types of dots of different sizes. Accordingly, an image of the same level as an image formed with a resolution of about 1200 dpi×1200 dpi can be expressed with 2 gray scales of forming dots or not forming dots with respect to one pixel by using only dots of the same size, although it is with a resolution of 600 dpi×600 dpi. Further, because the image is formed with a resolution of 600 dpi×600 dpi, the image can be formed with a time shorter than that of the case of forming an image with a resolution of about 1200 dpi×1200 dpi.
As a method of forming dots of different sizes, gray scale recording by multipulse printing has been widely used. According to this method, the number of pulses to be applied to a pressure generating element in one printing cycle is varied or a pulse with a different ejection amount of a liquid drop is selected and applied to the pressure generating element, to vary the ejection amount of the liquid drop for each printing cycle, and when a plurality of pulses are applied to the pressure generating element, a plurality of ejected liquid drops are combined in the air to land on the recording medium or all the ejected liquid drops land close to each other without combining these in the air, thereby forming dots for one pixel (for example, see U.S. Pat. No. 5,285,215 and Japanese Patent No. 3264422).
Further, for example, in Japanese Patent No. 3419372, there is disclosed a driving method of an inkjet head in which a drive waveform of a plurality of pulses is formed by pulses with different ejection amounts by a varying voltage or a shape such as a pulse width for each pulse, and one pulse or a plurality of pulses is selected among these pulses and applied to a pressure generating element, thereby ejecting liquid drops with different ejection amounts for each printing cycle.
In the driving method, such a design is possible by a drive voltage and the pulse width that a rate of a liquid drop to be ejected next becomes faster than that of a liquid drop previously ejected to combine these ejected liquid drops in the air or to land these liquid drops close to each other. Therefore, an interval between pulses can be designed to be wide, and the drive waveform can be turned off at a long time after an electric current of the drive waveform approximately stops, even between the pulses. Recently, however, along with the speeding up of recording, one printing cycle has become shorter, and it is required to narrow the interval between pulses.
Furthermore, when it is attempted to eject large liquid drops as much as possible with a limited voltage applicable to a pressure generating element, or for simplifying a waveform generating unit to realize cost reductions, there has been a demand to eject liquid drops with different sizes for each printing cycle from the same nozzle by forming each pulse in a voltage waveform of the same shape and varying the number of pulses to be applied to the pressure generating element.
As a method of applying a plurality of pulses of a same shape, for example, Japanese Patent Application Laid-open No. 2001-315324 discloses a driving method of an inkjet head in which ON/OFF control signals having a 5-pulse drive waveform in one printing cycle and set to increase the number of pulses to be selected in order from a center pulse are prepared in six types, as shown in FIG. 2, from one selecting 0 pulses to the one selecting five pulses, so that liquid drops of 0 nanograms, 10 nanograms, 20 nanograms, 30 nanograms, 40 nanograms, and 50 nanograms can be ejected, and four types among these types are selected to perform gray scale recording.
However, in the invention described in Japanese Patent Application Laid-open No. 2001-315324, it is not taken into consideration to combine ejected liquid drops in the air, and it is difficult to have all the ejected liquid drops landed close to each other, unless relative movement speed of the inkjet head and the recording medium is low. To make the rate of liquid drops to be ejected next faster than that of a liquid drop previously ejected to combine these ejected liquid drops in the air or to have all the ejected liquid drops landed close to each other even if the relative movement speed of the inkjet head and the recording medium is fast, residual vibration due to previous ejection is used as described later in the pulses of the same shape. Accordingly, the interval between the pulses needs to be made narrow.
An electric circuit of an inkjet head includes a resistance component, a capacitor component, and a coil component, and a transmission line of a drive waveform also forms a resistor-capacitor (RC) circuit. Therefore, as shown in FIG. 3, there is a delay in the waveform of a drive current with respect to that of the drive voltage. A piezoelectric element frequently used as a pressure generating element of an inkjet head has a structure such that a ferroelectric substance is positioned between electrodes, and this tendency is noticeable because of characteristics of a capacitor.
A case of using a piezoelectric element as a pressure generating element is explained next. At a point in time shown in FIG. 4 when a drive waveform is turned off, charging of the piezoelectric element is substantially completed, and a sufficient displacement amount of the piezoelectric element has been obtained. Therefore, even if the drive waveform is turned off at this timing, ejection is hardly affected. However, if the drive waveform is turned off simultaneously with respect to many piezoelectric elements at a point in time in FIG. 4 when the drive waveform is turned off, a position where a charging current is still flowing in a considerable amount is abruptly stopped. Therefore, as shown in FIG. 4, a noise is generated in the waveform of the drive voltage due to an influence of the coil component in the transmission line of the drive waveform.
According to this principle, when an interval between pulses is made narrow due to the reasons described above, if the drive waveform is turned off between the pulses as in the conventional manner at the time of selecting some pulses from a plurality of pulses and applying these pulses to a pressure generating element by ON/OFF control of the drive waveform, a noise can be generated in the drive waveform because the current is still flowing in the transmission line of the drive waveform at an OFF timing due to the narrow interval. When the noise is generated in the drive waveform, the noise is not applied to a nozzle already turned off. However, the noise is directly applied to a nozzle being turned on at the time of noise generation, to affect ejection or cause a malfunction of an apparatus due to radiation noise.