Patent Description:
Inkjet heads for ejecting ink are known inventions. In general, inkjet heads drive actuators, causing changes in volumes of ink pressure chambers in order to eject liquid droplets of ink from nozzles connected to the ink pressure chambers. Such inkjet heads are most typically mounted in inkjet printers, which eject liquid droplets of ink from the inkjet heads to form images on the surfaces of recording media.

Various driving waveforms to drive an actuator for ink ejection have been used. There is one driving waveform by which an ink pressure chamber is only partially contracted to eject a droplet rather than fully after the volume of the ink pressure chamber has been expanded. This type of driving waveform is also known as a draw-draw (DD) driving waveform. However, when ink is actually ejected with such a driving waveform, misting often occurs particularly at the beginning of ink droplet formation (that is, the initial portion of the ejected droplet is often generates ink mist). The misting of ink can be a cause of deteriorating printing quality.

Hence, there is a need for an inkjet head capable of suppressing such ink misting.

To this end, there is provided an inkjet head according to claim <NUM>.

There is also provided an inkjet printer according to claim <NUM>.

According to an embodiment, an inkjet head includes a nozzle plate on which at least one nozzle is formed for ejecting ink or the like, an ink pressure chamber that is connected to the nozzle, an actuator that changes the volume of the ink pressure chamber, and an actuator driving circuit that applies a driving waveform to the actuator for ejecting ink. The driving waveform includes an ejection pulse portion that changes from a first voltage to a second voltage at which the ink pressure chamber expands and then from the second voltage to a third voltage at which the ink pressure chamber contracts. The third voltage is between the first and second voltages in potential. A potential difference between the second and third voltages is greater than a potential difference between the third and first voltages.

Hereinafter, certain example embodiments of an inkjet head and an inkjet printer will be described with reference to the accompanying drawings. In the drawings, the same reference numerals are given to the same configurations, components, elements, and the like.

An inkjet printer <NUM> of the first embodiment includes inkjet heads <NUM>, <NUM>, <NUM>, and <NUM> mounted therein. <FIG> illustrates a schematic configuration of the inkjet printer <NUM>. The inkjet printer <NUM> includes a cassette <NUM> that accommodates a sheet of paper S which is an example of a recording medium, an upstream conveyance path <NUM> for the sheet S, a conveyance belt <NUM> that conveys the sheet S from the cassette <NUM>, a plurality of inkjet heads (inkjet heads <NUM>, <NUM>, <NUM>, and <NUM>) that eject liquid drops of ink toward the sheet S on the conveyance belt <NUM>, a downstream conveyance path <NUM> for the sheet S, a discharge tray <NUM>, a control substrate <NUM>, and a casing <NUM>. The inkjet printer <NUM> further includes an operation unit <NUM> as a user interface provided on an upper side of the casing <NUM>.

Image data to be printed on the sheet S is generated by, for example, a computer <NUM> which can be an external device communicably connected to the inkjet printer <NUM>. The image data generated by the computer <NUM> is transmitted to the control substrate <NUM> (also referred to as a control board or the like) of the inkjet printer <NUM> through a cable <NUM> and connectors <NUM> and <NUM>.

A pickup roller <NUM> supplies sheets S from the cassette <NUM> to the upstream conveyance path <NUM> one by one. The upstream conveyance path <NUM> includes a pair of feeding rollers <NUM>, a pair of feeding rollers <NUM>, a sheet guide plates <NUM>, and sheet guide plates <NUM>. The sheet S is fed to the conveyance belt <NUM> via the upstream conveyance path <NUM>. In <FIG>, the arrow <NUM> indicates a conveyance path for the sheet S from the cassette <NUM> to the conveyance belt <NUM>.

The conveyance belt <NUM> is, for example, a mesh belt having many through holes formed in its surface. A driving roller <NUM> and driven rollers <NUM> and <NUM> support the conveyance belt <NUM> so that the conveyance belt can rotate. A motor <NUM> rotates the conveyance belt <NUM> in a rotation direction <NUM> by rotating the driving roller <NUM>. The motor <NUM> is an example of a driving device. The sheet S moves in conjunction with the rotation of the conveyance belt <NUM> past the ink jet heads <NUM> to <NUM> in turn. A negative pressure container <NUM> is on a rear surface side of the conveyance belt <NUM>. The negative pressure container <NUM> is connected to a fan <NUM>. The fan <NUM> reduces pressure inside the negative pressure container <NUM> by forming an air flow <NUM> and thus sucks and holds the sheet S to the upper surface of the conveyance belt <NUM>.

The inkjet heads <NUM> to <NUM> are arranged to face the sheet S held on the conveyance belt <NUM> with small gaps of about <NUM>, for example, being left between sheet S surface and the inkjet heads <NUM> to <NUM> when the sheet S passes underneath each inkjet head in turn. The inkjet heads <NUM> to <NUM> eject ink droplets toward the sheet S. The inkjet heads <NUM> to <NUM> print an image when the sheet S passes under them. The respective inkjet heads <NUM> to <NUM> have the same structure as each other except that ink colors to be ejected are different. The ink colors are, for example, cyan, magenta, yellow, and black.

The inkjet heads <NUM> to <NUM> are respectively connected to ink tanks <NUM> to <NUM> and ink supply pressure adjustment devices <NUM> to <NUM> via ink flow passages <NUM> to <NUM>. The ink tanks <NUM> to <NUM> are provided above the inkjet heads <NUM> to <NUM>. At a device standby time, the ink supply pressure adjustment devices <NUM> to <NUM> respectively adjust the inside of the inkjet heads <NUM> to <NUM> to be a negative pressure, for example -<NUM> kPa, with respect to the atmospheric pressure so that ink does not leak from nozzles <NUM> (see <FIG>) of the inkjet heads <NUM> to <NUM>. At an image formation time, the ink of the ink tanks <NUM> to <NUM> is supplied to the inkjet heads <NUM> to <NUM> by the ink supply pressure adjustment devices <NUM> to <NUM>.

After the image is formed, the sheet S is sent from the conveyance belt <NUM> to the downstream conveyance path <NUM>. The downstream conveyance path <NUM> is formed by pairs of feeding rollers <NUM>, <NUM>, <NUM> and <NUM> and sheet guide plates <NUM> and <NUM> that regulate a conveyance path of the sheet S. The sheet S is discharged from a discharge port <NUM> to the discharge tray <NUM> via the downstream conveyance path <NUM>. In the drawing, arrow <NUM> indicates the conveyance path of the sheet S.

<FIG> each illustrate aspects of the inkjet head <NUM>. The inkjet heads <NUM>, <NUM>, and <NUM> have the same configuration as the inkjet head <NUM>.

As shown in <FIG>, the inkjet head <NUM> includes a nozzle plate <NUM>, a substrate <NUM>, an ink supply unit <NUM>, a flexible substrate <NUM>, and a driving circuit <NUM>. A plurality of nozzles <NUM> configured to eject ink in a form of droplets are provided to the nozzle plate <NUM>. The ink to be ejected from each nozzle <NUM> is supplied from the ink supply unit <NUM>. The ink flow passage <NUM> from the ink supply pressure adjustment device <NUM> is connected to an upper side of the ink supply unit <NUM>.

<FIG> illustrates an enlarged plan view of a portion surrounded by the frame P in <FIG>, the nozzles <NUM> are arranged two-dimensionally in columns and rows (X and Y axis directions in the drawing). In each column (X direction), the nozzles <NUM> are arranged obliquely (in the plan view) such that the centers of the nozzles <NUM> do not overlap with one another on in X axis direction. The nozzles <NUM> are disposed at an interval of a distance X1 in the X axis direction and offset at a distance Y1 in the Y axis direction from the respective center lines. The distances X1 and YI are set to achieve a specific recording density or printing resolution. As an example, the distance X1 can be <NUM> and the distance Y1 can be <NUM> to achieve <NUM> dots-per-inch (DPI) in both X and Y axis directions. The distance X1 may be set based on a relation between a rotational speed of the conveyance belt <NUM> and a time necessary for an ink droplet to land on a printing surface of the sheet S to achieve the intended resolution in the X axis direction.

In the present embodiment, four nozzles <NUM> are arranged in the X axis direction as one nozzle set, and a plurality of nozzle sets are provided next to each other along the Y axis direction. Although not separately illustrated, as an example, <NUM> sets of the nozzles <NUM> are arranged in the Y axis direction, and two groups, each consisting of the <NUM> nozzle sets, are arranged in the X axis direction (see <FIG>), with the number of nozzles <NUM> totaling <NUM> (= <NUM> x <NUM> x <NUM>).

An actuator <NUM> which is a driving source in an ink ejection operation is provided with respect to each nozzle <NUM>. A set of the nozzle <NUM> and actuator <NUM> forms one channel. Each actuator <NUM> has a ring shape and is arranged such that the nozzle <NUM> locates at a center of the actuator <NUM>. The actuator <NUM> has a size of, for example, an inner diameter of <NUM> and an outer diameter of <NUM>. A plurality of individual electrodes <NUM> are also provided, and the actuators <NUM> are electrically connected to the corresponding individual electrodes <NUM>, respectively. In the present embodiment, every four actuators <NUM> arranged in the X axis direction are electrically connected to a common electrode <NUM> arranged in the X axis direction. Both the individual electrode <NUM> and the common electrode <NUM> are electrically connected to a mounting pad <NUM>. The mounting pad <NUM> serves as an input port that gives a driving waveform to each actuator <NUM>. Of the plurality of individual electrodes <NUM>, although not separately illustrated, four individual electrodes <NUM> surrounded by the frame <NUM> in the drawing are respectively connected to the actuators <NUM> of another group illustrated in <FIG>. The same applies to the other individual electrodes <NUM> arranged in the X axis direction. As a modified example, the mounting pad <NUM> and the flexible substrate <NUM> may be added for the actuators <NUM> of the other group. Embodiments are not limited to the configuration in which the ink of the same color is ejected. For example, a flow passage of the ink may be separated from each group so that ink of another color is ejected. For ease of description, the actuators <NUM>, the individual electrodes <NUM>, and the common electrode <NUM> are illustrated with solid lines in <FIG> though some portions may be provided inside the nozzle plate <NUM> (see also <FIG> which illustrates a cross-sectional view of part of the inkjet head <NUM>). The position of each of the actuators <NUM> is not limited to the present embodiment and can be modified as appropriate.

The mounting pad <NUM> is electrically connected to a wiring pattern formed in the flexible substrate <NUM> via, for example, an anisotropic contact film (ACF). The wiring pattern of the flexible substrate <NUM> is electrically connected to the driving circuit <NUM>. The driving circuit <NUM> is, for example, an integrated circuit (IC). The driving circuit <NUM> selects a channel from which the ink is ejected in accordance with the image data to be printed and applies the driving waveform to the actuators <NUM> of the selected channel.

As illustrated in <FIG>, the nozzles <NUM> penetrate the nozzle plate <NUM> in the Z axis direction. Each nozzle <NUM> has a diameter of, for example, <NUM>. Inside the substrate <NUM>, an ink pressure chamber <NUM> that communicates with the corresponding nozzle <NUM> is provided. The ink pressure chambers <NUM> may be referred to as individual pressure chambers in some contexts. The ink pressure chamber <NUM> is, for example, a cylindrical space having an upper end portion that is open. The open upper portion of each ink pressure chamber <NUM> connects with a common ink chamber <NUM>. The ink flow passage <NUM> communicates with the common ink chamber <NUM> via an ink supply port <NUM>. The ink pressure chamber <NUM> and the common ink chamber <NUM> are filled with the ink. The common ink chamber <NUM> has, for example, a flow passage shape in which the ink is circulated. The ink pressure chamber <NUM> has, for example, a configuration in which a cylindrical hole with a diameter of, for example, <NUM> is formed in the substrate <NUM> of a monocrystalline silicon wafer with a thickness of, for example, <NUM>. The ink supply unit <NUM> has a configuration in which a space corresponding to the common ink chamber <NUM> is formed, for example, in alumina (Al<NUM>O<NUM>).

As shown in <FIG>, the nozzle plate <NUM> has a structure in which a protective layer <NUM>, the actuator <NUM>, and a vibration plate <NUM> are stacked on one another in order from a bottom surface side of the nozzle plate <NUM>. The actuator <NUM> has a structure in which an upper electrode <NUM>, a piezoelectric body <NUM>, and a lower electrode <NUM> are stacked on one another. The piezoelectric body <NUM> has a thin sheet shape. The lower electrode <NUM> is electrically connected to the individual electrode <NUM>, and the upper electrode <NUM> is electrically connected to the common electrode <NUM>. In a boundary area of the protective layer <NUM> and the vibration plate <NUM>, an insulation layer <NUM> that prevents short-circuiting between the individual electrodes <NUM> and the common electrode <NUM> is provided, being sandwiched between the protective layer <NUM> and the vibration plate <NUM>. The insulation layer <NUM> is formed of a silicon dioxide film (SiO<NUM>) with a thickness of, for example, <NUM>. The upper electrode <NUM> and the common electrode <NUM> are electrically connected by a contact hole <NUM> formed in the insulation layer <NUM>. The piezoelectric body <NUM> is formed of PZT (lead zirconate titanate) with a thickness of, for example, <NUM> or less. The lower electrode <NUM> and the upper electrode <NUM> are formed of platinum with a thickness of, for example, <NUM>. The individual electrodes <NUM> and the common electrode <NUM> are formed of gold (Au) with a thickness of, for example, <NUM>.

The vibration plate <NUM> is formed of an insulating inorganic material. The insulating inorganic material is, for example, silicon dioxide (SiO<NUM>). The thickness of the vibration plate <NUM> is in the range of, for example, <NUM> to <NUM> or may be in the range of, for example, <NUM> to <NUM>. The vibration plate <NUM> and the protective layer <NUM> are configured to bend or flex in Z direction towards the ink pressure chamber <NUM> when the piezoelectric body <NUM> deforms in a d31 mode upon voltage application. If the application of the voltage to the piezoelectric body <NUM> is stopped, the vibration plate <NUM> and the protective layer <NUM> return to the original shape (relax). This reversible deformation process enables the volume of the ink pressure chamber <NUM> to be expanded and contracted. When the volume of the ink pressure chamber <NUM> changes, the ink pressure inside the ink pressure chamber <NUM> will also change. The ink is ejected from the nozzles <NUM> using both the expansion and contraction of the volume of the ink pressure chamber <NUM> and the resulting change in the ink pressure.

In the present embodiment, the nozzle <NUM>, the actuator <NUM>, and the ink pressure chamber <NUM> form or function as an ink ejection unit of the inkjet head <NUM>.

The protective layer <NUM> is formed of polyimide with a thickness of, for example, <NUM>. The protective layer <NUM> covers one surface of the bottom surface side of the nozzle plate <NUM> facing the sheet S and also covers an inner circumferential surface of a hole of the nozzle <NUM>.

As shown in <FIG>, the control substrate <NUM> that serves as a control unit of the inkjet printer <NUM> includes a central processing unit (CPU) <NUM>, a read-only memory (ROM) <NUM>, a random-access memory (RAM) <NUM>, an input and output (I/O) port <NUM>, and an image memory <NUM> mounted thereon. The CPU <NUM> controls the motor <NUM>, the ink supply pressure adjustment devices <NUM> to <NUM>, the operation unit <NUM>, and various sensors through the I/O port <NUM>. The image data from the computer <NUM> is transmitted to the control substrate <NUM> through the I/O port <NUM> and stored in the image memory <NUM>. The CPU <NUM> transmits the image data stored in the image memory <NUM> to the driving circuits <NUM> of the respective inkjet heads <NUM> to <NUM> in order of forming the image on the sheet S. The transmitted data may include gradation data for designating gradations of dots based on the image data.

The driving circuit <NUM> includes a data buffer <NUM>, a decoder <NUM>, and a driver <NUM>. The data buffer <NUM> chronologically stores therein the image data for each actuator <NUM>. The decoder <NUM> controls the driver <NUM> based on the image data stored in the data buffer <NUM> for each actuator <NUM>. The driver <NUM> outputs a driving signal for operating each actuator <NUM> under the control of the decoder <NUM>. The driving signal is a voltage applied to the actuator <NUM> in accordance with a driving waveform.

That is, the driving circuit <NUM> for each of the inkjet heads <NUM> to <NUM> functions as an actuator driving circuit that provides the driving waveform to each actuator <NUM>.

<FIG> illustrates a driving waveform <NUM> to drive the actuator <NUM> such that ink will be ejected. The driving waveform <NUM> is an example of a so-called pull-striking driving waveform. If a dot is to be formed by ejecting ink once, the actuator <NUM> can be driven with the driving waveform <NUM>. If gradation printing is to be performed to form a dot with two or more ejections, the actuator <NUM> would be driven with a multi-drop driving waveform instead of the driving waveform <NUM>.

In the driving waveform <NUM>, if voltages V1 and V3 are positive voltages (where V1 > V3) and a voltage V2 is set to <NUM> V, as illustrated in <FIG>, the voltage V1, as an example of a first voltage, is applied to the actuator <NUM> as a bias voltage. For example, the voltage V1 is applied to the lower electrode <NUM> of the actuator <NUM> through the individual electrode <NUM>. The common electrode <NUM> connected to the upper electrode <NUM> of the actuator <NUM> is set to <NUM> V. As a first expansion pulse for further expanding the ink pressure chamber <NUM> from the state or volume thereof at the voltage V1, the voltage V2 is applied to the actuator <NUM> through the individual electrode <NUM> at a start time for a time Ta. Thereafter, as a first contraction pulse for contracting the ink pressure chamber <NUM> from the expanded state, the voltage V3 is applied to the actuator <NUM> through the individual electrode <NUM> after the first time Ta for another time Ta. The first contraction pulse is a contraction pulse for ejecting ink. Subsequently, as a second contraction pulse for further contracting the ink pressure chamber <NUM>, the voltage V1 is applied to the actuator <NUM> through the individual electrode <NUM>. The second contraction pulse is a contraction pulse for attenuating residual vibration (oscillation).

In the driving waveform <NUM>, the first extraction pulse and the first contraction pulse together form an "ejection pulse portion" for ejecting ink. That is, the ejection pulse portion of this example includes a draw-draw (DD) driving waveform for ejecting an ink droplet by initially expanding the ink pressure chamber <NUM> and then contracting the ink pressure chamber <NUM>, but only partially (for example, midway).

After the application of the voltage V1 of the second contraction pulse, the voltage V3 is applied to the actuator <NUM> through the individual electrode <NUM> for a time Ta as a second expansion pulse for expanding the ink pressure chamber <NUM>. Thereafter, the voltage V1 is again applied to the actuator <NUM> through the individual electrode <NUM>. The voltage V1 can be applied as the bias voltage until a subsequent first expansion pulse starts (that is, when another ejection event begins). The actuator <NUM> can be driven with the same driving waveform <NUM> during a driving period after the initial ejection.

In the driving waveform <NUM>, the second contraction pulse and the second expansion pulse together form a "cancellation pulse portion" for attenuating residual vibration after the first contraction pulse of the ejection pulse portion. In this way, in the driving waveform <NUM>, the ejection pulse portion and the cancellation pulse portion may be combined to form a draw-draw-release-draw (DDRD) driving waveform.

In the present embodiment, the relation of magnitude among the voltages V1 to V3 satisfies V1 > V3 > V2. In the driving waveform <NUM>, the potential difference Δ(V2 - V3) between the voltages V2 and V3 in the ejection pulse portion is greater than the potential difference Δ(V3 - V1) between the voltages V3 and V1 in the same ejection pulse portion, that is Δ(V2 - V3) > Δ(V3 - V1). In some instances, the ratio of Δ(V2 - V3) to Δ(V3 - V1) is preferably within a range of <NUM> : <NUM> to <NUM> : <NUM>. In some instances, the ratio of Δ(V2 - V3) to Δ(V3 - V1) is more preferably set to <NUM> : <NUM> when Δ(V1 - V2) is set to <NUM>, such that Δ(V2 - V3) : Δ(V3 - V1) = <NUM> : <NUM>. As an example, the voltages V1 to V3 can be set as follows: V1 = <NUM> V; V3 = <NUM> V; and V2 = <NUM> V. The voltage of the common electrode <NUM> can be set to be constant at <NUM> V. Thus, the ratio of Δ(V2 - V3) to Δ(V3 - V1) is <NUM> : <NUM>, when Δ(V2 - V3) = <NUM> V and Δ(V3 - V1) = <NUM> V.

Each pulse width (that is, the time Ta) is set to a half period (λ/<NUM>) of a first natural vibration period λ of the actuator <NUM> in a state in which the ink pressure chamber <NUM> and the nozzle <NUM> are filled with ink. The natural vibration period λ can be measured, for example, by detecting a change in impedance of the actuator <NUM> while the ink pressure chamber <NUM> and the nozzle <NUM> are ink-filled. The impedance can be detected using, for example, an impedance analyzer. As another method of measuring the natural vibration period λ, an electrical signal with a stepped waveform can be input from the driving circuit <NUM> to the actuator <NUM> and vibration of the actuator <NUM> may be measured with a laser Doppler vibrometer or the like. The vibration amounts may be obtained through computer simulation. As a modified example, Ta for each pulse may be a multiple of λ/<NUM> or may be shorter than λ/<NUM>. Furthermore, in some examples, the time Ta (pulse width) for each of the respective pulses may be different from each other. The value λ/<NUM> is also sometimes referred to as an acoustic length (AL).

While in the example of <FIG>, V1 and V3 are set to positive voltages (where V1 > V3) and V2 is set to <NUM> V, embodiments or examples of the disclosure are not limited thereto. As a modified example, V1 and V3 may be set to positive voltages and V2 may be set to a negative voltage. The negative voltage of V2 may be equal to or greater than a polarization reversal voltage of the piezoelectric body <NUM>. For example, V1, V3, and V2 may be <NUM> V, <NUM> V, and -<NUM> V, respectively.

<FIG> schematically illustrate the ink ejection operations when the actuator <NUM> is driven with the driving waveform <NUM> of <FIG>. The reference numeral "M" in the figures denotes the meniscus (meniscus M) of ink. When the bias voltage V1 is applied to the actuator <NUM> in its standby state (see <FIG>), an electrical field in the thickness direction of the piezoelectric body <NUM> will be generated and the piezoelectric body <NUM> will deform in the d31 mode (see <FIG>). Specifically, in the case of a ring-shaped piezoelectric body <NUM>, the piezoelectric body <NUM> expands in the thickness direction and contracts in the radial direction. Due to the deformation of the piezoelectric body <NUM>, bending stress will be generated in the vibration plate <NUM>, and the actuator <NUM> will curve inward or warp towards the ink pressure chamber <NUM> as illustrated in <FIG>. That is, the actuator <NUM> deforms to a concave shape centered at the nozzle <NUM>, and the volume of the ink pressure chamber <NUM> is reduced (contracts).

Subsequently, when the voltage V2 of the first expansion pulse is applied, the actuator <NUM> returns to its state (see <FIG>) before the deformation. At this time, as the ink pressure chamber returns to the original state and its volume expands from the previously contracted state, the ink pressure inside the ink pressure chamber <NUM> decreases. As the ink starts to flow into the pressure-decreased ink pressure chamber <NUM> from the common ink chamber <NUM>, the ink pressure increases again. Thereafter, the supply of the ink to the ink pressure chamber <NUM> stops, and the increase in the ink pressure also stops. With this operation, a so-called pull state is achieved.

Subsequently, when the voltage V3 of the first contraction pulse is applied, the piezoelectric body <NUM> of the actuator <NUM> will further deform and the volume of the ink pressure chamber <NUM> will contract again as illustrated in <FIG>. This causes the ink pressure inside the ink pressure chamber <NUM> to increase. Thus, the ink is ejected from the nozzle <NUM> due to the contraction of the volume of the ink pressure chamber <NUM> in conjunction with the increase of the ink pressure inside the pressure chamber <NUM>.

After the ejection pulse portion of the driving waveform <NUM>, when the voltage V1 of the second contraction pulse in the cancellation pulse portion is applied, the volume of the ink pressure chamber <NUM> will further contract as illustrated in <FIG>. The ejection of the ink reduces the ink pressure inside the ink pressure chamber <NUM> whereas the residual vibration of the ink remains present in the ink pressure chamber <NUM>. The further contraction of the volume of the ink pressure chamber <NUM> by the application of the voltage V1 attenuates the residual vibration of the ink.

When the voltage V3 of the second expansion pulse is applied (see <FIG>), the deformation amount of the piezoelectric body <NUM> of the actuator <NUM> will be less than that when the voltage V1 is applied (see in <FIG>) while the ink pressure chamber <NUM> remains in a partially expanded state (as illustrated in <FIG>). This cancellation pulse portion attenuates the residual vibration.

Subsequent to this cancellation pulse portion, when the voltage V1 is again applied (<FIG>), the volume of the ink pressure chamber <NUM> will contract again and the same state as depicted in <FIG> is achieved again.

Experiments of the ink ejection operation using the driving waveform <NUM>, a DD driving waveform, and another driving waveform were conducted, and their results are shown in <FIG> as Example, Comparative Example <NUM>, and Comparative Example <NUM>, respectively. For each experiment, a driving waveform used in the experiment, a change in a flow velocity of the ink in the ink ejection operation, and a change in a position of a meniscus M of the ink are illustrated. The flow velocity of the ink is indicated by a negative value for a direction of inflow to the ink pressure chamber <NUM> and a positive value for a direction of outflow from the ink pressure chamber <NUM>. For the position of the meniscus, a change towards the ejection direction is indicated as a positive value and a change in the inward direction is indicated as a negative value, using the position of the meniscus M in an initial state (see <FIG>) as a reference.

In the DD driving waveform of Comparative Example <NUM>, the potential difference Δ(V2 - V3) between the voltages V2 and V3 is less than the potential difference Δ(V3 - V1) between the voltages V3 and V1. If the potential difference Δ(V1 - V2) between the voltages V1 and V2 is set to <NUM> then Δ(V2 - V3) : Δ(V3 - V1) = <NUM> : <NUM>. With such a DD driving waveform, ink at the beginning of a droplet formation can be misted at the time of ink ejection as shown in <FIG>. Taking into consideration the change in the position of the meniscus M as illustrated in <FIG>, the possible reason why the ink at the beginning of droplet formation is misted using the DD driving waveform of Comparative Example <NUM> may be that the meniscus M, which normally protrudes from a nozzle surface due to pressure vibration, is not formed upon the ejection of the ink droplet (see the portion surrounded by a circle in the meniscus position drawing of Comparative Example <NUM> in <FIG>).

On the other hand, if, as in Comparative Example <NUM>, a DD driving waveform is not used, the misting at the beginning of droplet formation is unlikely to occur. However, if the DD driving waveform is not used, any shift in a half period (λ/<NUM>) of the natural vibration period λ of the inkjet head <NUM> from an optimum value of λ/<NUM> may cause larger residual vibration after the ink ejection. A magnitude of the residual vibration according to the shift from the optimum λ/<NUM> value is shown in <FIG>. As shown in <FIG>, if a DD driving waveform is not used (Comparative Example <NUM>), the residual vibration increases substantially according to the shift from the optimum λ/<NUM>. Once the residual vibration increases beyond a non-negligible extent, it will likely affect a subsequent ejection state of the ink or cause crosstalk with another channel.

Normally, for a driving waveform, the optimum λ/<NUM> is calculated based on the structure, size, and the like for the inkjet head and this is used at the pulse width value (that is, time Ta). For example, the optimum λ/<NUM> can be about <NUM>. However, due to limits in manufacturing precision (manufacturing tolerances) for inkjet heads, the half period (λ/<NUM>) of the actual natural vibration period λ can be shifted away from the calculated optimum λ/<NUM> value in some cases. For example, the inkjet head printer <NUM> of <FIG> includes the plurality of inkjet heads <NUM> to <NUM> each having nominally the same shape and the same size, but the natural vibration periods λ of the respective inkjet heads <NUM> to <NUM> may not necessarily be the same as one another for a variety of reasons.

On the other hand, when the driving waveform <NUM> is used according to the present embodiment, ink misting was prevented by setting the potential difference Δ(V2 - V3) for the voltages V2 and V3 in the ejection pulse portion to be greater than the potential difference Δ(V3 - V1) for the voltages V3 and V1. If the potential difference Δ(V1 - V2) is set to <NUM> then Δ(V2 - V3) : Δ(V3 - V1) = <NUM> : <NUM>. Taking into consideration the change in the position of the meniscus M as illustrated in <FIG>, the possible reason why the ink misting can be prevented may be that the meniscus M, which normally protrudes from a nozzle surface due to pressure vibration, is formed when the ink droplet is ejected.

In this way, the driving waveform <NUM> can effectively prevent ink misting by setting Δ(V2 - V3) in the ejection pulse portion to be greater than Δ(V3 - V1). However, since the potential difference Δ(V3 - V1) of the driving waveform <NUM> is less than that of the driving wave form of Comparative Example <NUM> and Comparative Example <NUM>, the residual vibration attenuation of the second contraction pulse alone becomes less effective. Therefore, the driving waveform <NUM> is set to form the DDRD driving waveform by the combination of the ejection pulse portion and the cancelation pulse portion according to the present embodiment so that the residual vibration is effectively attenuated.

Further, as illustrated in <FIG>, for the driving waveform <NUM> ("EXAMPLE"), the residual vibration does not become as large as that in Comparative Example <NUM> even when the half period (λ/<NUM>) of the natural vibration period λ shifts from the calculated optimum λ/<NUM>. As summarized in the comparative table shown in <FIG>, the driving waveform <NUM> has well-balanced advantages of both prevention of the ink misting and stabilization of the residual vibration. By setting the ratio of Δ(V2 - V3) to Δ(V3 - V1) to <NUM> : <NUM> (that is, if Δ(V1 - V2) is set to <NUM> then Δ(V2 - V3) : Δ(V3 - V1) = <NUM> : <NUM>), a minimum number of voltages V1, V2 and V3 is necessary to form the DDRD driving waveform. Further, such ratio setting can be regarded as having selected the optimum point at which the well-balanced effects of the ink misting prevention and the residual vibration stabilization are achieved.

In the example of <FIG>, by forming the driving waveform <NUM> with just the three voltages V1, V2 and V3 necessary for the DDRD driving waveform, a circuit configuration can be made simpler. The number of voltages for the DDRD driving waveform is not, however, limited to the present embodiment. The driving waveform <NUM> may be formed with four or more different voltages. As an example, a fourth voltage can be set to be lower than the voltage V1 or higher than the voltage V1. As another example, a fifth voltage can be set to be lower than the voltage V3 or higher than the voltage V3.

The inkjet heads <NUM> to <NUM> according to a second embodiment utilizes a multi-drop driving waveform with which gradation printing can be performed. The configurations of the inkjet heads <NUM> to <NUM> of the second embodiment are the same or substantially the same as those of the first embodiment except for the generation and application of the driving waveform.

<FIG> illustrates multi-drop driving waveforms of "<NUM> Drop," "<NUM> Drop," and "<NUM> Drop. " A "<NUM> Drop" driving waveform is also depicted. The <NUM> Drop driving waveform is for printing two gradations by ejecting ink twice and includes a first-drop ejection pulse and a second-drop ejection pulse. For the multi-drop driving waveforms, the voltage V1 serving as a bias voltage is initially applied to the actuator <NUM>. From that state, as an expansion pulse (or a first-drop expansion pulse), the voltage V2 is applied to the actuator <NUM> for the time duration of <NUM> Ta. As a contraction pulse (or a first-drop contraction pulse), the voltage V3 is then applied to the actuator <NUM> for the time duration of Ta to eject a first-drop ink. That is, the first-drop ejection pulse is the same as the ejection pulse portion in the driving waveform <NUM> of the first embodiment (see <FIG>) except that the application time of the voltage V3 is adjusted as appropriate for the <NUM> Drop case. Subsequently, as a further contraction pulse (or a first-drop further contraction pulse), the voltage V1 is applied to the actuator <NUM> to attenuate residual vibration.

The second-drop ejection pulse including a second-drop expansion pulse and a second-drop contraction pulse is the same as the ejection pulse portion of the driving waveform <NUM>. The cancellation pulse following the second-drop ejection pulse is also the same as the cancellation pulse portion of the driving waveform <NUM>. In both the first and second drop ejection pulses of the <NUM> Drop driving waveform, the potential difference Δ(V2 - V3) between the voltages V2 and V3 in the contraction pulse at which the ink is ejected is greater than the potential difference Δ(V3 - V1) between the voltages V3 and V1, that is Δ(V2 - V3) > Δ(V3 - V1).

An intermediate time period Tm is provided between the first and second drops, that is between the first and second drop ejection pulses. The intermediate time period Tm is, for example, <NUM> Ta. An ejection speed of the second-drop ink is increased by setting the pulse width of the first-drop expansion pulse of the first-drop ejection pulse to <NUM> Ta and setting the pulse width of the second-drop expansion pulse of the second-drop ejection pulse to Ta. If Ta is set to λ/<NUM>, the pulse width of the first-drop expansion pulse is <NUM>(λ/<NUM>), the pulse width of the second-drop expansion pulse is λ/<NUM>, and the intermediate time period Tm is λ. If the ejection speed of final-drop ink that is the second-second-drop ink in the case of the <NUM>-Drop driving wave is increased and the intermediate time period Tm is provided, satellites of ink can be prevented from occurring.

The <NUM> Drop driving waveform is for printing three or more gradations by ejecting ink three times and includes a first-drop ejection pulse, a second-drop ejection pulse, and a third-drop ejection pulse. In the multi-drop driving waveform (<NUM> Drop), the voltage V1 serving as a bias voltage is initially applied to the actuator <NUM>. From that state, as a first-drop expansion pulse, the voltage V2 is applied to the actuator <NUM> for the time duration of <NUM> Ta. As a first-drop contraction pulse, the voltage V3 is applied to the actuator <NUM> to eject first-drop ink. That is, the first-drop ejection pulse is the same as the ejection pulse portion of the driving waveform <NUM> of the first embodiment (see <FIG>) except that the application time of the voltage V3 has been adjusted as appropriate for the <NUM> Drop case.

Subsequently, as a second-drop expansion pulse, the voltage V2 is applied to the actuator <NUM> for the time duration of <NUM> Ta. As a second-drop contraction pulse, the voltage V3 is applied to the actuator <NUM> to eject second-drop ink. Further, as a second-drop further contraction pulse, the voltage V1 is applied to the actuator <NUM> to attenuate residual vibration.

The third-drop ejection pulse is the same as the ejection pulse portion of the driving waveform <NUM>. The cancellation pulse following the third-drop ejection pulse is also the same as the cancellation pulse portion of the driving waveform <NUM>. In the <NUM> Drop driving waveform, the potential difference Δ(V2 - V3) between the voltages V2 and V3 in the contraction pulse at which ink is ejected is greater than the potential difference
Δ(V3 - V1) between the voltages V3 and V1, that is Δ(V2 - V3) > Δ(V3 - V1) for all of the first to third drops.

An intermediate time period Tm is provided between the second and third drops, that is between the second and third drop ejection pulses. The intermediate time period Tm is, for example, <NUM> Ta. An ejection speed of the third-drop ink which is a final drop is increased by setting the pulse width of the first-drop expansion pulse of the first drop ejection pulse to <NUM> Ta, setting the pulse width of the second-drop expansion pulse of the second drop ejection pulse to <NUM> Ta, and setting the pulse width of the third-drop expansion pulse of the third drop ejection pulse to Ta. If Ta is set to λ/<NUM>, the pulse width of the first-drop expansion pulse is <NUM>(λ/<NUM>), the pulse width of the second-drop expansion pulse is <NUM>(λ/<NUM>), the pulse width of the third-drop expansion pulse is λ/<NUM>, and the intermediate time period Tm is λ. If the ejection speed of the final-drop ink is increased and the intermediate time period Tm is provided, satellites of ink can be prevented from occurring. The interval between the first-drop expansion pulse and the second-drop expansion pulse is set such that the time duration between a center time of the first-drop expansion pulse and a center time of the second-drop expansion pulse (see the vertical dotted lines of the respective pulses in <FIG>) is equal to Ta.

The <NUM> Drop driving waveform has the same configuration as the <NUM> Drop driving waveform except that it includes a fourth-drop ejection pulse following the third-drop ejection pulse. Although not separately illustrated, a multi-drop driving waveform by which ink is ejected <NUM> or more times can be similarly configured.

If the depicted driving waveforms illustrated in <FIG> are used, the width of the expansion pulse can be set to <NUM> Ta in the driving waveform portion by which ink is ejected so that the ejection state of the ink is maintained.

In the configurations according to the first and second embodiments, the driving signal (waveform) is applied to the lower electrode <NUM> of the actuator <NUM> through the individual electrode <NUM> (see <FIG>). That is, the voltage application direction matches with the polarization direction of the piezoelectric body <NUM> (which is, the direction from the lower electrode <NUM> toward the upper electrode <NUM>). As a modified example, the driving signal may instead be applied to the upper electrode <NUM> of the actuator <NUM>, such that the voltage application direction does not match with the polarization direction of the piezoelectric body <NUM>. In this case, as illustrated in <FIG>, a driving waveform <NUM> (which corresponds to the driving waveform <NUM> being vertically inverted) would be applied. The same modification applies to the multi-drop waveforms.

In a modification of the above-described inkjet heads <NUM> to <NUM>, both the actuator <NUM> and the nozzle <NUM> may not be disposed on the surface of the nozzle plate <NUM>. The inkjet head utilizing the above-described driving waveforms may include an actuator for any driving scheme or type, for example, a drop on-demand piezoelectric scheme, a shared-wall type, and a shear-mode type actuator.

According to the above-described embodiments, it is possible to provide inkjet heads capable of suppressing misting when ink is ejected.

Claim 1:
An inkjet head (<NUM>, <NUM>, <NUM>, <NUM>), comprising:
a nozzle plate (<NUM>) on which at least one nozzle (<NUM>) is formed;
an ink pressure chamber (<NUM>) connected to the nozzle (<NUM>);
an actuator (<NUM>) configured to change a volume of the ink pressure chamber (<NUM>) for ejecting ink from the nozzle (<NUM>); and
an actuator driving circuit (<NUM>) configured to supply a driving waveform (<NUM>) to the actuator (<NUM>), the driving waveform (<NUM>) including an ejection pulse portion that changes from a first voltage to a second voltage that causes the ink pressure chamber (<NUM>) to expand and then from the second voltage to a third voltage that causes the ink pressure chamber (<NUM>) to contract to eject ink from the nozzle (<NUM>), the third voltage being at a potential between a potential of the first voltage and a potential of the second voltages, a difference between the potentials of the second and third voltages being greater than a difference between the potentials of the third and first voltages, characterized in that the driving waveform (<NUM>) includes, after the ejection pulse portion, a cancellation pulse portion that changes from the third voltage to a fourth voltage that causes the ink pressure chamber (<NUM>) to contract and then changes from the fourth voltage to a fifth voltage that causes the ink pressure chamber (<NUM>) to expand, and in that the third voltage and the fifth voltage are the same voltage level and the first voltage and fourth voltage are the same voltage level ,
wherein each of the first to fifth voltages is applied for a time Ta, the time Ta being set to a half period (λ/<NUM>) of a first natural vibration period of the actuator (<NUM>) in a state in which the ink pressure chamber (<NUM>) and the nozzle (<NUM>) are filled with ink.