Liquid ejection apparatus for suppressing a decrease in speed of liquid droplets which are discharged from adjacent nozzles during the same discharge period

A liquid ejecting apparatus including a pressure generating unit capable of changing a pressure of liquid contained in the pressure chamber, a liquid ejecting head capable of discharging liquid droplets from a nozzle opening by actuating the pressure generating unit, a passage extending from a common liquid chamber through a pressure chamber to the nozzle opening and a driving signal generating unit that repeatedly generates a plurality of driving signals causing the liquid droplets to be discharged by actuating the pressure generating unit. In order to prevent the vibration resulting from a first driving pulse sent to a first pressure generating unit from interfering with the discharge in an adjacent pressure chamber, a second driving signal including a second discharge pulse, is generated at a period of time after to the first discharge pulse corresponding to a characteristic vibration period of the liquid contained in the pressure chamber.

BACKGROUND OF THE INVENTION

The entire disclosure of Japanese Patent Application No. 2006-145844, filed May 25, 2006 is expressly incorporated herein by reference.

1. Technical Field

The invention relates to a liquid ejecting apparatus and, more particularly, to a liquid ejecting apparatus capable of controlling the discharge of liquid droplets using a plurality of driving signals.

2. Related Art

Typically, a liquid ejecting apparatus has a liquid ejecting head capable of discharging liquid droplets of various liquids. An example of such a liquid ejecting apparatus is an ink jet recording apparatus, or printer, with an ink jet recording head (hereinafter, referred to as a recording head) which discharges liquid ink droplets from the recording head.

A liquid ejecting head is typically provided with pressure chambers such that a change in the pressure of the liquid contained in the pressure chamber occurs by actuating a pressure generating unit such as a piezoelectric vibrator. The ink then travels through a series of passages extending from the pressure chambers to a series of nozzles where it is discharged as ink droplets.

In recent years, ink jet recording apparatuses have been developed wherein a plurality of driving signals, comprised of discharge pulses which correspond to the different volumes of the ink droplets are sent to the piezoelectric vibrators (for example, see JP-A-2005-088582 (FIG. 5)). Advantageously, this allows for multi-valued gradation and improved speed in the recording process.

In recent years, however, the thicknesses of partitions between the pressure chambers has been decreased in order to decrease the weight and size of the recording head. As a result, a pressure vibration occurring in ink in one pressure chamber can reach the pressure chamber of a second nozzle and the velocity of ink droplets as they are being discharged from the second nozzle may be decreased. Particularly, when the ink droplets are discharged from adjacent nozzles using discharge pulses generated from different driving signals, there is a possibility that discharge of one nozzle will influence the discharge of the other nozzle.

When the velocity of the discharged ink droplets is decreased, the droplets may enter a mist state and fail to successfully hit the discharge target, thereby deteriorating the quality the resulting image.

BRIEF SUMMARY OF THE INVENTION

An advantage of some aspects of the invention is a liquid ejecting apparatus which can suppress the decrease in a the speed of liquid droplet which are discharged from adjacent nozzles during the same discharge period.

One aspect of the invention is a liquid ejecting apparatus including a pressure generating unit capable of changing the pressure of a liquid contained in the pressure chamber; a liquid ejecting head that can discharge liquid droplets from a nozzle opening by actuating the pressure generating unit; a passage extending from the pressure chamber to the nozzle; and a driving signal generating unit capable of generating a plurality of driving signals comprising a discharge pulse which causes the liquid droplets to be discharged by actuating the pressure generating unit, wherein the driving signal generating unit generates a first driving signal comprising a first discharge pulse and a second driving signal comprising a second discharge pulse, wherein the second discharge pulse is generated at a period of time after to the first discharge pulse, wherein the period of time between the beginning of the first discharge pulse and the end of the second discharge pulse corresponds to a characteristic vibration period of the liquid contained in the pressure chamber.

A second aspect of the present invention is a method for ejecting a liquid in a liquid ejecting apparatus including a pressure generating unit capable of changing a pressure of liquid contained in the pressure chamber, a liquid ejecting head capable of discharging liquid droplets from a nozzle opening by actuating the pressure generating unit, a passage extending from the pressure chamber to the nozzle, and a driving signal generating unit capable of generating a plurality of driving signals comprising discharge pulses which cause the liquid droplets to be discharged by actuating the pressure generating unit. The method comprises generating a first driving signal comprising a first discharge pulse and a second driving signal comprising a second discharge pulse, and delaying the time of the generation of the second discharge pulse so that the time between a start point of the first discharge pulse and an end point the second discharge pulse correspond to a characteristic vibration period of the liquid contained in the pressure chamber.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments for carrying out the invention will be described with reference to the accompanying drawings. Although various detailed examples of the invention are given in the embodiments described below, but the scope of the invention is not limited to the embodiments unless specific imitations are described. Hereinafter, an ink jet recording apparatus (referred to as a printer) is included as an example of a liquid ejecting apparatus which may be used in association with the present invention.

FIG. 1is a block diagram illustrating an electrical configuration of a printer. The exemplified printer includes a printer controller1and a printer engine2. The printer controller1is provided with an external interface (external I/F)3that transmits and receives data to and from an external apparatus such as a host computer (not shown), a RAM4that stores various kinds of data, a ROM5that stores a control program for processing various kinds of data, a control unit6including a CPU, an oscillation circuit7that generates a clock signal, a driving signal generating circuit9that generates driving signals (COM1and COM2) supplied to a recording head8, and an internal interface (internal I/F)10that transmits recording data and the driving signals to the printer engine2.

The external I/F3receives print data such as image data supplied from the host computer. Status signals such as a busy signal or an acknowledgement signal are output from the external I/F3to the external apparatus. The RAM4is used as a receiving buffer, an intermediate buffer, an output buffer, and a work memory unit. The ROM5stores various control programs which may be executed by the control unit6, font data and code for executing graphic functions, and various other procedures.

The driving signal generating circuit9is provided with a first driving signal generating unit9A capable of generating a first driving signal COM1and a second driving signal generating unit9B capable of generating a second driving signal COM2, which will be described more fully below.

The control unit6controls units of the printer in accordance with the control program stored in the ROM5or converts the print data supplied from external apparatuses to recording data that may be transmitted to the recording head8. At the time of converting the print data to the recording data the control unit6first reads the print data stored in the RAM4. Then the control unit6converts the read data into intermediate code data and stores the intermediate code data in an intermediate buffer provided in the RAM4. Next, the control unit6analyzes the intermediate code data read from the intermediate buffer and converts the intermediate code data into the recording data (dot pattern data) for each dot by referring to font data and code for executing graphic functions stored in the ROM5. The control unit6supplies a latch signal or a channel signal to the recording head8through the internal I/F10. A latch pulse and a channel pulse included in the latch signal and the channel signal define a supply timing of each of the pulses constituting the driving signals COM1and COM2.

Next, the print engine2will be described. As shown inFIG. 1, the printer engine2is provided with the recording head8, a carriage mechanism11, a paper feeding mechanism12, and a linear encoder13. The carriage mechanism11includes a carriage having the recording head, which is a kind of liquid ejecting head8, attached thereto and a driving motor (such as a DC motor) that drives the carriage through a timing belt (carriage and driving motor not shown), and transports the recording head8mounted on the carriage in a main scanning direction. The paper feeding mechanism12includes a paper feeding motor and a paper feeding roller. The paper feeding mechanism12discharges recording sheets onto a platen and performs vertical scanning. The linear encoder13outputs an encoder pulse, which indicates the scanning position of the recording head8mounted on the carriage to the control unit6to the internal I/F10in the main scanning direction. The control unit6is then able to store the position of the recording head8.

As shown inFIG. 2, the first driving signal COM1is a signal having a first discharge pulse DPM1sufficient to generate a first medium-sized printing dot and a second medium-size dot discharge pulse DPM2in a recording period T. The first driving signal COM1is generated each recording period T. In the embodiment, one recording period T of the first driving signal COM1is divided into two periods T11and T12. In the first driving signal COM1, the first medium-size dot discharge pulse DPM1is generated in the period T11and the second medium-size dot discharge pulse DPM2is generated in the period T12.

The second driving signal COM2is a signal having a small dot discharge pulse DPS and a large dot discharge pulse DPL within the recording period T. One recording period T of the second driving signal COM2is divided into two pulse generation periods of T21and T22. The small dot discharge pulse DPS is generated in the period T21and the large dot discharge pulse DPL is generated in the period T22. The driving signals COM1and COM2will be described in greater detail below.

FIG. 3is a cross-sectional view illustrating the main units of the recording head8. The recording head8according to the embodiment is provided with, a vibrator unit15including a piezoelectric vibrator portion12, a clamping plate13, and a flexible cable14, a head case16capable of housing the vibrator unit15, and a series of passages17extending from ink chambers, through pressure chambers, and then to nozzle openings.

First, the vibrator unit15will be described. Piezoelectric vibrators20within the piezoelectric vibrator portion12are formed in an elongated comb-like shape in the longitudinal direction. Each of the piezoelectric vibrators20has a very small width of approximately several tens of μms. Each of the piezoelectric vibrators20is a piezoelectric vibrator of the longitudinal vibration type which is capable of extending in the longitudinal direction. A fixing end portion is bonded onto the clamping plate13and a free end portion protrudes outside a leading edge of the clamping plate13, meaning that each of the piezoelectric vibrators20is fixed in a so-called cantilever state. A front end of the free end portion of each of the piezoelectric vibrators20is bonded to an island section34constituting a diaphragm section32in each of the passage units17as described below. The flexible cable14is electrically connected to the piezoelectric vibrator20on a side surface of a fixing end portion opposite the clamping plate13. The clamping plate13supporting each of the piezoelectric vibrators20is formed from a metallic plate material having a rigidity such that it can receive a reaction force from the piezoelectric vibrators20. In this embodiment, the clamping plate13is composed of a stainless steel plate having a thickness of approximately 1 mm.

Next, the passage unit17will be described. The passage17is formed in a nozzle plate22, a passage formation substrate23, and a vibrating plate24. The passage17is creating by disposing and laminating the nozzle plate22on one surface of the passage substrate23and disposing and laminating the vibrating plate24on the other surface of the passage formation substrate23bonding the nozzle plate22to the vibrating plate24.

The nozzle plate22is a thin plate formed of stainless steel with a plurality of nozzle openings25formed in an array with a pitch corresponding to a dot formation concentration. In the embodiment, for example,180nozzle openings25are formed in an array in order to create a nozzle array. Two nozzle arrays are provided parallel to each other.

The passage formation substrate23is a plate-like member forming an ink passage including a reservoir26, ink supply port27, and a pressure chamber28. Specifically, the passage formation substrate23is a plate-like member in which a plurality of null portions serve as pressure chambers28which are separated by partitions with nozzle openings25and null portions serving as ink supply ports27and reservoirs26. According to one embodiment, the passage formation substrate23is manufactured by etching a silicon wafer. The pressure chambers28are formed into elongated chambers in a direction orthogonal to the direction of the nozzle array of nozzle openings25. Each of the ink supply ports27are formed into a narrow portion having a small passage width, which allows the pressure chamber28to communicate with the reservoir26. Each of the reservoirs26is a chamber for transferring ink stored in an ink cartridge (not shown) into the corresponding pressure chamber28through the ink supply port27.

The vibrating plate24is a composite plate material having a two-layer structure in which a resin film31such as PPS (polyphenylene sulfide) is laminated on a metallic supporting plate30formed of a material such as stainless steel. The vibrating plate24has a diaphragm section32for varying the volume of the pressure chamber28by sealing one opening surface of the pressure chamber28along with a compliance section33for sealing one opening of the reservoir26. In the diaphragm section32, the island section34is formed by etching part of the supporting plate30corresponding to the pressure chamber28and by removing the surrounding portions. The island section34has an elongated block shape in the direction orthogonal to the direction of the array of nozzle openings25. The resin film31is a resilient body film located near the island section34. The portion corresponding to the reservoir26is referred to as the compliance section33, which is formed above the resin film31by removing a portion of the supporting plate30that is roughly the same size as the opening shape of the reservoir26using an etching process.

Next, the electrical configuration of the recording head8will be described. As shown inFIG. 1, the recording head8is provided with a shift register circuit including a first shift register41and a second shift register42, a latch circuit including a first latch circuit43and a second latch circuit44, a decoder45, a control logic circuit46, a level shifter circuit including a first level shifter47and a second level shifter48, a switch circuit including a first switch49and a second switch50, and the piezoelectric vibrator20. The shift registers41and42, the latch circuits43and44, the level shifters47and48, the switches49and50, and the piezoelectric vibrators20are included in a number equal to the number of the nozzle openings25.

The recording head8discharges ink droplets on the basis of recording data received from a printer controller1. In the embodiment, since a higher bit group of recording data and a lower bit group of recording data, each formed of two bits, are sent to the recording head8sequentially, the higher bit group of the recording data is set in the second shift register42. At each nozzle openings25, any higher bit group of recording data set in the second shift register42is shifted to the first shift register41and the lower bit group of the recording data is set in the second shift register42.

The first latch circuit43is electrically connected to an end of the first shift register41and the second latch circuit44is electrically connected to an end of the second shift register42. When a latch pulse from the printer controller1is sent to each of the latch circuits43and44, the first latch circuit43latches the higher bit group of the recording data and the second latch circuit44latches the lower bit group of the recording data. The recording data (higher bit group and lower bit group) latched by the latch circuits43and44are then outputted to the decoder45. The decoder45generates pulse selection data for selecting the pulses comprising the driving signals COM1and COM2based on the higher bit group and the lower bit group of the recording data.

According to one embodiment, pulse selection data is generated for each of the driving signals COM1and COM2. That is to say, first pulse selection data corresponding to the first driving signal COM1is configured by 2-bit data corresponding to the first medium-size dot discharge pulse DPM1(the period T11) and the second medium-size dot discharge pulse DPM2(the period T12). Second pulse selection data corresponding to the second driving signal COM2is comprised of 2-bit data corresponding to the small dot discharge pulse DPS (the period T21) and the large dot discharge pulse DPL (the period T22).

A timing signal from the control logic circuit46is also input into the decoder45. The control logic circuit46generates the timing signal in synchronization with input from the latch signal or the channel signal. The timing signal is also generated for each of the driving signals COM1and COM2. Each pulse selection data generated by the decoder45is input into a corresponding level shifter47or48sequentially from a higher bit side at a timing defined by the timing signal. The level shifters47and48function as a voltage amplifier. The level shifters47and48output an electrical signal raised to a voltage sufficient to drive the corresponding switches49and50. For example, a voltage of approximately several tens of volts may be used when the pulse selection data has a value of 1. When the first pulse selection data has a value of 1, the electrical signal may be output to the first switch49and when the second pulse selection data has a value of 1, the electrical signal may be output to the second switch50.

The first driving signal COM1is supplied from a first driving signal generating unit9A to a first switch49and the second driving signal COM2is supplied from a second driving signal generating unit9B a second switch50. In return, each of the piezoelectric vibrators20is connected to the corresponding switches49and50. That is to say, the first switch49switches supply the first driving signal COM1to the piezoelectric vibrator20and the second switch50switches supply the second driving signal COM2to the piezoelectric vibrator20. The first switch49and the second switch50selectively supply the driving signals.

The pulse selection data controls actuation of each of the switches49and50. Thus, while the pulse selection data input sent to the first switch49has the value of 1, the first switch49is in a conduction state and a first driving signal COM1is supplied to the piezoelectric vibrator20. Similarly, while the pulse selection data input sent to the second switch50has the value of 1, a second driving signal COM2is supplied to the piezoelectric vibrator20. On the other hand, when the pulse selection data input sent to the switches49and50has a value of 0, each of the switches49and50is in a cut-off state and no driving signal is supplied to the piezoelectric vibrator20. In other words, when the pulse data has the value of 1 a pulse is supplied to the piezoelectric vibrator20for a specified period of time.

Next, the discharge pulse included in each of the driving signals COM1and COM2, which is generated by the driving signal generating circuit9will be described, in reference toFIGS. 2 and 10.FIG. 10will describe the discharge pulses generally in reference to printing apparatuses currently used in the art, whileFIG. 2will explain aspects of the invention in greater detail.

FIG. 10illustrates a configuration in which a generation time ta1of a first discharge pulse DPA1that is first generated in one driving signal COM1is different from the generation time tb1of the first discharge pulse DPB1generated in another driving signal COM2. Because the spacing of the discharge pulses in the driving signals is reduced as much as possible in order to speed up the recording operation by shortening the length of one recording period T, the generation time tm1of a discharge pulse DPA2generated after the discharge pulse DPA1may not match the generation timing tm2of a discharge pulse DPB2. Thus, the discharge pulse DPB2of the driving signal COM2is generated later than the pulse DPA2of the driving signal COM1by Δt.

Disadvantageously, in situations where discharge pulse DPA1and DPA2are used in adjacent nozzles, there is a possibility that discharge of the other nozzle will have an influence on discharge of the one nozzle.

By way of contrast, the configuration of the present invention will be described in more detail, usingFIG. 2as a reference. The first driving signal COM1comprises a first medium-sized dot discharge pulse DPM1which is generated in the period T11along with a second medium-size dot discharge pulse DPM2which is generated in the period T12. The discharge pulses DPM1and DPM2each have waveforms of the same shape and include an expansion component P11(corresponding to a pressure chamber expansion), an expansion hold component P12, a contraction component P13(corresponding to the contraction of the pressure chamber), damping hold component P14, and an expansion damping component P15. The first expansion component P11is a waveform component in which a potential is raised to an expansion potential VH1from a reference intermediate potential VHB at a comparatively constant low rate so as not to discharge the ink droplets. The first expansion hold component P12is a waveform component in which the first expansion potential VH1is constantly held. The first contraction component P13is a waveform component in which the potential drops to a contraction potential VL1from the expansion potential VH1at a comparatively high rate. The damping hold component P14is a waveform component in which the contraction potential VL1is held for a predetermined period. The expansion damping component P15is a waveform component in which the potential is recovered to the intermediate potential VHB from the first contraction potential VL1at a comparatively constant low rate so as not to discharge the ink droplets.

When the first medium-size dot discharge pulse DPM1or the second medium-size dot discharge pulse DPM2described above is supplied to the piezoelectric vibrator20, the piezoelectric vibrator20is contracted in a longitudinal direction by the first expansion component P11and the pressure chamber28expands from the reference volume corresponding to the intermediate potential VHB to an expansion volume corresponding to the expansion potential VH1. During the expansion, ink is supplied to the pressure chamber28from the reservoir26through the ink supply port27. This state is held during the expansion hold component P12of the pulse. During the contraction component P13, the piezoelectric vibrator20is extended by contracting the pressure chamber28rapidly from the expansion volume to contraction volume corresponding to the contraction potential VL1. The ink of the pressure chamber28is pressurized by the rapid contraction of the pressure chamber28and thus, ink droplets having a volume corresponding to that of medium-size dots are discharged from the nozzle openings25.

The contraction state of the pressure chamber28is held during the damping hold component P14and the pressure of the pressure chamber28, which has been decreased by the discharge of the ink droplets is raised again by natural vibration. During the expansion damping component P15, the pressure chamber28is expanded back to the reference volume and thus, pressure variation of the ink in the pressure chamber28is absorbed.

In the second driving signal COM2, a small dot discharge pulse DPS is generated in the period T21, which includes a first expansion component P21, a first expansion hold component P22, a contraction component P23, a contraction hold component P24, a second expansion component P25, a second expansion hold component P26, a second contraction component P27, a damping hold component P28, and a expansion damping component P29. The first expansion component P21is a waveform component in which the potential is raised to the first expansion potential VH2from the intermediate potential VHB and the first expansion hold component P22is a waveform component in which the first expansion potential VH2is constantly held. The first contraction component P23is a waveform component in which the potential drops rapidly from the first expansion potential VH2to first intermediate potential VM1. The contraction hold component P24is a waveform component in which the first intermediate potential VM1is constantly held, the second expansion component P25is a waveform component in which the potential is raised to second intermediate potential VM2from the first intermediate potential VM1, and the second expansion hold component P26is a waveform in which the second intermediate potential VM2is constantly held. The second contraction component P27is a waveform component in which the potential consistently drops to the contraction potential VL2from the second intermediate potential VM2at a comparatively high rate. The second damping hold component P28is a waveform component in which the contraction potential VL2is constantly held. The expansion damping component P29is a waveform component in which the potential is constantly recovered to the intermediate potential VHB from the contraction potential VL2at a comparatively low rate so as not to discharge the ink droplets.

When the small dot discharge pulse DPS is supplied to the piezoelectric vibrator20, the piezoelectric vibrator20is contracted sharply in a longitudinal direction by the first expansion component P21and thus, the island section34is displaced in a direction away from the pressure chamber28. Due to the displacement of the island section34, the pressure chamber28is expanded rapidly from the reference volume to expansion volume corresponding to the first expansion potential VH2. the expansion of the pressure chamber28causes a comparatively strong negative pressure in the pressure chamber28and causing the ink to travel from the reservoir26to the pressure chamber28. The expansion state of the pressure chamber28is held during supply of the first expansion hold component P22. Then, is the direction of the meniscus is changed during the first expansion hold component P22and the central part thereof is inflated into a column shape.

Thereafter, the first contraction component P23is supplied and the piezoelectric vibrator20is extended. During the extension of the piezoelectric vibrator20, the island section34is rapidly displaced in a direction adjacent to the pressure chamber28. Due to the displacement of the island section34, the pressure chamber28is contracted rapidly, decreasing the volume thereof from the expansion volume to a volume corresponding to the first intermediate potential VM1. The ink of the pressure chamber28is pressurized by the rapid contraction of the pressure chamber28. In addition, the contraction hold component P24is supplied and the discharge volume is held for a short time. The piezoelectric vibrator20is contracted by the second expansion component P25and thus, the volume of the pressure chamber28is slightly increased again. The piezoelectric vibrator20is extended by the second contraction component P27through the second expansion hold component P26and thus, the volume of the pressure chamber28is rapidly decreased again and the ink is discharged as ink droplets having a volume corresponding to that of the small dots during supply of the third contraction component P27from the contraction hold component P24. Thereafter, due to supply of the damping hold component P28and the expansion damping component P29, the pressure chamber28is expanded back to the reference volume and the pressure variation of the ink in the pressure chamber28is absorbed.

In the second driving signal COM2, the large dot discharge pulse DPL generated in the period T22includes an expansion component P31, a expansion hold component P32, a contraction component P33, a damping hold component P34, and a expansion damping component P35. The expansion component P31is a waveform component in which potential is raised to the expansion potential VH3from the intermediate potential VHB consistently at a comparatively low rate so as not to discharge the ink droplets. The expansion hold component P32is a waveform component in which the expansion potential VH3is constantly held. The contraction component P33is a waveform component in which the potential drops to contraction potential VL3from the expansion potential VH3consistently at a comparatively high rate. The damping hold component P34is a waveform component in which the contraction potential VL3is held for a short period. The expansion damping component P35is a waveform component in which the potential is recovered to the intermediate potential VHB from the contraction potential VL3.

When the large dot discharge pulse DPL configured as above is supplied to the piezoelectric vibrator20, first, the piezoelectric vibrator20is contracted in a longitudinal direction by the expansion component P31. The pressure chamber28then expands from the reference volume corresponding to the intermediate potential VHB to an expanded volume corresponding to the expansion potential VH3. During the expansion, the ink is drawn into the pressure chamber28from the reservoir26through the ink supply port27. The expansion state of the pressure chamber28is held during the supply of the expansion hold component P32. Thereafter, the contraction component P33is supplied and the piezoelectric vibrator20is extended. By the extension of the piezoelectric vibrator20, the pressure chamber28is contracted rapidly from the expansion volume to contraction volume corresponding to the contraction potential VL3. The ink in the pressure chamber28is pressurized by the rapid contraction of the pressure chamber28and thus, ink droplets having a volume corresponding to that of large dots are discharged from the nozzle openings25. Thereafter, the damping hold component P34is supplied along with the expansion damping component P35, wherein the pressure chamber28is expanded back to the reference volume and the pressure variation of the ink in the pressure chamber28is absorbed.

In this embodiment, the start of the discharge pulse, referred to as the generation timing of the first medium-size dot discharge pulse DPM1in the first driving signal COM1corresponds with the generation timing of the small dot discharge pulse DPS in the second driving signal COM2. Unfortunately, however, the a generation timing tm1of the second medium-size dot discharge pulse DPM2and the generation timing tm2of the large dot discharge pulse DPL in the second riving signal COM2do not correspond. That is to say, as shown inFIG. 2, the large dot discharge pulse DPL is generated later than the second medium-size dot discharge pulse DPM2by a time represented by Δt.

The recording head8of the present invention has a decreased size and weight. Therefore, as previously mentioned, the thicknesses of partitions partitioning the pressure chambers28adjacent to each other is reduced. As a result, as shown inFIG. 4, pressure vibration produced in the ink of the pressure chamber28by driving the piezoelectric vibrator20may be transmitted to an adjacent pressure chamber28through the partition. In situations where the ink droplets are discharged from the nozzle openings25adjacent to each other at the same time, phases of the pressure vibrations on both sides agree with each other, meaning that there is no influence of the pressure vibration. However, as described above, in situations where the discharge timings of the nozzle openings25adjacent to each other are different, the pressure vibration may influence the discharging from adjacent nozzles.

For example, in a certain recording period, assuming that the piezoelectric vibrator20corresponding to one nozzle opening25, shown inFIG. 4as nozzle A, is driven by the second medium-size dot discharge pulse DPM2, and that a second piezoelectric vibrator20corresponding to a second nozzle B is driven by the large dot discharge pulse DPL, the discharge timing in the nozzle B will be later than that in the nozzle A. In this case, the vibration of pressure chamber28corresponding to the nozzle A is transmitted to the pressure chamber28corresponding to the nozzle B through the partition. Thus, the velocity of the droplets as they leave the nozzle B, known as the flying velocity Vm, may be slower than the flying velocity Va of the droplets without the interfering vibration.

Disadvantageously, when the flying velocity of the ink droplets is decreased, the ink droplets may enter a mist state and fail to accurately hit the discharge target, resulting in deteriorated image quality.

In order to overcome these problems, in the printer1according to the invention, the displacement (delay time) Δt on a time axis between the generation timing tm1of a medium-size dot discharge pulse DPM2in the first driving signal COM1and the generation timing tm2of the large dot discharge pulse DPL in an adjacent nozzle is optimized. This allows the flying velocities of the ink droplets discharged from both nozzle openings25to achieve the target flying velocity Va even when the ink droplets are discharged from adjacent nozzle openings25in the same recording period. Specifically, as shown inFIG. 5, the delay time Δt, or the time from the starting point tm1of the expansion component P11of the second medium-size dot discharge pulse DPM2to the starting point tm2of the expansion component P31of the large dot discharge pulse DPL, is set so that the displacement Δts between a start point of the contraction component P13and the end point of the contraction component P33corresponds with the characteristic vibration period Tc of the ink in the pressure chamber28.

FIG. 6is a graph illustrating the flying velocity Vm (m/s) of the ink droplets in the nozzle B at various delay times Δt (μs) between the generation timings of the second medium-size dot discharge pulse DPM2and the large dot discharge pulse DPL when the ink droplets are discharged adjacent nozzles during the same recording period, wherein the second medium-size dot discharge pulse DPM2is used for the nozzle A and the large dot discharge pulse DPL is used for the nozzle B. InFIG. 6, the flying velocity Vm is represented in a ratio (%) to the target flying velocity Va. When the delay time Δt has a value of 0, the second medium-size dot discharge pulse DPM2and the large dot discharge pulse DPL are generated at the same time and when the delay time Δt has a minus value, the large dot discharge pulse DPL is generated earlier than the second medium-size dot discharge pulse DPM2.

As shown inFIG. 6, the flying velocity Vm of the ink droplets varies periodically after the border point Pm, and is substantially similar to the target flying velocity Va (100%) when the delay time Δt is set to less than border point Pm. Thus, the discharge of the nozzle A has no influence on the nozzle B before the generation period tx of the large dot discharge pulse DPL, meaning that there is no interference before the generation period tx matches the generation period of the second medium-size dot discharge pulse DPM2. Conversely, the pressure vibration produced by the discharge of the nozzle A does have an influence on the nozzle B when the generation period tx matches the generation period of the second medium-size dot discharge pulse DPM2. Accordingly, the delay time Δt corresponding to the border point Pm is acceptable only before the generation period tx. The generation period tx can be written by tx=tc2+th2+td2when the tc2represents the generation period of the expansion component P31, th2represents the generation period of the expansion hold component P32, and td2represents the generation period of the contraction component P33.

When the delay time is set past the border point Pm, since the pressure vibration in the pressure chamber28is excited at the time when the piezoelectric vibrator20on the nozzle A side is driven by the second medium-size dot discharge pulse DPM2, the flying velocity Vm of the ink droplets is faster or slower than the target flying velocity Va depending on amplitude of the pressure vibration. That is to say, when the ink droplets are discharged from the nozzle B at a timing when the pressure vibration is displaced in a direction opposite the discharge direction, the flying velocity of the ink droplets is decreased, while when the ink droplets are discharged from the nozzle B at a timing when the pressure vibration is displaced in the discharge direction, the flying velocity of the ink droplets increases. A variation curve of the flying velocity Vm substantially agrees with a waveform of the pressure vibration produced in the ink of the pressure chamber28.

Assuming that the variation of the flying velocity Vm shown inFIG. 6corresponds to the pressure vibration produced in the ink of the pressure chamber28, the pressure chamber28is expanded by the first expansion component P11between the point Pm and a point Po, wherein pressure chamber28causes the ink to vibrate according to a natural vibration period Tc. After the point Po, a natural vibration period Tc is generated when the ink of the pressure chamber is pressurized and discharged by means of the first contraction component P13.

Here, the phase of the pressure vibration depends on the generation period tc1of the expansion component P11and the generation period th1of the expansion hold component p12of the second medium-size dot discharge pulse DPM2.FIGS. 7A to 7C,8A to8C, and9A to9C are diagrams illustrating various flying velocities Vm when the generation period of a waveform component of the second medium-size dot discharge pulse DPM2is changed, and may be referred to hereinafter as waveform diagrams of the pressure vibration produced in the ink of the pressure chamber.FIGS. 7A to 7Cillustrate the change in the flying velocity Vm when the generation period tc1of the first expansion component P11is changed,FIGS. 8A to 8Cillustrate the change of the flying velocity Vm when the generation period th1of the first expansion hold component P12is changed, andFIGS. 9A to 9Cillustrate the change of the flying velocity Vm when the generation period td1of the first contraction component P13is changed. The generation period of each of the components is increased in the order ofFIGS. 7A to 7C,8A to8C, and9A to9C, respectively.

The maximum value ep specified inFIGS. 7A to 7C,8A to8C, and9A to9C, changes in size and position when the generation period tc1of the first expansion component P11and the generation period th1of the first expansion hold component P12are changed. Specifically, as values of tc1and th1increase, the generation of the maximum value ep occurs later. That is to say, as the values of tc1and th1become larger, the variation curve phase occurs later. On the contrary, when the generation period td1of the first contraction component P13is changed, a phase of the variation curve is not significantly changed whereas amplitude of the variation curve is changed (FIGS. 9A to 9C).

In consideration of the configuration, the target flying velocity Va can be acquired (Vm 100% inFIG. 6) at a point Pp after the generation period tc1, the generation period th1, and the characteristic vibration period Tc from the border point Pm. That is to say, the amplitude of the pressure vibration becomes0at the point Pp. Accordingly, in the printer1according to the invention, the delay time Δt is determined by Δt=tc1+th1+Tc−(tc2+th2+td2).

In accordance with the expression, the displacement Δts (FIG. 5) on the time axis between the start point of the first contraction component P13of the second medium-size discharge pulse DPM2and the end point of the fourth contraction component P33which is the discharge component of the large dot discharge pulse DPL becomes the delay time Δt which agrees with the characteristic vibration period Tc.

Even when the ink droplets are discharged from each of the nozzles in the same recording period by using the second medium-size discharge pulse DPM2in a nozzle(the nozzle A) adjacent a second nozzle (nozzle B) using the large dot discharge pulse DPL, the amplitude of the pressure vibration produced by the discharge of one nozzle A becomes almost 0, when the delay time Δt calculated above is used between a generation timing of the large dot discharge pulse DPL as the second discharge pulse and a generation timing of the second medium-size discharge pulse DPM2is set.

Accordingly, it is possible to suppress the influence of the pressure vibration. As the result, the flying velocity of the ink droplets on the nozzle B can achieve the flying velocity of the ink droplets when the ink droplets are discharged without any interference from adjacent nozzles (target flying velocity Va). As the result, the ink droplets refrain from entering a mist state and the flying curve is suppressed, and it is possible to hit the ink droplets onto the discharge target with high precision.

Because the ink droplets have a small volume, the flying curve may be easily influenced by any pressure vibration produced by a discharge from the adjacent nozzle openings25. Accordingly, a large dot discharge pulse DPL corresponding to the second discharge pulse causes liquid droplets with a volume larger than that of ink droplets which are discharged by the second medium-size discharge pulse DPM2which correspond to the first discharge pulse. That is, the second medium-size discharge pulse DPM2results in liquid droplets which are comparatively smaller in volume than previously generated during the large dot discharge pulse DPL, making it possible to prevent the situation where the pressure vibration produced by the discharge of the adjacent nozzle openings25at the time of discharging results in ink droplets with a small volume. In situations where the ink droplets are discharged from the nozzle openings25in the middle of a discharge generation period, the delay time At is preferably determined by
Δt=tc1+th1+Tc−(tc2+th2+td2−α)
where α is set to a range represented by 0≦α≦td2.

That is to say, in the modified example, the delay time Δt corresponds to the generation time td2of the contraction component P33by means of α. The discharging timing of the ink droplets can agree with the timing when the amplitude of the pressure is almost 0 as much as possible by optimizing α, making it possible to suppress the influence of the pressure vibration more surely.

However, the invention is not limited to the embodiments, but various modifications may occur insofar as they are within the scope of the appended claims.

Waveform configurations of the driving signals COM1and COM2are not limited to those exemplified in the embodiments, but the invention can be applied to driving signals having various configurations. For example, when the first driving signal COM1may include a first discharge pulse that is a small dot discharge pulse, and a third discharge pulse, that is a medium-size discharge pulse, which causes liquid droplets with a larger volume than that of the liquid droplets discharged by the first discharge pulse and the second driving signal COM2includes a second discharge pulse that is a large dot discharge pulse, and a fourth discharge pulse which is a small dot discharge pulse, which causing liquid droplets with a smaller volume than the larger discharge pulse, it is efficient to have the first discharge pulse be generated later than the third discharge pulse in the first driving signal COM1and the second discharge pulse be generated later than the fourth discharge pulse in the second driving signal COM2, with the third discharge pulse of the driving signal COM1and the fourth discharge pulse of the second driving signal COM2being generated at the same time.

That is to say, in this configuration, it is assumed that the ink droplets are discharged from the nozzle openings25during the same recording period by using the third discharge pulse and the fourth discharge pulse for adjacent nozzle openings25, so that the discharging timings of the both nozzles substantially agree with each other. Thus, it is further assumed the ink droplets discharged using the first discharge pulse and the second discharge pulse for adjacent nozzle openings25, so that the discharge timing of the ink droplets on the nozzles agree with the timing when the amplitude of the pressure vibration from the one nozzle opening is almost 0. This makes it is possible to prevent the influence of the pressure vibration from the nozzles where the ink droplets are of smaller volume.

The invention can be also applied to a configuration in which one driving signal includes three or more discharge pulses.

The invention may be used in any liquid ejecting apparatus capable of performing a discharge control by using the plurality of driving signals, meaning that the invention is not limited to a printer, and may be applied to various ink jet recording apparatus such as plotters, facsimile equipment, copy machines, as well as liquid ejecting apparatuses other than the recording apparatuses such as display manufacturing apparatuses, electrode manufacturing apparatuses, and chip manufacturing apparatuses.