阅读说明：本技术 喷墨记录装置以及喷墨头驱动方法 (Ink jet recording apparatus and ink jet head driving method ) 是由 马渡健儿 于 2018-10-04 设计创作，主要内容包括：本发明是从多个喷墨头喷出墨的记录装置,应用于如下情况,即：进行针对1个像素喷出1个液滴或者多个液滴并使多个液滴合体的驱动。驱动信号包含由N个(N为2以上的整数)的驱动波形元素构成的驱动波形,在将根据喷墨头的结构求出的固有振动周期设为Tc时,作为从驱动波形的起点至下一个后续的驱动波形的起点的时间Ts,使其满足1.1Tc≤Ts≤1.4Tc的关系。由此,在将喷墨头以多灰度等级驱动的情况下,抑制由驱动喷墨头的压电致动器的谐振频率的偏差引起的速度偏离。(The present invention is a recording apparatus that ejects ink from a plurality of ink jet heads, and is applied to a case where: the driving is performed to discharge 1 droplet or a plurality of droplets for 1 pixel and combine the droplets. The drive signal includes a drive waveform composed of N (N is an integer of 2 or more) drive waveform elements, and when Tc is a natural vibration period obtained from the structure of the ink jet head, a time Ts from the start of the drive waveform to the start of the next subsequent drive waveform satisfies a relationship of 1.1Tc ≦ Ts ≦ 1.4 Tc. Thus, when the inkjet head is driven at a plurality of gradation levels, a speed deviation caused by a deviation in resonance frequency of a piezoelectric actuator that drives the inkjet head is suppressed.)

1. An ink jet recording apparatus includes:

a plurality of ink jet heads having nozzles that eject ink;

a plurality of actuators for applying pressure changes to the inks by a predetermined driving operation to the plurality of ink jet heads; and

a drive unit configured to generate and apply a drive signal for discharging 1 droplet or a plurality of droplets for 1 pixel and combining the plurality of droplets to each of the plurality of actuators,

the drive signal generated by the drive unit includes a drive waveform composed of N drive waveform elements, where N is an integer of 2 or more,

when the natural vibration period obtained by the structure of the ink jet head is Tc, the time Ts from the starting point of the driving waveform to the starting point of the next subsequent driving waveform satisfies the relation of 1.1 Tc-Ts-1.4 Tc.

2. The inkjet recording apparatus according to claim 1, wherein,

the drive waveform element N is 5 or more.

3. The inkjet recording apparatus according to claim 1, wherein,

in the case of 1 droplet for 1 pixel, the drive waveform is outputted in the 1 st cycle from the end, and in the case of 2 droplets for 1 pixel, the drive waveform is outputted in the 1 st cycle and the 3 rd cycle from the end,

when the number of ejected droplets is M for 1 pixel, a drive waveform is output in the 1 st to M th periods from the last, where M is not less than 3.

4. The inkjet recording apparatus according to claim 1 or 2, wherein,

the N drive waveform elements of the drive signal include an expansion pulse for expanding a volume of a pressure chamber acting on the ink jet head and a contraction pulse for contracting the volume of the pressure chamber to eject ink from the nozzle,

the period from the start point of the expansion pulse to the start point of the contraction pulse of the last drive waveform element is 1.1 to 1.4 AL.

5. The inkjet recording apparatus according to claim 1 or 2, wherein,

the N drive waveform elements of the drive signal include an expansion pulse for expanding a volume of a pressure chamber acting on the ink jet head and a contraction pulse for contracting the volume of the pressure chamber to eject ink from the nozzle,

the last of the last drive waveform elements includes a drive pulse that is not ejected.

6. A method of driving an ink jet head, in which ink is ejected from a plurality of ink jet heads by applying pressure changes to the ink by a plurality of actuators using predetermined driving signals, and 1 droplet or a plurality of droplets are ejected to 1 pixel and combined with each other for each of the plurality of actuators,

the drive signal includes a drive waveform composed of N drive waveform elements, where N is an integer of 2 or more,

when the natural vibration period obtained by the structure of the ink jet head is Tc, the time Ts from the starting point of the driving waveform to the starting point of the next subsequent driving waveform satisfies the relation of 1.1 Tc-Ts-1.4 Tc.

Technical Field

The invention relates to an ink jet recording apparatus and an ink jet head driving method.

Background

An ink jet recording apparatus that ejects ink from a nozzle and lands on a medium to record an image or the like has been developed and commercialized.

In an inkjet recording apparatus, shading is generally expressed according to the coverage area of ink per unit area. As a method of controlling the ink coverage area, a method of changing the liquid amount per one ink droplet is known.

When the liquid amount of each ink droplet is appropriately changed, for example, the following is performed: the timing, speed, etc. of the ejection of a plurality of droplets ejected by a plurality of successive droplet ejection operations are adjusted so as to combine the droplets before landing on the medium, thereby obtaining individual droplets having a liquid amount corresponding to the number of original droplets. The amount of the liquid is adjusted according to the number of the original liquid drops, thereby expressing the gradation. However, if the droplet discharge operation is continuously performed, unnecessary fine droplets (satellites) may be easily generated due to the influence of the previous droplet discharge operation, and the quality of recording may be deteriorated due to the landing of the fine droplets on the medium.

Patent document 1 describes a technique of suppressing fluctuations in the ejection speed and the amount of droplets ejected from a nozzle by setting a variable when a multi-gradation waveform is expanded and contracted as a whole to a peak value in the ejection speed obtained from the waveform. By applying the technique described in patent document 1, even when there is a deviation in resonance frequency between the piezoelectric actuators that drive the inkjet head, the recording quality can be improved.

Patent document 1: japanese patent No. 4117162

The technique described in patent document 1 effectively functions with a small number of gradation levels. However, in a multi-gradation waveform having a large number of gradation levels (for example, a multi-gradation waveform having 5 gradation levels or more), the value of the velocity peak also changes due to the variation in the resonance frequency. Therefore, in the multi-gradation waveform, there is a problem that even if a set waveform generation frequency is used in a certain reference channel, a speed deviation due to a resonance deviation between channels cannot be suppressed.

Disclosure of Invention

The present invention aims to suppress a speed deviation caused by a deviation of a resonance frequency of a piezoelectric actuator that drives an inkjet head when the inkjet head is driven at a plurality of gradation levels, thereby improving the image quality of a recorded image.

An inkjet recording apparatus of the present invention includes: a plurality of ink jet heads having nozzles that eject ink; a plurality of actuators for applying pressure variations to the inks by a predetermined driving operation to the plurality of ink jet heads; and a driving unit configured to generate and apply, to each of the plurality of actuators, a driving signal for discharging 1 droplet or a plurality of droplets for 1 pixel and combining the plurality of droplets.

Here, the drive signal generated by the drive unit includes a drive waveform composed of N (N is an integer of 2 or more) drive waveform elements, and when Tc is a natural vibration period obtained from the structure of the inkjet head, a time Ts from the start point of the drive waveform to the start point of the next subsequent drive waveform satisfies a relationship of 1.1Tc ≦ Ts ≦ 1.4 Tc.

In the inkjet head driving method according to the present invention, the plurality of actuators are driven to apply pressure changes to the ink by predetermined driving signals, thereby ejecting the ink from the plurality of inkjet heads, and the plurality of actuators are driven to eject 1 droplet or a plurality of droplets for 1 pixel and combine the plurality of droplets.

Here, the drive signal includes a drive waveform composed of N (N is an integer of 2 or more) drive waveform elements, and when Tc is a natural vibration period obtained from the structure of the inkjet head, a time Ts from a start point of the drive waveform to a start point of a next subsequent drive waveform satisfies a relationship of 1.1Tc ≦ Ts ≦ 1.4 Tc.

Drawings

Fig. 1 is a perspective view showing a schematic configuration of a main part of an inkjet recording apparatus according to an embodiment of the present invention.

Fig. 2 is a sectional view showing an example of an ink jet head according to an embodiment of the present invention.

Fig. 3 is a block diagram showing a configuration example of an inkjet recording apparatus according to an embodiment of the present invention.

Fig. 4 is a cross-sectional view showing an example of the state of ink droplets according to an embodiment of the present invention.

Fig. 5 is a cross-sectional view showing an example of the state of ink droplets according to an embodiment of the present invention.

Fig. 6 is a waveform diagram showing a drive waveform example (a) and a comparative example (B) according to an embodiment of the present invention.

Fig. 7 is a characteristic diagram showing an example of the relationship between the droplet velocity and the sub-droplet period according to the embodiment of the present invention.

Fig. 8 is a characteristic diagram showing a velocity distribution (a) according to an embodiment of the present invention and a comparative example (B) thereof.

Fig. 9 is a characteristic diagram illustrating an example of the attenuation characteristic of the pressure wave for explaining an embodiment of the present invention.

Fig. 10 is a waveform diagram showing an example (modification) of a drive waveform according to an embodiment of the present invention.

Detailed Description

Hereinafter, an embodiment of the present invention will be described.

[1. Structure of recording apparatus ]

Fig. 1 is a perspective view schematically showing a schematic configuration of an inkjet recording apparatus 1 according to an embodiment.

The inkjet recording apparatus 1 performs a recording process of recording an image or the like with ink on a recording medium P. The recording medium P is conveyed by the drive roller 11. In fig. 1, only one drive roller 11 is shown for the sake of simplicity of explanation, but a plurality of rollers are arranged in the actual inkjet recording apparatus 1.

The recording unit 20 includes a recording head 21, a carriage 22, and a carriage rail 23.

The recording head 21 ejects ink and lands it on the recording medium P. Here, 4 recording heads 21 that eject 4 colors of ink of CMYK (cyan, magenta, yellow, black) are provided, respectively. The 4 recording heads 21 are arranged in the width direction perpendicular to the conveying direction of the recording medium P, and are mounted on the carriage 22. The surface of the recording head 21 facing the recording medium P is an ink ejection surface on which openings (nozzle openings) of the nozzles 212 (fig. 2) are arranged, and the ink is ejected from the nozzle openings substantially perpendicularly to the recording medium P and lands on the recording medium P.

Fig. 2 is a sectional view showing a schematic structure of the inkjet head.

The recording head 21 includes nozzles 212 for ejecting ink from openings 212a at the tips, an ink flow path 213 including pressure chambers communicating with the nozzles 212, and actuators 211 for deforming the pressure chambers. The actuator 211 is constituted by a piezoelectric element that is deformed by a voltage.

The actuator 211 has the same polarity as the reference voltage, and introduces and flows the ink into the inside in a direction (volume increase) in which the pressure chamber expands by applying a voltage change that changes to a lower voltage. After that, the applied voltage of the actuator 211 is returned to the reference voltage to recover from the deformed state, and the volume of the pressure chamber is reduced to squeeze out the ink, and the ink is ejected from the nozzle 212.

In the recording head 21 of the present embodiment, a plurality of nozzles 212, ink channels 213, and actuators 211 shown in fig. 2 are arranged, respectively, and ink is efficiently ejected onto the recording medium P using the plurality of nozzles 212.

Returning to the explanation of fig. 1, the carriage 22 holds the recording head 21 and moves in the width direction along the carriage rail 23.

In a range of the maximum recordable width or more of the recording medium P, 2 (a pair of) carriage rails 23 are provided in parallel in a direction intersecting the conveying direction, here, the width direction. The carriage rail 23 supports the carriage 22 and enables the carriage 22 to move in the width direction. The movement of the carriage 22 is realized by, for example, a linear motor or the like. The position of the carriage 22 on the carriage rail 23 (position in the scanning direction) is detected by a linear encoder (not shown), and the detection result is output to the control unit 40.

The control unit 40 controls the timing of the conveyance of the recording medium P by the conveyance unit 10, the movement (scanning) of the recording head 21 in the width direction, and the ink ejection operation, and controls the image recording operation on the recording medium P. That is, in the inkjet recording apparatus 1, a scanning operation of moving the recording head 21 in the width direction and a conveying operation of moving the recording medium P in the conveying direction are combined to form a two-dimensional image.

Fig. 3 is a block diagram showing a functional configuration of the inkjet recording apparatus 1 of the present embodiment.

The inkjet recording apparatus 1 includes a control unit 40, a conveyance drive unit 12, a recording head 21, a head drive unit 24, a scan drive unit 25, an operation display unit 71, a communication unit 72, and a bus 90.

The head driving unit 24 outputs a driving voltage signal for ejecting ink from each nozzle of the recording head 21 at an appropriate timing to the actuator 211 corresponding to the selected nozzle 212, thereby operating the actuator 211. The head drive unit 24 includes a drive waveform signal output unit 241, a digital-to-analog converter (DAC)242, a drive circuit 243, and an output selection unit 244.

The drive waveform signal output unit 241 outputs digital data of drive waveforms corresponding to ejection and non-ejection of ink (including interruption and termination of image recording) in synchronization with a clock signal input from an oscillation circuit (not shown). The digital-to-analog converter 242 converts the drive waveform of the digital data into an analog signal and outputs the analog signal as an input signal Vin to the drive circuit 243.

The driving circuit 243 amplifies the input signal Vin to a voltage value corresponding to the driving voltage of the actuator 211. The drive circuit 243 outputs an output signal Vout, which is current-amplified in accordance with the flowing current, to the actuator 211 (electrodes at both ends) via the output selection unit 244.

The output selection unit 244 outputs a switching signal of the actuator 211 to be output as the output signal Vout, based on the pixel data of the image to be formed input from the control unit 40.

In the recording head 21, the actuator 211 is deformed by a driving voltage signal from the driving circuit 243 of the head driving section 24. By the deformation of the actuator 211, ink is ejected from the plurality of nozzles 212, and ink droplets land on positions on the recording medium corresponding to the operations of the conveyance drive unit 12 and the scanning drive unit 25.

The conveyance drive unit 12 obtains the recording medium P before image recording from the medium supply unit, and arranges the recording medium P at an appropriate position so as to face the ink ejection surface of the recording head 21, and ejects the recording medium P on which an image is recorded from a position facing the ink ejection surface. The conveyance drive unit 12 rotates a motor that rotates the drive roller 11 at an appropriate speed and timing.

The scanning drive section 25 moves the carriage 22 to an appropriate position in the width direction. The scan driving unit 25 rotates a motor that rotates the endless belt at an appropriate timing and speed, for example.

The operation display unit 71 displays status information, menus, and the like related to image recording, and accepts input operations from a user. The operation display unit 71 includes, for example, a liquid crystal display panel and a touch panel provided to be superimposed on a liquid crystal screen, and outputs an operation detection signal corresponding to a position touched by a user and a type of operation to the control unit 40. The operation display unit 71 is further provided with an led (light emitting diode) lamp, a push switch, and the like, which are used for warning display, display of a main power source, and operation.

The communication unit 72 transmits and receives data to and from the outside in accordance with a predetermined communication standard.

As the communication standard, various well-known systems are used, such as TCP/IP connection for communication using LAN (local Area network) cable, short-range wireless communication (IEEE802.15 or the like) such as wireless LAN (IEEE802.11) and Bluetooth (registered trademark), usb (universal Serial bus) connection, and the like. The communication unit 72 includes a connection terminal conforming to a usable communication standard and hardware such as a network card for performing communication under the communication standard.

The control unit 40 controls the overall operation of the inkjet recording apparatus 1. The control Unit 40 includes a Central Processing Unit (CPU) 41, a ram (random Access memory)42, and a storage Unit 43. The CPU41 performs various arithmetic processes for controlling the inkjet recording apparatus 1 in a unified manner. The RAM42 provides the CPU41 with a memory space for work, and stores temporary data. The storage unit 43 stores control programs, setting data, and the like executed by the CPU41, and temporarily stores image data to be formed. The storage unit 43 includes a volatile memory such as a DRAM and a nonvolatile storage medium such as an hdd (hard Disk drive) or a flash memory, and is used for different purposes.

The bus 90 is a communication path for connecting these components to each other to transmit and receive data.

Note that, although the inkjet recording apparatus 1 is described by taking as an example a scanning type apparatus that performs scanning of the recording head 21, a line head may be used as the recording head 21, and a two-dimensional image may be recorded only by moving the recording medium P in the conveyance direction with respect to the fixed recording head 21.

[2. ink Ejection action ]

Next, an ink ejection operation in the inkjet recording apparatus 1 according to the present embodiment will be described. As described above, the ink jet recording apparatus 1 causes the actuator 211 to expand (increase the volume of) the ink channel 213 (pressure chamber) by the head driving unit 24, and then performs a driving operation of deformation to recover the expansion, thereby ejecting the ink. The deformation of the actuator 211 is performed by increasing the driving voltage applied to the piezoelectric element, that is, the actuator 211, to the original reference voltage after once decreasing from the reference voltage and maintaining the same.

In the inkjet recording apparatus 1, ejection of a multiple number (a predetermined number of times greater than or equal to 2) of the unit ejection amount corresponding to the normal 1-drop amount is possible. In the case of the present embodiment, a multi-gradation discharge operation of 6 times the maximum discharge unit discharge amount can be performed.

In the inkjet recording apparatus 1, a series of driving operations are performed in which a predetermined driving waveform voltage is continuously applied at a predetermined timing for each cycle time, thereby generating a plurality of ink blocks in which squeezed ink and ink in an ink flow path are continuously formed without being separated. Then, after they are separated from the ink in the ink flow path 213, these plural ink blocks are combined to become individual ink droplets having a total liquid amount (liquid amount corresponding to the number of times of operation of the driving operation), and land on the recording medium.

The cycle time here is set within a range in which the ink droplets ejected from the nozzle openings are generated and finally separated into ink droplets and the ink droplets are combined. The cycle time of the voltage waveform is determined so as to absorb the variation in the natural vibration cycle Tc of each channel of the ink jet head prepared with a plurality of (a plurality of channels), and the speed variation within the head surface and between the heads is within 7%. The term "within 7% means that the standard ink jet printer specification limits the variation to within a half pixel of 600 dpi.

The amplitude of each drive waveform voltage is adjusted so that the velocity of the ink droplets after the ink masses are united is uniform regardless of the amount of the ink droplets, that is, the number of times the drive waveform voltage is applied to the actuator 211, and the timing of application of the final drive waveform voltage is determined in accordance with the ejection timing of the ink, that is, the landing timing of the ink on the recording medium P. When the liquid volume of the ink droplets is set to a predetermined number of times 2 or more of the unit discharge volume, a predetermined drive waveform voltage is added before the drive waveform voltage signal of the last cycle, and the drive waveform voltage of the total number of cycles is applied to the actuator 211. The term "multiple" used herein may not be an exact value. That is, an error may be present in the density of an image based on the ejected ink to such an extent that the problem does not occur.

As described above, in this case, as the time during which the driving operation can be performed, the time during which the ink droplets of the 6-stage liquid volume can be ejected is set to 6 cycles (the time during which the driving operation can be performed a predetermined number of times or more) for each ejection operation of the ink droplets. This enables the ink ejection operation to be performed at a uniform cycle corresponding to the 6-cycle time. In the head driving unit 24, the presence or absence of the driving operation at each timing of 6-cycle time is switched in the output selection unit 244 for each pixel position based on the density gradation data input from the storage unit 43, and the ink of the corresponding liquid amount is ejected to the pixel position and landed.

Fig. 4 (a) to (F) show examples of the drive waveform voltage signal applied to the actuator 211 at 1 to 6 times the unit ejection amount.

Fig. 4 (a) shows a drive waveform voltage signal in the case where the drive waveform voltage is applied to the actuator 211 only 1 time, and a liquid amount 1 time the unit ejection amount is ejected and landed. The period Ta of the drive waveform voltage signal indicates the voltage from the start of falling to the start of rising.

Fig. 4B shows a driving waveform voltage signal in the case where the driving waveform voltage is applied to the actuator 211 2 times, and a liquid amount 2 times the unit ejection amount is ejected and landed (the number of operations is 2).

Here, as shown in fig. 4B, the head driving unit 24 performs a driving operation of outputting the first (first) driving waveform voltage signal before 2 cycle times 2Tc (before 2 times the cycle time Tc) with respect to the output timing of the last driving waveform voltage signal.

Fig. 4 (C), (D), (E), and (F) show drive waveform voltage signals in the case where the drive waveform voltage is applied to the actuator 211 3 times, 4 times, 5 times, and 6 times, and the amount of liquid ejected is 3 times, 4 times, 5 times, and 6 times the unit ejection amount and landed.

However, when the operation is performed a plurality of times, the period Tb is set to a period from the start of the decrease to the start of the increase of the voltage other than the last drive waveform voltage signal. The potential lowered in each period Tb is set to a value smaller than the potential lowered in the period Ta of the last drive waveform voltage signal. The reason why the potential of the last drive waveform voltage signal is larger than the potential of the drive waveform voltage signal in the other periods is to adjust the time from the start of the drop to the start of the rise so that only the timing of the last ink ejection coincides with the actual phase of the ink vibration.

Fig. 5 is a diagram schematically showing an ink surface in the vicinity of a nozzle opening at the time of ink ejection. However, for the sake of easy understanding in description, the relationship between the size of the ink droplets and the size of the ink droplets in these drawings does not accurately reflect the actual ratio.

As the first voltage of the drive waveform voltage decreases, the actuator 211 deforms and the ink channel 213 (pressure chamber) expands, and the ink surface (meniscus) inside the nozzle 212 is guided inward from the nozzle opening. As the voltage rises (returns to the original voltage) thereafter, the ink surface inside the nozzle 212 flies out from the nozzle opening 212a as shown in fig. 5 a. Specifically, the timing of the phase 0 and the timing of pi of the natural vibration period of the ink in the nozzle from the pressure vibration of the ink in the nozzle with reference to the start of the voltage drop are slightly delayed by about 0.05 to 0.20 times (a phase difference of about pi/10 to 2 pi/5) the natural vibration period of the ink in the ink flow path 213 and the nozzle 212, respectively, due to the viscosity of the ink and the friction corresponding to the shape of the nozzle.

The ink ejected from the opening 212a of the nozzle 212 becomes an ink mass connected as an ink column without being separated from the ink in the nozzle 212 at this time. The ink block separates from the ink in the nozzle 212 into ink droplets as shown in fig. 5 (B) after the lapse of 3 cycle times from the output start timing of the last drive waveform voltage signal.

When ink droplets of 2 times the unit ejection amount are ejected, as described with reference to fig. 4 (B), the second-time drive waveform voltage signal is input to the actuator 211 after 2 cycle times have elapsed since the first-time drive waveform voltage signal was output. Accordingly, as shown in fig. 5 (C), 2 ink columns are generated from the opening 212a of the nozzle 212, the ink columns being connected at intervals of ink blocks. By separating the 2 ink blocks from the ink in the nozzle 212, ink droplets of 2 times the unit ejection amount are ejected as shown in fig. 5 (D). The separated ink droplets are further integrated (i.e., united) by viscosity (surface tension) or the like, and fly and land on the recording medium P. The base of the ink column from which the ink droplets are separated is pulled back into the nozzle 212 by the viscosity of the ink (the force of introduction into the nozzle 212 due to the swirling vibration).

At this time, the ringing vibration is superimposed on the vibration accompanying the last (second) driving waveform voltage signal. The larger the amplitude of this swirling vibration is, the larger the velocity of the ink mass flying out of the nozzle opening 212a at the last (second time) becomes. The ease of generation of unnecessary micro droplets, that is, satellites depends on the ejection speed of the final ink block, that is, the length of the tail of the ink block from the ink in the nozzle 212 to the separation.

As shown in fig. 4 (B), when the drive waveform voltage signal output at a timing after 2 cycle times has elapsed is input to the actuator 211, since the ringing vibration is attenuated at intervals corresponding to 1 cycle time, the generation of satellite droplets is suppressed according to the attenuation of the ringing vibration.

In the case of ejecting ink droplets 3 times the unit ejection amount, as shown in (C) of fig. 4, the drive waveform voltage signal is continuously input to the actuator 211 3 times in 3 cycles. Accordingly, as shown in fig. 5 (E), an ink column in which 3 ink blocks are connected is generated from the opening 212a of the nozzle 212. As shown in fig. 5 (F), these are separated from the ink in the nozzle 212, and ink droplets of 3 times the unit discharge amount are discharged.

When ink droplets 3 times the unit ejection amount are ejected, the ratio of the liquid amount of the last ink patch (i.e., the unit ejection amount) to the total liquid amount of the previous ink patches is small. As a result, the last ink block is more effectively pulled toward the preceding ink block than in the case where ink droplets of 2 times the unit ejection amount are ejected as described above. On the other hand, since the vibration of the ink on the side of the nozzle 212 also increases, the force in the direction of introduction into the nozzle 212 also increases. Therefore, even if the speed of the last ink patch is slightly increased, only the ink droplets are easily separated without generating satellites.

When ink droplets having a liquid amount 4 times or more the unit ejection amount are ejected, the total liquid amount of the preceding ink blocks is further increased, and therefore the generation of satellite droplets is more effectively suppressed.

When ink droplets of 2 times or more the unit ejection amount are ejected, the period Ta from the start of the drop to the start of the rise of the voltage of the driving waveform voltage signal other than the last driving waveform voltage signal is half (Tc/2) of the natural vibration period Tc. In the last driving waveform voltage signal (the last droplet driving operation), the time Ta from the start of the voltage drop to the start of the voltage rise is 0.55 to 0.70Tc, which is longer than half of the natural vibration period Tc. The value of 0.55 to 0.70Tc is 1.1 to 1.4 times the time Ta as shown by AL (Acoustic Length: equal to half of the natural vibration period Tc) showing the propagation time relating to the vibration of the liquid surface. This corresponds to a value obtained by delaying the voltage start rise by a magnitude (delay time) corresponding to a phase delay of actual vibration (displacement) of the ink with respect to the application timing (driving operation) of the driving waveform voltage.

As described above, even if the period time to which each waveform element is applied varies depending on the natural vibration period Tc of each channel, the speed difference between the channels must be set to 7% or less in order to prevent deterioration of image quality. Here, in the present embodiment, the period time Ts is 1.1 times or more the natural vibration period Tc, and thus the speed difference of the maximum grayscale level 6dpd can be set to 7% even for the channels having the natural vibration period Tc deviated by about 5%.

Therefore, the desired cycle time Ts is 1.1 times or more the natural vibration cycle Tc.

On the other hand, when the cycle time Ts is continuously increased, the driving efficiency is decreased by the distance from resonance, and the droplets ejected by the waveform elements are not combined. Therefore, the desired cycle time Ts is 1.4 times or less the natural vibration cycle Tc.

Fig. 6 shows an example of the drive waveform voltage signal supplied to the actuator 211 of each channel by the head drive unit 24 which has been described so far.

Fig. 6 (a) shows a drive waveform voltage signal of the present embodiment, and fig. 6 (B) shows a comparative example (conventional example).

Here, the reference voltage Vref is set to 34V.

The drive waveform voltage signal in fig. 6 (a) is in the case of 6-cycle (6-droplet) drive.

Further, the period time Ts (sub-droplet period) is obtained by adding the above-described time Tb 'to the time Tb' from the rise start timing of the pulse to the fall start timing of the next pulse in each period. In the example of fig. 6 (a), Ta is 3.9 μ s, Tb is 3.6 μ s, and Tb' is 3.6 μ s, and only the time Ta of the last pulse is set to be long.

In fig. 6 a, the value V1 is the potential of the pulse decrease in the last cycle, the value V2 is the potential of the pulse decrease from the last 2 cycles onward, and the value V3 is the potential of the pulse decrease other than these (from the last 1 cycle onward, 3 cycles onward, 4 cycles onward, and 5 cycles onward).

Here, the potential V2 is set to 0.82 times the potential V1, and the potential V3 is set to 0.66 times the potential V1.

Fig. 6 (B) shows an example of a drive waveform voltage signal obtained by comparing the conventional drive waveform voltage signal with fig. 6 (a).

When the drive waveform voltage signal shown in fig. 6 (B) is not subjected to the processing as in the present embodiment, Ta is 3.9 μ s, Tb is 3.0 μ s, and Tb' is 3.0 μ s. The potential V2 of the pulse is 0.7 times the potential V1, and the potential V3 is 0.58 times the potential V1. In the example shown in fig. 6 (B), the sub-droplet period Ts is made to coincide with the natural vibration period Tc.

Fig. 7 is an example of the relationship of the sub-drop period Ts to the drop velocity measured for 3 channels (sample A, B, C). The vertical axis of fig. 7 represents the droplet velocity [ m/s ], and the horizontal axis represents the sub-droplet period Ts of the driving waveform voltage signal as an integral multiple of the natural vibration period Tc.

The characteristic of the sample a (indicated by "o" in the figure) is the natural vibration period Tc of 6.08 μ s, the characteristic of the sample B (indicated by "Δ" in the figure) is the natural vibration period Tc of 6.20 μ s, and the characteristic of the sample C (indicated by "x" in the figure) is the natural vibration period Tc of 6.38 μ s.

As can be seen from fig. 7, when the sub-droplet period Ts is in the range of 1.1 to 1.4 times the natural vibration period Tc, the droplet velocities of the 3 channels are substantially equal to each other. More preferably, the droplet velocity is equalized when the sub-droplet period Ts is in a range of 1.2 to 1.4 times the natural vibration period Tc.

Fig. 8 shows a characteristic example of the velocity distribution in a plurality of channels. Fig. 8 (a) shows a velocity distribution in the case where the sub-droplet Ts is 1.2 times the natural vibration period Tc of the inkjet head. Fig. 8 (B) shows a velocity distribution in which the sub-droplet period Ts is 1.0 times the natural vibration period Tc of the inkjet head. In fig. 8 (a) and (B), the vertical axis represents the droplet velocity, and the horizontal axis represents time. Here, in the case of driving 64-channel heads, the ejection timing differs depending on the arrangement position of the heads.

The circles, the Δ, and the × in fig. 8 indicate the cases where the discharge amount is 1 time, 3 times, and 6 times the unit discharge amount.

In the case of the voltage waveform of the present embodiment shown in fig. 8 (a), the droplet velocity is substantially constant in all 64 channels at an arbitrary liquid amount. On the other hand, in the case of the conventional voltage waveform shown in fig. 8 (B), the variation in droplet velocity is large, and particularly, a larger variation in droplet velocity occurs in the 6-fold liquid amount (× characteristic).

In this way, the droplet velocities of the ink jet heads passing through all the channels are substantially equal, so that all the ink jet heads are driven with the same characteristics, and the recording image quality is improved.

Here, the principle that the droplet velocities are substantially equal when the relationship between the natural vibration period Tc and the sub-droplet period Ts is 1.2Tc ≦ Ts ≦ 1.4Tc will be described with reference to fig. 9.

In general, in order to drive the inkjet head at a higher speed and with a lower voltage with high efficiency, it is preferable that the waveform element forming the drive signal has a waveform that utilizes the natural vibration period (resonance) of the system determined by the structure of the inkjet head including the ink.

In order to utilize resonance, the waveform elements are provided with an expansion pulse, a hold pulse, and a contraction pulse in this order, and the hold pulse time is adjusted so that the contraction pulse is applied at a timing just after 1/2 of the natural vibration period Tc has elapsed from the application of the expansion pulse.

When an expansion pulse and a contraction pulse are applied to the ink inside the head pressure chamber, pressure waves having opposite phases are generated as shown in fig. 9.

The voltage waveform Va shown in fig. 9 is the applied voltage to the actuator, and the characteristic P1 shows the pressure wave velocity caused by the expansion pulse, and the characteristic P2 shows the pressure wave velocity caused by the contraction pulse.

These 2 pressure waves shown in fig. 9 are damped vibrations that vibrate at the natural cycle of the system and are damped due to resistance caused by the ink flow path structure, or the like.

Therefore, by setting the time from the start of applying the expansion pulse to the start of applying the contraction pulse to 1/2 times Tc, the phases of the respective pressure waves are aligned, and the driving efficiency is maximized.

Since the vibration cycle and the attenuation factor of the pressure wave depend on the flow path structure, if a subtle change in the flow path structure occurs due to a structural component or an assembly variation of the inkjet head, both the natural vibration cycle and the attenuation factor change.

The natural vibration period and the damping rate change depending on the flow channel structure can also be understood from the following expression.

When the resistance of the system is R, the inertia is L, and the compliance is C, the resonance period Tc of the system is Tc 2 pi √ (L √ C), and the Q value, which is an index showing the generality of the attenuation (the smaller the Q value, the larger the attenuation), is represented by Q (2 pi/R) √ (L/C). That is, the larger the compliance C becomes, the longer the resonance period Tc becomes, and the smaller the Q value becomes. The resistance R has no influence on the resonance period Tc, but has a larger influence on the Q value than the inertia L and the compliance C.

In the case of driving with only one waveform element, the influence of the deviation of the natural vibration period from the attenuation rate on the droplet velocity is of a negligible level. On the other hand, in the case where a plurality of waveform elements are applied by multi-gradation driving, since the deviation of the vibration period overlaps with the attenuation of the pressure wave, the speed deviation affects the print image quality as the pixel deviation.

Here, by setting the relationship between the natural vibration period Tc and the sub-droplet period Ts to 1.2Tc ≦ Ts ≦ 1.4Tc, the droplet velocities are substantially equalized as described with reference to fig. 7 and 8.

As described above, according to the inkjet head recording apparatus to which the inkjet head driving method of the present embodiment is applied, it is possible to absorb variations in the natural vibration periods of the plurality of inkjet heads, and all the inkjet heads are driven with the same characteristics, thereby improving the recording image quality.

[3. modification ]

In the drive voltage waveform shown in fig. 6, the pulse width Tb of each waveform element is 1/2 of the sub-droplet period Ts (Tb + Tb' in the example of fig. 6). In contrast, the pulse width Tb of each waveform element is not necessarily limited to 1/2 of the sub-droplet period Ts, and may be arbitrarily determined within a range in which the combined velocities of the droplets after combination match.

For example, the drive voltage waveform shown in fig. 10 may be used. The drive voltage waveform shown in fig. 10 is assumed to have a sub-droplet period Ts of 1.1Tc, Tb of 0.7Tc, and Tb' of 0.4 Tc.

The configuration of the ink jet recording apparatus 1 shown in fig. 1 and 2 is shown as an example, and the driving method of the present embodiment may be applied to an ink jet recording apparatus having another configuration.

Reference characters of the drawings

An inkjet recording apparatus; a handling section; driving a roller; a transport drive section; a recording portion; a recording head; an actuator; a nozzle; an opening; an ink flow path; a carriage; a carriage rail; a head drive section; a drive waveform signal output section; an analog conversion section; 243.. drive circuitry; an output selection portion; 25.. a scan drive; a control portion; a CPU; a RAM; a storage portion; 71.. an operation display part; a communication portion; 90..