Inkjet head

Abstract

To suppress generation of small droplets.SOLUTION: A driving circuit for driving an actuator ejects ink droplets by driving the actuator using driving waveforms which include: an extended waveform for driving the actuator in a direction in which a volume of a pressure chamber extends, a contracted waveform for driving the actuator in a direction in which the volume of the pressure chamber contracts, a first weak contracted waveform between the extended waveform and the contracted waveform for driving the actuator in a direction in which the volume of the pressure chamber contracts more weakly than that by the contracted waveform; and a second weak contracted waveform for driving the actuator in a direction in which the volume of the pressure chamber contracts more weakly than that by the contracted waveform after the contracted waveform, where the number of ink droplets in minimum gradation is set to two or more in performing gradation expression using the number of ink droplets.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] An embodiment of the present invention relates to an inkjet head. [Background technology]

[0002] Inkjet heads sometimes produce small droplets called satellites or ink mist accompanying the main ink droplets ejected from the nozzles. These small droplets can lead to a decrease in print quality. For this reason, there is a demand for the development of inkjet heads that suppress the production of small droplets. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6242361 Summary of the Invention [Problem to be solved by the invention]

[0004] The problem to be solved by the embodiments of the present invention is to provide an inkjet head that suppresses the generation of small droplets. [Means for solving the problem]

[0005] In one embodiment, the inkjet head includes pressure chambers that store ink, a nozzle plate with nozzles that communicate with the pressure chambers, actuators that are provided corresponding to the pressure chambers and that displace the volume of the pressure chambers, and a drive circuit that drives the actuator. The drive circuit drives the actuator with drive waveforms that include an expansion waveform that drives the actuator in a direction that expands the volume of the pressure chamber, a contraction waveform that drives the actuator in a direction that contracts the volume of the pressure chamber, a first weak contraction waveform that drives the actuator in a direction that contracts the volume of the pressure chamber less than the contraction caused by the contraction waveform between the expansion waveform and the contraction waveform, and a second weak contraction waveform that drives the actuator in a direction that contracts the volume of the pressure chamber less than the contraction caused by the contraction waveform after the contraction waveform, and when gradation is expressed by the number of ink droplets, the number of ink droplets for the lowest gradation is two or more. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a perspective view showing an inkjet head according to an embodiment. [Figure 2] FIG. 2 is a plan view showing the main body of the inkjet head. [Figure 3] FIG. 3 is a longitudinal cross-sectional view of the main body of the inkjet head taken along the line AA. [Figure 4] FIG. 4 is a longitudinal BB cross-sectional view of the main body of the inkjet head. [Figure 5] FIG. 5 is a diagram used to explain the operating principle of the inkjet head. [Figure 6] FIG. 6 is a block diagram showing the hardware configuration of the inkjet printer. [Figure 7] FIG. 7 is a diagram showing the main circuit configuration of the head drive circuit in the inkjet printer. [Figure 8] FIG. 8 is a block diagram showing the main circuit configuration of a waveform generating circuit included in the head driving circuit. [Figure 9] FIG. 8 is a diagram showing the correspondence between the state data and the drive pattern data related to the waveform generating circuit. [Figure 10] FIG. 10 is an explanatory diagram of the driving waveforms used in this embodiment. [Figure 11] FIG. 11 is a timing diagram showing the drive waveform, the pressure waveform of the pressure chamber, and the ink flow velocity waveform. [Figure 12] FIG. 12 is an explanatory diagram of a reference example of a drive waveform used when forming one dot with one to three drops. [Figure 13] FIG. 13 is an explanatory diagram of a first embodiment of a driving waveform used when forming one dot with one to three drops. [Figure 14] FIG. 14 is a photograph showing the ink flying state according to the reference example and the first example. [Figure 15] FIG. 15 is an explanatory diagram of a second embodiment of a driving waveform used when one dot is formed with one to three drops. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, the embodiments will be described with reference to the drawings. In this embodiment, a piezo inkjet head is exemplified as an on-demand inkjet head.

[0008] 1 is a perspective view showing a piezoelectric inkjet head 100. The inkjet head 100 is of a shared-wall type. Hereinafter, the inkjet head 100 will be referred to as head 100.

[0009] The head 100 is composed of a head main body 3 equipped with multiple nozzles 2 for ejecting ink, a head driver 4 that generates drive signals, and a manifold 7 equipped with an ink supply port 5 and an ink discharge port 6. The head driver 4 is equipped with two driver ICs 41 and 42. Each driver IC 41 and 42 has the same circuit configuration. Each driver IC 41 and 42 includes a head drive circuit 101, which will be described later.

[0010] The head 100 ejects ink supplied from an ink supply port 5, which is an ink supply means, from the nozzles 2 in response to a drive signal generated by a head driver 4. The head 100 also ejects, from an ink outlet 6, any ink that has flowed in from the ink supply port 5 but has not been ejected from the nozzles 2.

[0011] Fig. 2 is a plan view of the head body 3. Fig. 3 is a vertical cross-sectional view of the head body 3 shown in Fig. 2 taken along line AA, and Fig. 4 is a horizontal cross-sectional view of the head body 3 shown in Fig. 3 taken along line BB. As shown in FIG. 2, the head body 3 is composed of a piezoelectric member 14, a base substrate 15, a nozzle plate 16, and a frame member 17. The head body 3 is based on the base substrate 15. The frame member 17 is then bonded onto the base substrate 15, and the piezoelectric member 14 is bonded inside the frame member 17. The head body 3 has the nozzle plate 16 bonded onto the frame member 17. As shown in FIG. 3, the central space of the head body 3 surrounded by the base substrate 15, the piezoelectric member 14, and the nozzle plate 16 forms an ink supply channel 18. The peripheral space of the head body 3 surrounded by the base substrate 15, the piezoelectric member 14, the frame member 17, and the nozzle plate 16 forms an ink discharge channel 19. The nozzle plate 16 has a plurality of nozzles 2 formed in a predetermined pattern.

[0012] 3, the base substrate 15 has a hole 22 that communicates with the ink supply path 18 and a hole 23 that communicates with the ink discharge path 19. The hole 22 communicates with the ink supply port 5 via the manifold 7. The hole 23 communicates with the ink discharge port 6 via the manifold 7.

[0013] 4, the piezoelectric member 14 is formed by laminating a first piezoelectric member 141 with a second piezoelectric member 142 having a polarity opposite to that of the first piezoelectric member 141. The first piezoelectric member 141 and the second piezoelectric member 142 are bonded together.

[0014] As shown in FIG. 3, the piezoelectric member 14 has a plurality of parallel long grooves 26 formed therein, which connect the ink supply path 18 to the ink discharge path 19. As shown in FIG. 4, an electrode 21 is disposed on the inner surface of each long groove 26. As shown in FIG. 2, each electrode 21 is connected to the head driver 4 via a wiring 20. The space surrounded by each long groove 26 and the back surface of the nozzle plate 16, which is bonded onto the second piezoelectric member 142 so as to cover each long groove 26, forms each pressure chamber 24. The nozzles 2 are connected to each pressure chamber 24 in a one-to-one correspondence.

[0015] As shown in FIG. 4, the piezoelectric member 14, which forms a partition between adjacent pressure chambers 24, is sandwiched between the electrodes 21 of each pressure chamber 24. In the head body 3, the piezoelectric member 14 and the electrodes 21 on both sides thereof constitute an actuator 25. When an electric field is applied to the actuator 25 by a drive signal generated by the head drive circuit 101, the actuator 25 undergoes shear deformation into a dogleg shape, with the junction between the first piezoelectric member 141 and the second piezoelectric member 142 at its apex. This deformation of the actuator 25 displaces the volume of the pressure chamber 24, pressurizing the ink inside the pressure chamber 24. The pressurized ink is ejected from the nozzle 2 that communicates with that pressure chamber 24. In other words, the head drive circuit 101 functions as a drive circuit that drives the actuator 25.

[0016] A set of one pressure chamber 24, an electrode 21 disposed in that pressure chamber 24, and a nozzle 2 communicating with that pressure chamber 24 is referred to as a channel. In other words, the head 100 has channels equal to the number of pressure chambers 24. Hereinafter, the channels equal to the number of pressure chambers 24 will be referred to as a channel group 102 (see FIG. 6).

[0017] Next, the operating principle of the head 100 configured as above will be explained with reference to FIG. 5(a) shows a state in which the potentials of the electrodes 21 disposed on the wall surfaces of the central pressure chamber 242 and the pressure chambers 241 and 243 adjacent to this pressure chamber 242 are all at ground potential GND. In this state, neither the actuator 251 sandwiched between the pressure chambers 241 and 242 nor the actuator 252 sandwiched between the pressure chambers 242 and 243 is subjected to any strain.

[0018] 5(b) shows a state in which a negative voltage -V is applied to the electrode 21 of the central pressure chamber 242, and a positive voltage +V is applied to the electrodes 21 of the adjacent pressure chambers 241, 243. In this state, an electric field twice the voltage V acts on each of the actuators 251, 252 in a direction perpendicular to the polarization direction of the piezoelectric members 141, 142. This action causes each of the actuators 251, 252 to deform outward so as to expand the volume of the pressure chamber 242.

[0019] 5(c) shows a state in which a positive voltage +V is applied to the electrode 21 of the central pressure chamber 242, and a negative voltage -V is applied to the electrodes 21 of the adjacent pressure chambers 241, 243. In this state, an electric field that is twice the voltage V acts on each of the actuators 251, 252 in the opposite direction to that in FIG. 5(b). Due to this action, each of the actuators 251, 252 deforms inward so as to contract the volume of the pressure chamber 242.

[0020] When the volume of the pressure chamber 242 is expanded or contracted, a pressure vibration occurs within the pressure chamber 242. This pressure vibration increases the pressure within the pressure chamber 242, causing an ink droplet to be ejected from the nozzle 2 communicating with the pressure chamber 242.

[0021] In this way, the actuator 251 separating the pressure chamber 241 from the pressure chamber 242 and the actuator 252 separating the pressure chamber 242 from the pressure chamber 243 apply pressure vibrations to the inside of the pressure chamber 242, which has the actuators 251, 252 as its wall surfaces. In other words, the pressure chamber 242 shares the actuator 25 with the adjacent pressure chamber 241 and pressure chamber 243, respectively. For this reason, the head drive circuit 101 cannot drive each pressure chamber 24 individually. The head drive circuit 101 drives each pressure chamber 24 by dividing it into (n+1) groups, where n is an integer of 2 or more, and drives the pressure chambers 24. In this embodiment, the head drive circuit 101 divides each pressure chamber 24 into three groups, where every two pressure chambers 24 are divided and driven, which is called three-division driving. Note that three-division driving is merely an example, and four-division driving or five-division driving may also be used.

[0022] Next, a description will be given of an inkjet printer 200 that uses the head 100. Hereinafter, the inkjet printer 200 will be referred to as the printer 200. 6 is a block diagram showing the hardware configuration of printer 200. Printer 200 includes processor 201, ROM (Read Only Memory) 202, RAM (Random Access Memory) 203, operation panel 204, communication interface 205, conveyance motor 206, motor drive circuit 207, pump 208, pump drive circuit 209, and head 100. Printer 200 also includes bus lines 210 such as an address bus and a data bus. In printer 200, processor 201, ROM 202, RAM 203, operation panel 204, communication interface 205, motor drive circuit 207, pump drive circuit 209, and drive circuit 101 of head 100 are connected to this bus line 210 directly or via input / output circuits.

[0023] The processor 201 corresponds to the central part of the computer. The processor 201 controls each part to realize various functions of the printer 200 in accordance with an operating system and application programs. The processor 201 is, for example, a CPU (Central Processing Unit).

[0024] The ROM 202 corresponds to the read-only main memory of the computer. The ROM 202 stores the operating system and application programs. The ROM 202 may also store data necessary for the processor 201 to execute processes for controlling each unit.

[0025] The RAM 203 corresponds to the rewritable main memory of the computer. The RAM 203 stores data necessary for the processor 201 to execute processing. The RAM 203 is also used as a work area where information is rewritten as needed by the processor 201. The work area includes an image memory where print data is expanded.

[0026] The operation panel 204 has an operation section and a display section. The operation section has function keys such as a power key, a paper feed key, and an error reset key. The display section can display various statuses of the printer 200.

[0027] The communication interface 205 receives print data from a client terminal connected via a network such as a LAN (Local Area Network). For example, when an error occurs in the printer 200, the communication interface 205 transmits a signal notifying the client terminal of the error.

[0028] A motor drive circuit 207 controls the driving of the transport motor 206. The transport motor 206 functions as a drive source for a transport mechanism that transports recording media such as printing paper. When the transport motor 206 starts, the transport mechanism begins transporting the recording media. The transport mechanism transports the recording media to a printing position using the head 100. After printing, the transport mechanism ejects the recording media from an ejection port (not shown) to the outside of the printer 200.

[0029] A pump drive circuit 209 controls the driving of the pump 208. When the pump 208 is driven, ink in an ink tank (not shown) is supplied to the head 100.

[0030] A head driving circuit 101 drives a channel group 102 of a head 100 based on print data. 7 is a diagram showing the main circuit configuration of the head drive circuit 101. The head drive circuit 101 includes a charge / discharge circuit 300, a waveform generation circuit 400, and a power supply circuit 500. The charge / discharge circuit 300 electrically connects the waveform generation circuit 400 and the power supply circuit 500. Note that the waveform generation circuit 400 and the power supply circuit 500 may not be included in the head drive circuit 101, but may be located physically separate from the head 100 and electrically connected to the charge / discharge circuit 300.

[0031] The power supply circuit 500 connects a first voltage source 501 and a second voltage source 502 in series. More specifically, the power supply circuit 500 connects the negative electrode of the first voltage source 501 and the positive electrode of the second voltage source 502, and grounds the connection point to zero [V]. Both the first voltage source 501 and the second voltage source 502 output a DC voltage E / 2 [V], which is half the maximum voltage E [V] that is the charging target of the charge / discharge circuit 300. Therefore, the power supply line La connected to the positive electrode of the first voltage source 501 is a positive power supply line of +E / 2 [V]. The power supply line Lb connected to the negative electrode of the second voltage source 502 is a negative power supply line of -E / 2 [V]. The power supply line Lc connected to the connection point between the negative electrode of the first voltage source 501 and the positive electrode of the second voltage source 502 is a ground line of zero [V].

[0032] The charge / discharge circuit 300 is connected to the first voltage source 501 and the second voltage source 502 of the power supply circuit 500 via the power supply line La, the power supply line Lb, and the power supply line Lc. The charge / discharge circuit 300 is also connected to the +24V reference power supply VBG via the power supply line Ld.

[0033] The charge / discharge circuit 300 connects a large number of switch series circuits between the positive power supply line La and the negative power supply line Lb. More specifically, the charge / discharge circuit 300 connects a switch series circuit of switch element 611 and switch element 612, a switch series circuit of switch element 621 and switch element 622, ..., a switch series circuit of switch element 691 and switch element 692 between the positive power supply line La and the negative power supply line Lb.

[0034] Furthermore, the charge / discharge circuit 300 connects a switch element 613, a switch element 623, ..., a switch element 693 between the switch element interconnection point of each switch series circuit and the ground line Lc. Furthermore, the charge / discharge circuit 300 connects capacitive actuators 251, 252 (not shown), ..., 258 made of piezoelectric elements between the switch element interconnection points of adjacent switch series circuits.

[0035] In this way, the actuators 251, 252, ..., 258 are connected between the interconnection points of the switch elements of adjacent switch series circuits, and therefore the number of actuators 251, 252, ..., 258 is one less than the number of switch series circuits. Note that although the symbols "611", "612", ..., "613", "621", "622", ..., "623", and "691", "692", ..., "693" are used to indicate the switch elements of the switch series circuits, it goes without saying that the number of switch series circuits is not limited to nine, and the number of actuators indicated by the symbols "251", "252", ..., "258" is not limited to eight.

[0036] Of the switch elements in each switch series circuit, switch elements 611, 621, ..., 691 connected to the positive power supply line La are P-channel MOS transistors. Of the switch elements in each switch series circuit, switch elements 612, 622, ..., 692 connected to the negative power supply line Lb are N-channel MOS transistors. Therefore, in the charge / discharge circuit 300, a large number of series circuits, each consisting of the source-drain of a P-channel MOS transistor and the source-drain of an N-channel MOS transistor, are connected between the positive power supply line La and the negative power supply line Lb.

[0037] The switch elements 613, 623, ..., 693 are N-channel MOS transistors. Therefore, in the charge / discharge circuit 300, the source-drain of each N-channel MOS transistor is connected between the switch element interconnection point of each switch series circuit and the ground line Lc.

[0038] The back gates of the P-channel MOS transistors (switch elements 611, 621, ..., 691) are connected to a +24 V reference power supply line Ld. The back gates of the N-channel MOS transistors (switch elements 612, 622, ..., 692 and switch elements 613, 623, ..., 693) are connected to a -E / 2 V negative power supply line Lb. The gates of the P-channel MOS transistors (switch elements 611, 621, ..., 691) and the gates of the N-channel MOS transistors (switch elements 612, 622, ..., 692 and switch elements 613, 623, ..., 693) are all connected to the waveform generating circuit 400.

[0039] The waveform generating circuit 400 generates a control waveform for controlling the on / off switching of each of the switch elements 611, 621, ..., 691, 612, 622, ..., 692, and 613, 623, ..., 693. Each of the switch elements 611, 621, ..., 691, 612, 622, ..., 692, and 613, 623, ..., 693 is switched on / off by the control waveform output from the waveform generating circuit 400. This on / off switching charges and discharges each of the actuators 251, 252, ..., 258.

[0040] Here, switch elements 611, 612, and 613, and switch elements 621, 622, and 623, which are connected to each other with actuator 251 in between, form a current path for charging and discharging actuator 251. Although not shown, switch elements 621, 622, and 623, and switch elements 631, 632, and 633, which are connected to each other with actuator 252 in between, form a current path for charging and discharging actuator 252. The same applies to the other actuators 253 to 258. Therefore, hereinafter, the description of this embodiment will continue, focusing on actuator 251 and the six switch elements 611, 612, 613, 621, 622, and 623 that form the current path to this actuator 251.

[0041] 8 is a block diagram showing the main circuit configuration of the waveform generating circuit 400. The waveform generating circuit 400 includes a time setting register 401, a selector 402, a timer 403, a state counter 404, and a drive pattern memory 405.

[0042] The time setting register 401 includes a first setting register 4011, a second setting register 4012, a third setting register 4013, a fourth setting register 4014, a fifth setting register 4015, a sixth setting register 4016, and a seventh setting register 4017. A time Ta is set in the first setting register 4011. A time Tb is set in the second setting register 4012. A time Tc is set in the third setting register 4013. A time Td is set in the fourth setting register 4014. A time Te is set in the fifth setting register 4015. A time Tf is set in the sixth setting register 4016. A time Tg is set in the seventh setting register 4017.

[0043] The selector 402 selects, in order, from the times Ta, Tb, Tc, Td, Te, Tf, and Tg set in the first to seventh setting registers 4011 to 4017, respectively, in accordance with the state data ST output from the state counter 404. The selector 402 sets the selected time in the timer 403.

[0044] The timer 403 counts the time set by the selector 402. When the counting of the time has finished, the timer 403 outputs a state update signal SA to the state counter 404.

[0045] The state counter 404 is an octal counter, and in the initial state, resets the state data ST to "0". In this state, when a trigger signal for starting waveform output is input from the printer 200, the state counter 404 counts up the state data ST by "1". Thereafter, each time a state update signal SA is input from the timer 403, the state counter 404 counts up the state data ST by "1". Then, when the state data ST reaches the upper limit value ("7" because it is an octal counter), the state counter 404 resets the state data ST to "0" in response to the input of the subsequent state update signal SA. The state counter 404 outputs the state data ST to the selector 402 and the drive pattern memory 405.

[0046] In the following, the state data ST in its initial state will be referred to as state data STa. The state data ST after counting up by "1" will be referred to as state data STb. Thereafter, the state data counted up by "1" will be referred to as state data STc, STd, STe, and STf, and the state data ST after counting up to the upper limit value of "7" will be referred to as state data STh.

[0047] The drive pattern memory 405 stores drive pattern data in association with each of the state data STa to STh. The drive pattern data is data for controlling the on / off of six switch elements 611, 612, 613, 621, 622, and 623 that form a current path to the actuator 251. The drive pattern data is also data for controlling the on / off of six switch elements 621, 622, 623, 631, 632, and 633 that form a current path to the actuator 252.

[0048] Each time state data STa to STh is input from the state counter 404, the drive pattern memory 405 generates a drive waveform for controlling the on / off switching of each of the switch elements 611, 612, 613, 621, 622, 623, . . . in accordance with the drive pattern data corresponding to the state data STa to STh.

[0049] 9 is a diagram showing the correspondence relationship between the state data STa to STh and the drive pattern data. In the initial state of the state data STa, the drive pattern data is data that turns on the switch elements 623 and 613 and turns off the switch elements 621, 622, 611, and 612.

[0050] In this state, when a trigger signal to start waveform output is input to state counter 404 and the state data is updated from STa to STb (time ta), switch element 613 is turned off and switch element 612 is turned on by the drive waveform of drive pattern data corresponding to state data STb output from drive pattern memory 405. At this time, a closed circuit of first voltage source 501 → switch element 611 → actuator 251 → switch element 623 → first voltage source 501 is formed. As a result, actuator 251 is energized in the forward direction at a voltage E / 2 [V] and charged.

[0051] In this way, in the first half of charging, the positive polarity first voltage source 501 is used to charge the actuator 251 halfway at an intermediate voltage E / 2 [V] that is half the maximum voltage E [V] that is the charging target.

[0052] When the state data is updated from STa to STb, the selector 402 selects the first setting register 4011. As a result, the timer 403 measures the time Ta. After the time Ta is measured and the timer 403 times out, the state data is updated from STb to STc.

[0053] When the state data is updated from STb to STc (time tb), the switch element 623 is turned off and the switch element 622 is turned on by the drive waveform of the drive pattern data corresponding to the state data STc. At this time, a closed circuit of the first voltage source 501 → switch element 611 → actuator 251 → switch element 622 → second voltage source 502 → first voltage source 501 is formed. As a result, the actuator 251 is energized in the forward direction at the maximum voltage E [V] and is further charged.

[0054] In this way, in the latter half of charging, the actuator 251 is charged with the maximum voltage E [V] using the positive polarity first voltage source 501 and the negative polarity second voltage source 502. By charging the actuator 251 with the maximum voltage E [V], the actuator 251 is fully charged.

[0055] When the state data is updated from STb to STc, the selector 402 selects the second setting register 4012. As a result, the timer 403 counts the time Tb. After the time Tb has been counted, the timer 403 times out, and the state data is updated from STc to STd.

[0056] When the state data is updated from STc to STd (time tc), the switch element 622 is turned off and the switch element 623 is turned on by the drive waveform of the drive pattern data corresponding to the state data STd. At this time, a closed circuit of the actuator 251 → switch element 611 → first voltage source 501 → switch element 623 → actuator 251 is formed. As a result, the actuator 251 discharges.

[0057] In this way, in the first half of the discharge, the charge is returned from the actuator 251 to the positive polarity first voltage source 501, and while the first voltage source 501 is being charged, the actuator 251 is discharged.

[0058] When the state data is updated from STc to STd, the selector 402 selects the third setting register 4013. As a result, the timer 403 counts the time Tc. After the time Tc has been counted, the timer 403 times out, and the state data is updated from STd to STe.

[0059] When the state data is updated from STd to STe (time td), the switch element 611 is turned off and the switch element 613 is turned on by the drive waveform of the drive pattern data corresponding to the state data STe. At this time, a closed circuit of the actuator 251 → switch element 613 → switch element 623 → actuator 251 is formed. As a result, the actuator 251 continues to discharge.

[0060] In this way, in the latter half of the discharge, the actuator 251 is completely discharged by looping between the terminals of the actuator 251.

[0061] Through the above charging and discharging operations, the head 100 expands the volume of the pressure chamber, refills it with ink, and then returns the volume of the pressure chamber to its original state. This operation causes pressure vibrations in the pressure chamber, causing ink droplets to be ejected from the nozzle. The ejection occurs during the discharging operation.

[0062] When the state data is updated from STd to STe, the selector 402 selects the fourth setting register 4014. As a result, the timer 403 counts the time Td. After the time Td has been counted and the timer 403 times out, the state data is updated from STe to STf.

[0063] When the state data is updated from STe to STf (time te), the switch element 623 is turned off and the switch element 621 is turned on by the drive waveform of the drive pattern data corresponding to the state data STf. At this time, a closed circuit of the first voltage source 501 → switch element 621 → actuator 251 → switch element 613 → first voltage source 501 is formed. As a result, the actuator 251 is energized in the reverse direction at E / 2 [V] and charged.

[0064] In this way, in the first half of the reverse charging, the positive polarity first voltage source 501 is used to charge the actuator 251 in the reverse direction up to half the charge at an intermediate voltage E / 2 [V] that is half the maximum voltage E [V].

[0065] When the state data is updated from STe to STf, the selector 402 selects the fifth setting register 4015. As a result, the timer 403 counts the time Te. After the time Te has been counted, the timer 403 times out, and the state data is updated from STf to STg.

[0066] When the state data is updated from STf to STg (at time tf), the switch element 613 is turned off and the switch element 612 is turned on by the drive waveform of the drive pattern data corresponding to the state data STg. At this time, a closed circuit of the first voltage source 501 → switch element 621 → actuator 251 → switch element 612 → second voltage source 502 → first voltage source 501 is formed. As a result, the actuator 251 is energized in the reverse direction at E [V] and is further charged.

[0067] In this way, in the latter half of the reverse charging, the actuator 251 is charged in the reverse direction at the maximum voltage E [V] using the positive polarity first voltage source 501 and the negative polarity second voltage source 502. By charging the actuator 251 in the reverse direction at the maximum voltage E [V], the actuator 251 is completely charged in the reverse direction.

[0068] When the state data is updated to STf to STg, the selector 402 selects the sixth setting register 4016. As a result, the timer 403 counts the time Tf. After counting the time Tf, when the timer 403 times out, the state data is updated from STg to STh.

[0069] When the state data is updated from STg to STh (time tg), the switch element 612 is turned off and the switch element 613 is turned on by the drive waveform of the drive pattern data corresponding to the state data STh. At this time, a closed circuit of the actuator 251 → switch element 621 → first voltage source 501 → switch element 613 → actuator 251 is formed. As a result, the actuator 251 discharges.

[0070] In this way, in the first half of the discharge, the charge is returned from the actuator 251 to the positive polarity first voltage source 501, and while the first voltage source 501 is being charged, the actuator 251 is discharged.

[0071] When the state data is updated from STg to STh, the selector 402 selects the seventh setting register 4017. As a result, the timer 403 counts the time Tg. After the time Tg has been counted and the timer 403 times out, the state data returns from STh to STa.

[0072] When the state data returns from STh to STa (time point th), the switch element 621 is turned off and the switch element 623 is turned on by the drive waveform of the drive pattern data corresponding to the state data STa. At this time, a closed circuit of the actuator 251 → switch element 623 → switch element 613 → actuator 251 is formed. As a result, the actuator 251 continues to discharge.

[0073] In this way, in the latter half of the discharge, the actuator 251 is completely discharged by looping between the terminals of the actuator 251.

[0074] By the reverse charging and discharging operation described above, the head 100 contracts the volume of the pressure chamber and then returns it to its original state. This operation cancels out any residual vibration in the pressure chamber.

[0075] Thereafter, the waveform generating circuit 400 repeatedly executes the same operation each time a trigger signal for starting waveform output is input to the state counter 404. By such operation of the waveform generating circuit 400, the charging / discharging circuit 300 switches on and off the six switch elements 611, 612, 613, 621, 622, and 623 that form the current path to the actuator 251.

[0076] Here, the electrode 21, the applied voltage of which is controlled by turning on and off the three switch elements 621, 622, and 623, is the electrode of one channel (hereinafter referred to as ejection channel Ch.X) that ejects ink. The electrode 21, the applied voltage of which is controlled by turning on and off the remaining three switch elements 611, 612, and 613, is the electrode of the channel adjacent to ejection channel Ch.X (hereinafter referred to as adjacent channel Ch.X-1). The actuator 251 is sandwiched between the electrode 21 of ejection channel Ch.X and the electrode 21 of the adjacent channel Ch.X-1. Therefore, the actuator 251 is driven by repeated charging and discharging depending on the difference between the voltage applied to the electrode 21 of ejection channel Ch.X and the voltage applied to the electrode 21 of the adjacent channel Ch.X-1. By appropriately controlling the driving of this actuator 251, one ink droplet can be ejected from the nozzle 2 of ejection channel Ch.X. In this way, the waveform that controls the driving of the actuator 251 is called a driving waveform.

[0077] 10 is an explanatory diagram of the drive waveforms used in this embodiment. In this embodiment, a first drive waveform (I) and a second drive waveform (II) are used as the drive waveforms.

[0078] The first drive waveform (I) includes an expansion waveform in section D, a holding waveform in section R, and a contraction waveform in section P. The expansion waveform applies a first pulse Pa that changes from a steady state of 0 [V] to a negative maximum voltage -E [V] to the actuator 251. When the first pulse Pa is applied to the actuator 251, the actuator 251 drives the pressure chamber 24 of the ejection channel Ch.X in a direction that expands it.

[0079] The expansion waveform reaches a steady state of 0 [V] after the time Dt corresponding to section D has elapsed. When the voltage applied to the actuator 251 reaches the steady state of 0 [V], the actuator 251 drives the pressure chamber 24 in a direction to restore its original state.

[0080] Thus, in section D, the pressure chamber 24 of the ejection channel Ch.X first expands, maintains that expanded state, and then returns to its original state. This change in the volume of the pressure chamber 24 causes an ink droplet to be ejected from the nozzle 2 communicating with this pressure chamber 24. Incidentally, if the time Dt during which the pressure chamber 24 corresponding to section D maintains its expanded state is set to 1 / 2 the pressure oscillation period 2AL (Acoustic Length) of the pressure chamber 24, the ink ejection volume will be maximized. The time Dt can be obtained by adjusting the time Ta set in the first setting register 4011 and the time Tb set in the second setting register 4012. Here, the expansion waveform of section D is referred to as a compression pulse, an ejection pulse, etc.

[0081] When the expansion waveform reaches a steady state of 0 [V], the first drive waveform (I) becomes a hold waveform. The hold waveform holds the steady state of 0 [V] for a time Rt corresponding to the section R. When the steady state of 0 [V] is held for the time Rt, the first drive waveform (I) becomes a contraction waveform.

[0082] The contraction waveform applies a second pulse Pb that changes from a steady state of 0 [V] to a positive maximum voltage +E [V] to the actuator 251. When the second pulse Pb is applied to the actuator 251, the actuator 251 drives the pressure chamber 24 of the ejection channel Ch.X in a direction that contracts.

[0083] The contraction waveform reaches a steady state of 0 [V] after the time Pt ​​corresponding to the section P has elapsed. When the voltage applied to the actuator 251 reaches the steady state of 0 [V], the actuator 251 drives the pressure chamber 24 in a direction to restore its original state.

[0084] Thus, in section P, the pressure chamber 24 of the ejection channel ch.x first contracts, maintains the contracted state, and then returns to its original state. This change in volume of the pressure chamber 24 can cancel the residual vibration of the pressure chamber 24. Specifically, by adjusting the time Rt corresponding to the section of the hold waveform and the time Pt ​​corresponding to section P of the contraction waveform to appropriate values, the residual vibration of the pressure chamber 24 is canceled at the trailing edge of the contraction waveform. The time Rt can be adjusted by adjusting the time Td set in the fourth setting register 4014. The time Pt ​​can be adjusted by adjusting the times Te, Tf, and Tg set in the fifth setting register 4015, sixth setting register 4016, and seventh setting register 4017. Here, the contraction waveform of section P is referred to as a contraction pulse, a cancel pulse, etc.

[0085] In this way, the first drive waveform (I) can cancel the residual vibration of the pressure chamber 24 in the ejection channel ch.x, thereby achieving good ejection efficiency and also improving the landing performance of ink droplets.

[0086] However, when ink droplets are ejected from the nozzles 2 of the head 100, they usually leave a trailing ink droplet. When the ink droplet separates from the ink in the nozzles 2, this trailing ink droplet, known as a liquid column, becomes a spherical satellite and follows the main ink droplet (main droplet). Because these satellite droplets are tiny droplets, they fly slower than the main ink droplet. This can lead to degradation of print quality, such as uneven density and ghosting, by separating from the main ink droplet and landing on the recording medium. Furthermore, some satellite droplets lose speed and become what is known as ink mist, floating within the printer 200. If the ink mist adheres to the head 100 or its surrounding circuit components, it could cause malfunction of the printer 200. The first drive waveform (I) is not capable of suppressing the generation of small droplets, such as the satellites and ink mist, described above.

[0087] The second drive waveform (II) includes an expansion waveform in section D, a holding waveform in section R', a first weak contraction waveform in section H, a contraction waveform in section P', and a second weak contraction waveform in section W. The expansion waveform is the same as the expansion waveform of the first drive waveform (I). That is, the expansion waveform applies a first pulse Pa that changes from a steady state of 0 [V] to a negative maximum voltage -E [V] to the actuator 251, and after the time Dt corresponding to section D has elapsed, the steady state of 0 [V] is reached.

[0088] In the second drive waveform (II), in section D, the pressure chamber 24 of the ejection channel Ch.X first expands, maintains that expanded state, and then returns to its original state. This change in the volume of the pressure chamber 24 causes an ink droplet to be ejected from the nozzle 2 communicating with this pressure chamber 24. Incidentally, if the time Dt during which the pressure chamber 24 maintains its expanded state, which corresponds to section D, is set to 1 / 2 the pressure oscillation period 2AL of the pressure chamber 24, the ejection volume of ink will be at its maximum.

[0089] When the expansion waveform reaches a steady state of 0 [V], the second drive waveform (II) becomes a holding waveform. The holding waveform holds the steady state of 0 [V] for a time R't corresponding to the section R'. When the time R't corresponding to the section R' of the holding waveform has elapsed, the second drive waveform (II) becomes a first weak contraction waveform.

[0090] The first weak contraction waveform applies a third pulse Pc to the actuator 251, which changes from a steady state of 0 [V] to an intermediate voltage +E / 2 [V], which is half the maximum positive voltage. When the third pulse Pc is applied to the actuator 251, the actuator 251 drives the pressure chamber 24 of the ejection channel Ch.X in a direction to contract. However, the degree of contraction is smaller than the degree to which the pressure chamber 24 contracts due to the second pulse Pb of the first drive waveform (I). Hereinafter, the degree of contraction of the pressure chamber 24 due to the third pulse Pc will be referred to as weak contraction, and this weak contraction state will be referred to as a weak contraction state.

[0091] After the time Ht corresponding to the section H of the weak contraction waveform has elapsed, the second drive waveform (II) becomes a contraction waveform. The contraction waveform applies a fourth pulse Pd, which changes from an intermediate voltage +E / 2 [V] to a positive maximum voltage +E [V], to the actuator 251. By applying the fourth pulse Pd to the actuator 251, the actuator 251 drives the pressure chamber 24 of the ejection channel Ch.X in a direction that further contracts. The degree of contraction is equal to the degree to which the pressure chamber 24 contracts due to the second pulse Pb of the first drive waveform (I).

[0092] When the time P't corresponding to the section P' of the contraction waveform has elapsed, the second drive waveform (II) becomes a second weak contraction waveform. The second weak contraction waveform applies a fifth pulse Pe, which changes from a maximum voltage +E [V] to an intermediate voltage +E / 2 [V], to the actuator 251. When the fifth pulse Pe is applied to the actuator 251, the actuator 251 drives the pressure chamber 24 of the ejection channel Ch.X in a direction to restore the pressure chamber 24. However, the pressure chamber 24 does not completely restore. When the voltage applied to the actuator 251 becomes the intermediate voltage +E / 2 [V], the pressure chamber 24 enters a weak contraction state.

[0093] When the time Wt corresponding to the section W of the second weak contraction waveform has elapsed, the second drive waveform (II) becomes a steady state of 0 [V]. When the voltage applied to the actuator 251 becomes a steady state of 0 [V], the pressure chamber 24, which was in a weak contraction state, is completely restored.

[0094] The second drive waveform (II) configured as described above can suppress the generation of small droplets called satellites, ink mist, etc. Specifically, the time R't corresponding to the section R' of the hold waveform, the time Ht corresponding to the section H of the first weak contraction waveform, the time P't corresponding to the section P' of the strong contraction waveform, and the time Wt corresponding to the section W of the second weak contraction waveform are adjusted to appropriate values. By doing so, the generation of small droplets called satellites, ink mist, etc. can be suppressed. The time R't can be adjusted by adjusting the time Td set in the fourth setting register 4014. The time Ht can be adjusted by adjusting the time Te set in the fifth setting register 4015. The time P't can be adjusted by adjusting the time Tf set in the sixth setting register 4016. The time Wt can be adjusted by adjusting the time Tg set in the seventh setting register 4017.

[0095] Next, appropriate values ​​for the time R't, the time Ht, the time P't, and the time Wt will be explained. Time R't is the time from time tc when the first pulse Pa starts discharging the actuator 251, which has been charged to a negative maximum voltage -E [V], to time te when the third pulse Pc starts charging the actuator 25 at an intermediate voltage E / 2 [V]. Time Ht is the time from time te when the third pulse Pc starts charging the actuator 25 at an intermediate voltage E / 2 [V] to time tf when the fourth pulse Pd starts charging the actuator 25 at a positive maximum voltage +E [V]. Time P't is the time from time tf when the fourth pulse Pd starts charging the actuator 25 at a positive maximum voltage +E [V] to time tg when the fifth pulse Pe starts discharging the actuator 25. Time Wt is the time from time tg when the fifth pulse Pe starts discharging the actuator 25 to time th when discharging is completed.

[0096] By setting these time elements so that they satisfy the relationships of the following formulas (1) to (3), it is possible to suppress the generation of small droplets known as satellites, ink mist, and the like.

[0097] R´t+Ht=Rt+(0.4~0.6) …(1) Wt = Dt + (-0.5~0.5) … (2) P´t=4*Dt-(R´t+Ht)-Wt …(3) In equation (1), Rt is the time corresponding to the section R of the hold waveform in the first drive waveform (I).

[0098] That is, the sum of time R't and time Ht is time Rt plus 0.4 μs to 0.6 μs. Time Wt is time Dt, which corresponds to section D of the extended waveform, plus or minus -0.5 μs to 0.5 μs. Time P't is time obtained by subtracting the sum of time R't and time Ht plus time Wt from four times time Dt.

[0099] FIG. 11 is a timing diagram showing the pressure waveform of the pressure chamber 24 in the ejection channel Ch.X and the ink flow velocity waveform when a second drive waveform (II) is applied to the actuator 251, in which the sum of the time R't and the time Ht is set to time Rt+0.5 μs and the time Wt is set to time Dt+0.1 μs. In FIG. 11, the solid line "drive voltage" is the voltage waveform of the second drive waveform (II). The dashed-dotted line "pressure" is the pressure waveform generated in the pressure chamber 24. The dashed-dotted line "flow velocity" is the flow velocity waveform of the ink flowing into the nozzle 2. The horizontal axis represents the passage of time (μs). The vertical axis represents the drive voltage, pressure, flow velocity, and waveform magnitude, with the values ​​normalized.

[0100] 11, the pressure inside the pressure chamber 24, which had decreased as a result of the expansion of the pressure chamber 24 at the leading edge (first pulse Pa) of the expansion waveform in the second drive waveform (II) at time ta and time tb, increases while maintaining the expanded state. Then, when the pressure chamber 24 returns to its original state at the trailing edge of the expansion waveform at time tc and time td, the pressure increases rapidly. As a result, an ink droplet is ejected from the nozzle 2 communicating with the pressure chamber 24.

[0101] After the ink droplet is ejected, the pressure reaches a positive peak at time te of the leading edge (third pulse Pc) of the first weak contraction waveform in the second drive waveform (II). The pressure that reached the positive peak decreases and turns to negative pressure while the pressure chamber 24 maintains its weak contraction state, reaches a negative peak, and then increases again. The pressure then turns to positive pressure at time Tf of the leading edge (fourth pulse Pd) of the contraction waveform in the second drive waveform (II). The pressure that has turned to positive pressure reaches a second positive peak while the pressure chamber 24 maintains its contraction state, then decreases again and turns to negative pressure. The pressure that has reached the second negative peak then rises again and turns to positive pressure. The pressure that turned to positive pressure becomes negative pressure at time tg of the leading edge (fifth pulse Pe) of the second weak contraction waveform in the second drive waveform (II). The negative pressure rises while the pressure chamber 24 maintains the weak contraction state, and then turns positive again.

[0102] On the other hand, the flow velocity of the ink flowing into the nozzle 2 reaches a positive peak value after the ink droplet is ejected. Thereafter, the flow velocity decreases and reaches a negative peak value at time Tf of the leading edge (fourth pulse Pd) of the contraction waveform in the second drive waveform (II). After reaching the negative peak value, the flow velocity begins to increase, reaches a second positive peak value while the pressure chamber 24 maintains its contracted state, decreases again, and reaches a second negative peak value at time tg of the leading edge (fifth pulse Pe) of the second weak contraction waveform in the second drive waveform (II). After reaching the negative peak value, the flow velocity begins to increase again. Then, at time th when the flow velocity becomes zero, that is, at time th when the discharge of the actuator 25 is completed, the pressure chamber 24 completely recovers from the weak contraction state. At this time, the pressure inside the pressure chamber 24, which had turned positive, decreases and becomes approximately zero.

[0103] In this way, the second drive waveform (II) causes the pressure chamber 24 to be in a weak contraction state for a time period Ht after ejecting an ink droplet. Furthermore, in order to cancel residual vibration of the pressure chamber, the pressure chamber 24 is also kept in a weak contraction state for a time period Wt after being put into a contracted state. Due to this change in the state of the pressure chamber 24, the ink meniscus rises in the nozzle 2 communicating with the pressure chamber 24 to the extent that an ink droplet is not ejected. This meniscus rise shortens the tail, which is the main cause of satellite generation. As a result, the generation of small droplets that become satellites or ink mist is suppressed. Furthermore, the residual vibration of the pressure chamber 24 is also canceled by restoring the state of the pressure chamber 24 from the contracted state. Thus, by using the second drive waveform (II) as the drive waveform that controls the drive of the actuator 25, it is possible to suppress residual vibration while also suppressing the generation of small droplets. As a result, there is no risk of satellites landing on the recording medium, causing print quality degradation such as uneven density or ghosting, or ink mist adhering to the head 100 or surrounding circuit components, causing malfunction of the printer 200.

[0104] However, the second drive waveform (II) has a longer waveform length than the first drive waveform (I). Therefore, in the case of a multi-drop method in which gradation is expressed by forming one dot by the number of ink droplets (drops) ejected in succession, if the ejection of all ink droplets is handled by the second drive waveform (II), it will take time to form one dot, which raises concerns about the impact on the drive frequency.

[0105] Therefore, in the case of the multi-drop method, one dot is formed by combining ink droplets ejected by the first drive waveform (I) and ink droplets ejected by the second drive waveform (II). As an example, a combination of drive waveforms for a multi-drop method in which input image data has three gradations of "01H", "02H", and "03H" will be described using Figures 12 and 13.

[0106] 12 is a reference example, and is a data table in matrix format with the columns representing input image data and the number of drops corresponding to that data, and the rows representing frame numbers. In this reference example, the number of drops is "1 Drop" for input image data "01H," "2 Drops" for input image data "02H," and "3 Drops" for input image data "03H." The frame numbers consist of "1 Frame" indicating the first of the three drops, "2 Frame" indicating the second of the three drops, and "3 Frame" indicating the third of the three drops.

[0107] In the case of input image data "01H", one drop forms one dot. This one drop uses "frame 3", which is the third drop in a three-drop set. In this reference example, ink droplets in "frame 3" are ejected using the second drive waveform (II).

[0108] In the case of input image data "02H", one dot is formed with two drops. In this case, the first drop uses "frame 2", which is the second drop in a case of three drops, and the second drop uses "frame 3", which is the third drop in a case of three drops. In this reference example, the ink droplets in "frame 2" and the ink droplets in "frame 3" are each ejected using the second drive waveform (II).

[0109] In the case of input image data "03H", one dot is formed using three drops. In this case, the first drop uses "1 frame", the second drop uses "2 frames", and the third drop uses "3 frames". In this reference example, ink droplets in "1 frame" are ejected using the first drive waveform (I). Ink droplets in "2 frames" and "3 frames" are each ejected using the second drive waveform (II).

[0110] In the reference example, the occurrence of satellites is almost completely suppressed compared to when all drops are ejected using the first drive waveform when the input image data is "01H," "02H," and "03H." This prevents print quality degradation, such as uneven density or ghosting, and improves print quality. Furthermore, the occurrence of ink mist is also suppressed, eliminating concerns about malfunction of the printer 200.

[0111] However, when printing on a medium that includes irregularities, i.e., when there is a large gap in the distance from the inkjet head to the printing surface of the medium, there is a concern that satellites or ink mist may occur, particularly in the low gradation region of the input image data "01H." Therefore, next, we will explain a first embodiment that can suppress the occurrence of satellites or ink mist even when there is a large gap in the distance from the inkjet head to the printing surface of the medium.

[0112] FIG. 13 is a matrix-format data table applied in the first embodiment. Similar to FIG. 12, the columns of the data table represent input image data and the drop count corresponding to that data, and the rows represent frame numbers. The drop count is set to "2 Drops" for input image data "01H," "3 Drops" for input image data "02H," and "4 Drops" for input image data "03H." In other words, each time the gradation of the image data increases, one more ink droplet is ejected than in the reference example. Therefore, the frame numbers are "1 Frame" indicating the first drop of four drops, "2 Frame" indicating the second drop of four drops, "3 Frame" indicating the third drop of four drops, and "4 Frame" indicating the fourth drop of four drops.

[0113] Generally, the multi-drop method expresses gradation by forming one or more ink droplets per pixel. The drive device outputs an ejection pulse signal to the actuator in succession for the number of drops corresponding to the gradation value of the pixel. To pre-vibrate the actuator, the drive device outputs an auxiliary pulse signal immediately before the ejection pulse signal for the first drop. This type of auxiliary pulse signal is called a boost signal (BST). Alternatively, to absorb actuator vibration after ink ejection, the drive device outputs an auxiliary pulse signal immediately after the ejection pulse signal for the final drop. This type of auxiliary pulse signal is called a damp signal (DMP). In the first embodiment, the damp signal is replaced with an ejection pulse signal for the fourth frame. This does not affect the overall drive time.

[0114] In the first embodiment, when the input image data is "01H", one dot is formed using two drops. In this case, the first drop uses "frame 3", which would be the third drop in a four-drop sequence, and the second drop uses "frame 4", which would be the fourth drop in a four-drop sequence. In the first embodiment, the ink droplet in "frame 3" of the first drop and the ink droplet in "frame 4" of the second drop are each ejected using the second drive waveform (II).

[0115] For input image data "02H," one dot is formed using three drops. In this case, the first drop is used in "frame 2," which would be the second drop in a four-drop set. The second drop is used in "frame 3," which would be the third drop in a four-drop set. The third drop is used in "frame 4," which would be the fourth drop in a four-drop set. In the first embodiment, the ink droplet in "frame 2," the first drop, is ejected using the first drive waveform (I). The ink droplet in "frame 3," the second drop, and the ink droplet in "frame 4," the third drop, are each ejected using the second drive waveform (II). In this way, even if the first drop is ejected using the first drive waveform (I), the satellites generated by the ejection are extremely small compared to when all three drops are ejected using the first drive waveform (I). Furthermore, the ink mist may adhere to the ink droplets of the second or third drop and land on the recording medium. Therefore, there is no degradation in print quality. Moreover, the time required to form one dot does not affect the drive frequency.

[0116] For input image data "03H," one dot is formed using four drops. In this case, the first drop uses "frame 1," the second drop uses "frame 2," the third drop uses "frame 3," and the fourth drop uses "frame 4." In the first embodiment, the ink droplet for the first drop in "frame 1" and the ink droplet for the second drop in "frame 2" are each ejected using the first drive waveform (I). The ink droplet for the third drop in "frame 3" and the ink droplet for the fourth drop in "frame 4" are each ejected using the second drive waveform (II). In this way, even if the first and second drops are ejected using the first drive waveform (I), the satellites generated by their ejection are significantly smaller than when all four drops are ejected using the first drive waveform (I). In addition, the ink mist may adhere to the ink droplet for the third or fourth drop and land on the recording medium. Therefore, there is no degradation in print quality. Furthermore, by using a dump signal frame as the ejection pulse signal for the fourth frame, the time required to form one dot does not affect the drive frequency.

[0117] In this way, in the reference example, one drop is ejected for the lowest gradation input image data "01H", but in the first embodiment, two drops are ejected, and both of these drops are ejected using the second drive waveform (II).

[0118] For input image data "02H" that is one step higher than the lowest gradation, two drops are ejected in the reference example, but three drops are ejected in the first embodiment. Of the three drops, the first drop is ejected using the first drive waveform (I), and the remaining two drops are ejected using the second drive waveform (II).

[0119] For input image data "03H" that is one step higher than the lowest gradation, three drops are ejected in the reference example, but four drops are ejected in the first embodiment. Of the four drops, the first and second two drops are ejected using the first drive waveform (I), and the remaining two drops are ejected using the second drive waveform (II).

[0120] According to the first embodiment having such a configuration, even when printing on a medium that includes irregularities, that is, when there is a large gap in the distance from the inkjet head to the printing surface of the medium, it is possible to suppress the generation of satellites or ink mist.

[0121] FIG. 14 is a photograph showing the ink flight state. A typical UV ink is used. In the figure, photograph PHa shows the ink flight state when the second drive waveform (II) is applied and one drop is ejected using a single-drop method. Photograph PHb shows the ink flight state when the second drive waveform (II) is applied and two-drop multi-drop method is ejected. Photograph PHc shows the ink flight state when the first drive waveform (I) is applied and the first drop is ejected, followed by the second and third drops using a three-drop multi-drop method. In other words, photographs PHa to PHc are for the case where the reference example is applied.

[0122] On the other hand, photo PHd shows the ink flight state when ejecting using a two-drop multi-drop method with the second drive waveform (II). Photo PHe shows the ink flight state when ejecting using a three-drop multi-drop method, in which the first drop is ejected using the first drive waveform (I), followed by the second and third drops with the second drive waveform (II). Photo PHf shows the ink flight state when ejecting using a four-drop multi-drop method, in which the first drop is ejected using the first drive waveform (I), followed by the third and fourth drops with the second drive waveform (II). In other words, photos PHd to PHf are for the case where the first embodiment is applied.

[0123] In particular, comparing photos PHa and PHd, in the reference example, satellites are present away from the main ink droplets, which may result in poor print quality, such as uneven density or ghosting. In contrast, in the first embodiment, the occurrence of satellites is almost completely suppressed. Therefore, poor print quality, such as uneven density or ghosting, is not encountered, and print quality can be improved. Furthermore, the occurrence of ink mist is suppressed, eliminating the risk of printer 200 malfunctioning. Next, a second embodiment will be described, which can suppress the occurrence of satellites or ink mist even during wide gap printing.

[0124] Fig. 15 is a matrix-format data table applied in the second embodiment. As with Figs. 12 and 13, the columns of the data table represent input image data and the number of drops corresponding to that data, and the rows represent frame numbers. The number of drops is set to "2 Drops" for input image data "01H," "2 Drops" for input image data "02H," and "3 Drops" for input image data "03H." Therefore, the frame number consists of "1 Frame" indicating the first drop of the three drops, "2 Frame" indicating the second drop of the three drops, and "3 Frame" indicating the third drop of the three drops.

[0125] In the second embodiment, when the input image data is "01H", one dot is formed using two drops. In this case, the first drop uses "frame 2", which is the second drop in a three-drop formation, and the second drop uses "frame 3", which is the third drop in a three-drop formation. In the second embodiment, the ink droplet in "frame 2" of the first drop is ejected using the second drive waveform (II). The ink droplet in "frame 3" of the second drop is also ejected using the second drive waveform (II).

[0126] In the case of input image data "02H", one dot is also formed with two drops. That is, just like in the case of input image data "01H", the first drop uses "2 frames" and the second drop uses "3 frames". Then, the ink droplets in "2 frames" of the first drop are ejected with the second drive waveform (II). The ink droplets in "3 frames" of the second drop are also ejected with the second drive waveform (II).

[0127] In the case of input image data "03H", one dot is formed using three drops. In this case, the first drop uses "1 frame", the second drop uses "2 frames", and the third drop uses "3 frames". In the second embodiment, ink droplets in "1 frame" are ejected using the first drive waveform (I). Ink droplets in "2 frames" and "3 frames" are each ejected using the second drive waveform (II).

[0128] In this way, the second embodiment differs from the reference embodiment only in the case of input image data "01H." That is, in the reference embodiment, one dot is formed with one drop when the input image data is "01H," but in the second embodiment, one dot is formed with two drops.

[0129] In the case of input image data "01H", in the first embodiment, one dot is formed with two drops, which is more effective in reducing ink mist and satellites than the reference example. In the second embodiment, like the first embodiment, one dot is formed with two drops when the input image data is "01H", so it goes without saying that it is more effective in reducing ink mist and satellites than the reference example.

[0130] Furthermore, in the second embodiment, the "fourth frame" is unnecessary compared to the first embodiment, so the previous damp signal can be applied as the "fourth frame." However, because the number of drops is the same when the input image data is "01H" and when the input image data is "02H," it becomes necessary to maintain the gradation characteristics by changing the image data for printing. Specifically, gradation correction is performed on the input image data "01H" with the lowest gradation. Gradation correction can be performed using a tone curve or a density pattern method for pseudo-halftoning. Gradation correction can also be performed using other technical means. It is also possible to maintain the gradation characteristics by performing gradation correction on the input image data "02H."

[0131] Although the inkjet head has been described above as an embodiment, the embodiment is not limited to this.

[0132] In the above embodiment, when gradation is expressed by the number of ink droplets, the number of ink droplets of the lowest gradation is set to 2. It goes without saying that by setting the number of ink droplets of the lowest gradation to 3 or more, it is possible to suppress the occurrence of satellites or ink mist even during wide gap printing.

[0133] In the above embodiment, for the second drive waveform (II), the time elements of the retention time R't, the first weak contraction time Ht, the contraction time P't, and the second weak contraction time Wt are set to have the relationships shown in the above formulas (1) to (3). In another embodiment, the above formula (1) may be set to have the relationship shown in the following formula (4).

[0134] Ht = Rt + (0.4~0.6) … (4) That is, in the second drive waveform (II), the time R't of the hold section indicated by section R' may be set to 0. Even with such a drive waveform, the amount of satellites accompanying ink droplets ejected from the nozzles can be reduced by adjusting the time elements of the first weak contraction time Ht, the contraction time P't, and the second weak contraction time Wt.

[0135] The first drive waveform (I) is not limited to that shown in Fig. 10. Even if another drive waveform is used as the first drive waveform (I), by applying the second drive waveform (II) to the ejection of at least two ink droplets, the final drop and the drop before it, it is possible to achieve the effect of suppressing the generation of small droplets such as satellites and ink mist even during wide gap printing.

[0136] The head 100 is not limited to the shared wall type, and this embodiment can also be applied to other types of piezo inkjet heads.

[0137] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope of the invention and the scope of the inventions and their equivalents as defined in the claims. The inventions described in the original claims of this application are set forth below. [1] An inkjet head comprising: a pressure chamber that stores ink; a nozzle plate having a nozzle communicating with the pressure chamber; an actuator that is provided corresponding to the pressure chamber and displaces the volume of the pressure chamber; and a drive circuit that drives the actuator, wherein the drive circuit drives the actuator with a drive waveform that includes: an expansion waveform that drives the actuator in a direction that expands the volume of the pressure chamber; a contraction waveform that drives the actuator in a direction that contracts the volume of the pressure chamber; a first weak contraction waveform that drives the actuator in a direction that contracts the volume of the pressure chamber more weakly than the contraction caused by the contraction waveform between the expansion waveform and the contraction waveform; and a second weak contraction waveform that drives the actuator in a direction that contracts the volume of the pressure chamber more weakly than the contraction caused by the contraction waveform after the contraction waveform, and wherein when gradation is expressed by the number of ink droplets, the number of ink droplets of the lowest gradation is 2 or more. [2] The inkjet head according to appendix [1], wherein the number of ink droplets for the lowest gradation is set to 2, and the number of ink droplets is increased each time the gradation of the image data increases. [3] An inkjet head according to appendix [1], wherein the number of ink droplets for the lowest gradation is set to 2, and when the number of ink droplets for the gradation one level higher than the lowest gradation is the same, gradation correction is performed on the image data for the lowest gradation. [4] The inkjet head according to appendix [3], wherein the gradation correction is gradation correction using a tone curve. [5] The inkjet head according to appendix [3], wherein the tone correction is performed by a density pattern method of pseudo-halftoning. [Explanation of symbols]

[0138] 2...nozzle, 3...head body, 4...head driver, 14,141,142...piezoelectric member, 16...nozzle plate, 21...electrode, 24,241,242,243...pressure chamber, 25,251,252,258...actuator, 100...inkjet head (head), 101...head drive circuit, 102...channel group, 200...inkjet printer (printer), 201...processor, 202...ROM, 203...RAM, 204...operation panel, 205...communication interface, 206...conveyor motor, 207...motor drive circuit, 208...pump, 209...pump drive circuit, 300...charge / discharge circuit, 400...waveform generation circuit, 401...time setting register, 402...selector, 403...timer, 404...state counter, 405...drive pattern memory, 500...power supply circuit.

Claims

1. a pressure chamber that contains ink; a nozzle plate having a nozzle communicating with the pressure chamber; an actuator provided corresponding to the pressure chamber, for displacing the volume of the pressure chamber; a drive circuit for driving the actuator; Equipped with the drive circuit drives the actuator with a first drive waveform including an expansion waveform that drives the actuator in a direction in which the volume of the pressure chamber expands and a contraction waveform that drives the actuator in a direction in which the volume of the pressure chamber contracts, and a second drive waveform that includes an expansion waveform that drives the actuator in a direction in which the volume of the pressure chamber expands and a contraction waveform that drives the actuator in a direction in which the volume of the pressure chamber contracts, a first weak contraction waveform that drives the actuator in a direction in which the volume of the pressure chamber contracts less than the contraction caused by the contraction waveform between the expansion waveform and the contraction waveform, and a second weak contraction waveform that drives the actuator in a direction in which the volume of the pressure chamber contracts less than the contraction caused by the contraction waveform after the contraction waveform, to eject ink droplets; When gradation is expressed by the number of ink droplets, the inkjet head ejects two ink droplets with the second drive waveform for the lowest gradation of the image data.

2. An inkjet head as described in claim 1, wherein when expressing gradation by the number of ink droplets, the number of ink droplets is increased by the first drive waveform each time the gradation of the image data increases.

Images