JP7640353B2 - Inkjet head - Google Patents

Description

An embodiment of the present invention relates to an inkjet head.

There is a type of inkjet head in which the partitions of adjacent pressure chambers act as actuators. In this type of inkjet head, when a drive pulse including an expansion pulse and a contraction pulse is applied to the actuator, the partitions deform in a direction that expands or contracts the pressure chamber, generating pressure vibrations within the pressure chamber. This pressure vibration changes the volume within the pressure chamber, causing ink droplets to be ejected from the nozzle that communicates with that pressure chamber.

In this way, the inkjet head ejects ink droplets from the nozzles by deforming the partitions of the pressure chambers, so that adjacent nozzles that are connected to adjacent pressure chambers cannot eject ink droplets simultaneously. Therefore, the inkjet head divides each pressure chamber into three groups, for example, every two, and changes the phase of the drive pulse for each group. For this reason, depending on the image pattern, a state may occur in which only one nozzle ejects ink and no ink is ejected from the other nozzles (hereinafter referred to as a single-nozzle drive state), a state in which only one nozzle in one group ejects ink and no ink is ejected from the nozzles in the other groups (hereinafter referred to as a multiple-nozzle simultaneous drive state), or a state in which ink is ejected in a time-division manner from nozzles in at least two groups (hereinafter referred to as a multiple-nozzle continuous drive state).

In recent years, there has been a demand for high-resolution inkjet heads. To achieve high-resolution inkjet heads, it is necessary to make the actuator smaller. To eject ink at a specified flight speed using a small actuator, the electric field applied to the actuator must be strong. If the electric field applied to the actuator is strong, the nonlinearity of the actuator cannot be ignored.

Inkjet heads in which each of these actuators is shared by adjacent pressure chambers, each pressure chamber is divided into three groups of two, and the phase of the drive pulse is changed for each group to drive them sequentially, produce a large difference in ejection speed between the multiple nozzles being driven simultaneously and the multiple nozzles being driven continuously. This is because, in the multiple nozzles being driven simultaneously and the multiple nozzles being driven continuously, the direction of the previous drive history of the actuator shared with adjacent pressure chambers is reversed depending on whether ejection has occurred from the adjacent pressure chamber.

To solve this problem, a technique is known in which an auxiliary pulse including an expansion pulse and a contraction pulse that are large enough to prevent ink droplets from being ejected from the nozzle is applied to the actuator before the drive pulse that ejects the first drop, resetting the actuator's drive direction history and stabilizing the ejection speed of multiple drops regardless of the drive state.

However, this technology uses two auxiliary pulses, an expansion pulse and a contraction pulse, which increases the number of times the device is driven, raising concerns about increased heat generation.

JP 2017-013487 A JP 2018-114642 A

The problem that the embodiment of the present invention aims to solve is to provide an inkjet head capable of high-quality printing by using a single pulse as an auxiliary pulse, without the risk of increased heat generation, and capable of stabilizing the multi-drop ejection speed regardless of the driving state.

In one embodiment, the inkjet head includes a pressure chamber that contains 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. The drive circuit applies to the actuator a pulse that drives the pressure chamber in a contracting direction to increase the pressure of the pressure chamber, and then maintains the contracted state of the pressure chamber until the pressure decreases and turns negative and increases again and turns positive, as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, and then returns the pressure chamber to its original state . The auxiliary pulse returns the pressure chamber from the contracted state in stages in a plurality of times at the trailing edge of the auxiliary pulse .

FIG. 2 is an exploded perspective view of a part of the inkjet head. FIG. 2 is a vertical cross-sectional view of the front portion of the inkjet head. FIG. 2 is a cross-sectional view of the front portion of the inkjet head. 3A and 3B are diagrams for explaining the operation principle of an inkjet head. FIG. 1 is a block diagram showing the hardware configuration of an inkjet printer. FIG. 2 is a block diagram showing a specific configuration of a head driving circuit in an inkjet printer. 4 is a schematic circuit diagram of a buffer circuit and a switch circuit included in the head drive circuit. FIG. 4 is a waveform diagram showing an example of a drive pulse for ejecting ink. FIG. 4 is a schematic diagram of each nozzle of the head as viewed from the print paper transport path side. FIG. 11 is an explanatory diagram of a multi-nozzle continuous drive state. FIG. 13 is an explanatory diagram of a multiple nozzle simultaneous driving state. FIG. 4 is a diagram showing a voltage waveform applied to a PZT test piece. 13 is a graph showing the charge and displacement when the voltage waveform of FIG. 12 is applied to a PZT test piece. FIG. 13 is a graph showing an example of the ejection velocity for each drop when an auxiliary pulse waveform is not applied. FIG. 11 is a waveform diagram showing a first embodiment of an auxiliary pulse that is output prior to a drive pulse and does not eject ink. 16 is a diagram showing a simulation result when ink droplets are ejected by a multi-drop method using the drive pulse of FIG. 8 and the auxiliary pulse of FIG. 15 . FIG. 11 is a waveform diagram showing a modified example of the auxiliary pulse in the first embodiment. 18 is a diagram showing a simulation result when ink droplets are ejected by a multi-drop method using the drive pulse of FIG. 8 and the auxiliary pulse of FIG. 17. 18 is a diagram showing another simulation result in the case where ink droplets are ejected by a multi-drop method using the drive pulses in FIG. 8 and the auxiliary pulses in FIG. 17. FIG. 11 is a waveform diagram showing a second embodiment of an auxiliary pulse that is output prior to a drive pulse and does not eject ink. 21 is a diagram showing a simulation result when ink droplets are ejected by a multi-drop method using the drive pulse of FIG. 8 and the auxiliary pulse of FIG. 20. FIG. 13 is a waveform diagram showing a modified example of the auxiliary pulse in the second embodiment. 23 is a diagram showing a simulation result when ink droplets are ejected by a multi-drop method using the drive pulse of FIG. 8 and the auxiliary pulse of FIG. 22. 23 is a diagram showing another simulation result in the case where ink droplets are ejected by a multi-drop method using the drive pulses in FIG. 8 and the auxiliary pulses in FIG. 22 . FIG. 11 is a waveform diagram showing a third embodiment of an auxiliary pulse that is output prior to a drive pulse and does not eject ink. 26 is a diagram showing a simulation result when ink droplets are ejected by a multi-drop method using the drive pulse of FIG. 8 and the auxiliary pulse of FIG. 25 . FIG. 13 is a waveform diagram showing a modified example of an auxiliary pulse in the third embodiment. 28 is a diagram showing a simulation result when ink droplets are ejected by a multi-drop method using the drive pulse of FIG. 8 and the auxiliary pulse of FIG. 27. 28 is a diagram showing another simulation result when ink droplets are ejected by a multi-drop method using the drive pulses in FIG. 8 and the auxiliary pulses in FIG. 27.

The inkjet head according to the embodiment will be described below with reference to the drawings. In this embodiment, a share mode shared wall type inkjet head 100 (see FIG. 1) is used as an example of the inkjet head.

First, the configuration of the inkjet head 100 (hereinafter abbreviated as head 100) will be described with reference to Figs. 1 to 3. Fig. 1 is a perspective view showing a part of the head 100 in an exploded state, Fig. 2 is a vertical cross-sectional view of the front part of the head 100, and Fig. 3 is a horizontal cross-sectional view of the front part of the head 100.

As shown in Figs. 1 to 3, the head 100 has a base substrate 9. The head 100 has a first piezoelectric member 1 bonded to the upper surface of the front side of the base substrate 9, and a second piezoelectric member 2 bonded on top of the first piezoelectric member 1. The bonded first piezoelectric member 1 and second piezoelectric member 2 are polarized in opposite directions along the plate thickness direction, as shown by the arrows in Fig. 2.

The base substrate 9 is formed using a material that has a small dielectric constant and a small difference in thermal expansion coefficient from the piezoelectric members 1 and 2. Suitable materials for the base substrate 9 include alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), aluminum nitride (AlN), lead zirconate titanate (PZT), etc. On the other hand, suitable materials for the piezoelectric members 1 and 2 include lead zirconate titanate (PZT), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), etc.

The head 100 has a large number of long grooves 3 extending from the tip end to the rear end of the joined piezoelectric members 1 and 2. The grooves 3 are parallel and spaced at regular intervals. The tip end of each groove 3 is open, and the rear end is inclined upward.

The head 100 has electrodes 4 on the side walls and bottom surface of each groove 3. The electrodes 4 have a two-layer structure of nickel (Ni) and gold (Au). The electrodes 4 are formed uniformly in each groove 3 by, for example, a plating method. The method of forming the electrodes 4 is not limited to plating. Other methods such as sputtering and vapor deposition can also be used.

As shown in Figures 1 and 3, the head 100 comprises a top plate 6 and a nozzle plate 7. The top plate 6 covers the top of each groove 3. The nozzle plate 7 covers the tip of each groove 3. In the head 100, a plurality of pressure chambers 15 are formed by each groove 3 surrounded by the top plate 6 and the nozzle plate 7. The pressure chambers 15 are, for example, 300 μm deep and 80 μm wide, and are arranged in parallel at a pitch of 169 μm. Such pressure chambers 15 are also called ink chambers.

The top plate 6 has a common ink chamber 5 at the rear inside. The nozzle plate 7 has nozzles 8 drilled in positions facing each groove 3. The nozzles 8 communicate with the opposing groove 3, i.e., the pressure chambers 15. The nozzles 8 are tapered from the pressure chamber 15 side to the ink ejection side on the opposite side. The nozzles 8 are formed in a set corresponding to three adjacent pressure chambers 15, and are offset at a regular interval in the height direction of the groove 3 (the vertical direction on the paper in Figure 2).

As shown in FIG. 1, the head 100 has an extraction electrode 10 extending from the rear end of each groove 3 toward the rear upper surface of the second piezoelectric member 2. The extraction electrode 10 extends from the electrode 4. The head 100 also has a printed circuit board 11, on which a conductive pattern 13 is formed, bonded to the rear upper surface of the base substrate 9. The head 100 has a drive IC 12 mounted on this printed circuit board 11, which implements a head drive circuit 101, which will be described later. The drive IC 12 is connected to the conductive pattern 13. The conductive pattern 13 is connected to each extraction electrode 10 with a conductor 14 by wire bonding.

The set of pressure chamber 15, electrode 4, and nozzle 8 that the head 100 has is called a channel. That is, the head 100 has channels ch.1, ch.2, ..., ch.N, the number of which is N, the number of grooves 3.

Next, the operating principle of the head 100 configured as above will be described with reference to FIG.
4A shows a state in which the potentials of the electrodes 4 disposed on the wall surfaces of the central pressure chamber 152 and the adjacent pressure chambers 151, 153 on both sides adjacent to this pressure chamber 152 are all at the ground potential GND. In this state, neither the partition wall 161 sandwiched between the adjacent pressure chambers 151, 152 nor the partition wall 162 sandwiched between the adjacent pressure chambers 152, 153 is subjected to any distortion action.

Figure 4 (b) shows a state in which a negative voltage -V is applied to the electrode 4 of the central pressure chamber 152. The potentials of the electrodes 4 of both adjacent pressure chambers 151, 153 remain at ground potential GND. In this state, an electric field of voltage V acts on each of the partitions 161, 162 in a direction perpendicular to the polarization direction of the piezoelectric members 1, 2. This action causes each of the partitions 161, 162 to deform outward so as to expand the volume of the pressure chamber 152.

Figure 4(c) shows a state in which a positive voltage +V is applied to the electrode 4 of the central pressure chamber 152. The potentials of the electrodes 4 of both adjacent pressure chambers 151, 153 remain at ground potential GND. In this state, an electric field of voltage V acts on each of the partitions 161, 162 in the opposite direction to that in Figure 4(b). This action causes each of the partitions 161, 162 to deform inward so as to contract the volume of the pressure chamber 152.

When the volume of the pressure chamber 152 is expanded or contracted, a pressure vibration occurs within the pressure chamber 152. This pressure vibration increases the pressure within the pressure chamber 152, and ink droplets are ejected from the nozzle 8 that communicates with the pressure chamber 152.

In this way, the partition 161 separating the pressure chambers 151 and 152 and the partition 162 separating the pressure chambers 152 and 153 serve as actuators for applying pressure vibrations to the inside of the pressure chamber 152, which has the partitions 161 and 162 as its wall surfaces. That is, each pressure chamber 15 shares an actuator with the adjacent pressure chamber 15. For this reason, the head drive circuit 101 cannot drive each pressure chamber 15 individually. The head drive circuit 101 divides each pressure chamber 15 into (n+1) groups every n (n is an integer of 2 or more) chambers and drives them. In this embodiment, the head drive circuit 101 divides each pressure chamber 15 into three groups every two chambers and drives them, which is called three-division drive. Note that the three-division drive is merely an example, and four-division drive or five-division drive may also be used.
4A, 4B, and 4C, voltages -V and +V are applied to the electrode 4 of the central pressure chamber 152 in order to eject ink from the nozzle corresponding to the central pressure chamber 152. The example of ejecting ink from the nozzle corresponding to the central pressure chamber 152 is not limited to this. For example, a voltage -V is applied to the electrode 4 of the central pressure chamber 152, and a voltage +V is applied to the electrodes 4 of the adjacent pressure chambers 151 and 153. Conversely, a voltage +V is applied to the electrode 4 of the central pressure chamber 152, and a voltage -V is applied to the electrodes 4 of the adjacent pressure chambers 151 and 153. In this case as well, if the applied voltage V is halved, the operation of the actuator will be exactly the same as in the case of FIG. 4.

Next, the configuration of the inkjet printer 200 (hereinafter abbreviated as printer 200) will be described with reference to Figs. 5 to 7. Fig. 5 is a block diagram showing the hardware configuration of the printer 200, Fig. 6 is a block diagram showing the specific configuration of the head drive circuit 101, and Fig. 7 is a schematic circuit diagram of the buffer circuit 1013 and switch circuit 1014 included in the head drive circuit 101. The printer 200 is applicable to, for example, office printers, barcode printers, POS printers, industrial printers, etc.

The printer 200 includes a CPU (Central Processing Unit) 201, a ROM (Read Only Memory) 202, a RAM (Random Access Memory) 203, an operation panel 204, a communication interface 205, a conveying motor 206, a motor drive circuit 207, a pump 208, a pump drive circuit 209, and a head 100. The printer 200 also includes a bus line 211 such as an address bus and a data bus. The printer 200 connects the CPU 201, ROM 202, RAM 203, the operation panel 204, the communication interface 205, the motor drive circuit 207, the pump drive circuit 209, and the drive circuit 101 of the head 100 to this bus line 211 directly or via an input/output circuit.

The CPU 201 corresponds to the central part of the computer. The CPU 201 controls each part to realize the various functions of the printer 200 according to the operating system and application programs.

ROM 202 corresponds to the main memory of the computer. ROM 202 stores the operating system and application programs. ROM 202 may also store data necessary for CPU 201 to execute processes for controlling each part.

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

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.

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 sends a signal notifying the client terminal of the error.

The motor drive circuit 207 controls the drive of the transport motor 206. The transport motor 206 functions as the drive source for the transport mechanism that transports a recording medium such as printing paper. When the transport motor 206 is driven, the transport mechanism starts transporting the recording medium. The transport mechanism transports the recording medium to the printing position by the head 100. The transport mechanism ejects the recording medium after printing from an ejection port (not shown) to the outside of the printer 200.

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

The head drive circuit 101 drives the channel group 102 of the head 100 based on print data. As shown in FIG. 6, the head drive circuit 101 includes a pattern generator 1011, a logic circuit 1012, a buffer circuit 1013, and a switch circuit 1014.

The pattern generator 1011 generates waveform patterns such as the ejection target waveform, ejection adjacent waveforms, non-ejection target waveform, and non-ejection adjacent waveforms. The data of the waveform patterns generated by the pattern generator 1011 is supplied to the logic circuit 1012.

The logic circuit 1012 accepts the input of print data read out one line at a time from the image memory. When the print data is input, the logic circuit 1012 sets three adjacent channels ch.(i-1), ch.i, and ch.(i+1) of the head 100 as one set, and determines whether one of the channels, for example the central channel ch.i, is an ejection channel that ejects ink or a non-ejection channel that does not eject ink. If the channel ch.i is an ejection channel, the logic circuit 1012 outputs pattern data of the ejection waveform to this channel ch.i, and outputs pattern data of the ejection waveforms on both sides to the adjacent channels ch.(i-1) and ch.(i+1). If the channel ch.i is a non-ejection channel, the logic circuit 1012 outputs pattern data of the non-ejection waveform to this channel ch.i, and outputs pattern data of the non-ejection waveforms on both sides to the adjacent channels ch.(i-1) and ch.(i+1). Each pattern data output from the logic circuit 1012 is provided to the buffer circuit 1013.

The buffer circuit 1013 connects a power supply of a positive voltage Vcc to a power supply of a negative voltage -V. As shown in FIG. 7, the buffer circuit 1013 includes a pre-buffer PB for each of the channels ch.1, ch.2, ..., ch.N of the head 100. Note that FIG. 7 shows pre-buffers PB(i-1), PBi, and PB(i+1) corresponding to three adjacent channels ch.(i-1), ch.i, and ch.(i+1), respectively.

Each pre-buffer PB has three buffers Ba, Bb, and Bc, the first to third. Each buffer Ba, Bb, and Bc is connected to a power supply of a positive voltage Vcc and a power supply of a negative voltage -V.

In each pre-buffer PB, the output of the first to third buffers Ba, Bb, Bc changes according to the signal level of the pattern data supplied from the logic circuit 1012. The logic circuit 1012 supplies signals of different levels depending on whether the corresponding channel ch.k (1≦k≦N) is an ejection channel, a non-ejection channel, or a channel adjacent to an ejection channel or a non-ejection channel. The first to third buffers Ba, Bb, Bc supplied with a high-level signal output a signal of a positive voltage Vcc level. The first to third buffers Ba, Bb, Bc supplied with a low-level signal output a signal of a negative voltage -V level.

The output of each pre-buffer PB, i.e., the output signals of the first to third buffers Ba, Bb, and Bc, are provided to the switch circuit 1014.

The switch circuit 1014 connects a power supply of a positive voltage Vcc, a power supply of a positive voltage +V, a power supply of a negative voltage -V, and a ground potential GND. The positive voltage Vcc is higher than the positive voltage +V. Typical values are 24 volts for the positive voltage Vcc and 15 volts for the positive voltage +V. In this case, the negative voltage -V is -15 volts.

However, the appropriate values for the positive and negative voltages vary depending on the viscosity of the ink. The viscosity of the ink varies depending on the type of ink and the temperature at which it is used. For this reason, the positive voltage +V and the negative voltage -V are selected within the range of approximately ±15 volts to ±30 volts depending on the type of ink and the temperature at which it is used. In this case, the positive voltage Vcc must be higher than the positive voltage +V, so if the positive voltage +V and the negative voltage -V are a maximum of ±30 volts, then the positive voltage Vcc is set to, for example, 39 volts.

As shown in FIG. 7, the switch circuit 1014 has a driver DR for each of the channels ch.1, ch.2, ..., ch.N of the head 100. Note that FIG. 7 shows drivers DR(i-1), DRi, and DR(i+1) corresponding to three adjacent channels ch.(i-1), ch.i, and ch.(i+1), respectively.

Each driver DR includes a PMOS type field effect transistor Tra (hereinafter referred to as the first transistor Tra) and two NMOS type field effect transistors Trb and Trc (hereinafter referred to as the second transistor Trb and the third transistor Trc). Each driver DR connects a series circuit of the first transistor Tra and the second transistor Trb between a power supply of a positive voltage +V and a ground potential GND, and further connects the third transistor Trc between the connection point of the first transistor Tra and the second transistor Trb and a power supply of a negative voltage -V. Each driver DR also connects the back gate of the first transistor Tra to a power supply of a positive voltage Vcc, and connects the back gates of the second transistor Trb and the third transistor Trc to a power supply of a negative voltage -V. Furthermore, each driver DR connects the first buffer Ba of the corresponding pre-buffer PB to the gate of the second transistor Trb, the second buffer Bb to the gate of the first transistor Tra, and the third buffer Bc to the gate of the third transistor Trc. Each driver DR then applies the potential of the connection point between the first transistor Tra and the second transistor Trb to the electrode 4 of the corresponding channel ch.1, ch.2, ..., ch.N.

The first transistor Tra is therefore turned off when a signal of the positive voltage Vcc level is input from the second buffer Bb, and turned on when a signal of the negative voltage -V level is input. The second transistor Trb is turned on when a signal of the positive voltage Vcc level is input from the first buffer Ba, and turned off when a signal of the negative voltage -V level is input. The third transistor Trc is turned on when a signal of the positive voltage Vcc level is input from the third buffer Bc, and turned off when a signal of the negative voltage -V level is input.

When the first transistor Tra is turned on and the second transistor Trb and the third transistor Trc are turned off, the driver DR applies a positive voltage +V to the electrode 4 of the corresponding channel ch.1, ch.2, ..., ch.N. When the first transistor Tra and the third transistor Trc are turned off simultaneously and the second transistor Trb is turned on, the driver DR sets the potential of the electrode 4 of the corresponding channel ch.1, ch.2, ..., ch.N to the ground GND level. When the first transistor Tra and the second transistor Trb are turned off simultaneously and the third transistor Trc is turned on, the driver DR applies a negative voltage -V to the electrode 4 of the corresponding channel ch.1, ch.2, ..., ch.N.

Next, an example of a drive pulse for ejecting ink will be described. The drive pulse is a pulse that is applied to the electrode 4 of the channel that ejects ink (ejection channel ch.x), and thus to the actuator.

Figure 8 is an explanatory diagram of a DRP waveform, which is an example of a drive pulse. In Figure 8, the solid line "drive voltage" is a waveform that represents the voltage of the drive pulse. The dashed line "pressure" is a waveform that represents the change in pressure generated in the pressure chamber 15 by the drive pulse. The dashed line "flow velocity" is a waveform that represents the change in the flow velocity of ink flowing into the nozzle 8 by the drive pulse. The dashed line "meniscus" is a waveform that represents the change in the position of the ink liquid surface by the drive pulse. The horizontal axis represents the passage of time (s). The vertical axis represents the drive voltage, pressure, flow velocity, waveform, and meniscus size, and the numerical values are normalized. In the following, the natural vibration period 2AL (acoustic length) of the pressure in the pressure chamber 15 is defined as the pressure vibration period T. In this embodiment, the pressure vibration period T is 3.8 μs.

The drive pulse for ejection includes an expansion pulse (Draw) Pd, a retention time (Release) R, and a contraction pulse (Push) Pp. After applying an expansion pulse Pd from 0V, which is the steady state, the drive pulse maintains 0V for the retention time R, and then applies a contraction pulse Pp. After applying the contraction pulse Pp, the steady state of 0V is maintained.

The expansion pulse Pd is a pulse for suddenly expanding the pressure chamber 15 of the ejection channel ch.x at its leading edge. The expansion pulse Pd maintains the expanded state of the pressure chamber 15 for a time of T/2 (1.9 μs) to draw in ink and waits until the pressure of the ink in the pressure chamber 15 changes from negative to positive pressure. After the ink pressure changes from negative to positive pressure, the expansion pulse Pd suddenly returns the pressure chamber 15 from the expanded state at its trailing edge. This return further increases the pressure of the ink, which has become positive, and ink is ejected from the nozzle 8 connected to the pressure chamber 15.

The retention time R is the time required to wait for the ink pressure in the pressure chamber 15, which has returned to the restored state, to change from positive pressure to negative pressure, reach the maximum negative pressure, and then increase again.

The contraction pulse Pp, at its leading edge, contracts the pressure chamber 15 in which the ink pressure has started to increase due to the lapse of the retention time R, and then causes the pressure chamber 15 to return from the contracted state. This contraction and return cancels the residual vibration of the pressure chamber 15.

The DRP waveform first puts the pressure chamber 15 into an expanded state, and the ink ejection volume is maximized when the time for which the expanded state is maintained is 1/2 the pressure vibration period T of the pressure chamber 15. If the time for which the expanded state is maintained is longer or shorter than 1/2 the pressure vibration period T, the ink ejection volume decreases. Therefore, in the DRP waveform, the time for which the pressure chamber 15 is maintained in an expanded state may be shifted from 1/2 the pressure vibration period T for the purpose of adjusting the ejection volume, etc. The drive waveform for ejecting ink is not limited to the DRP waveform. Other drive waveform pulse waveforms may also be used as the drive pulse.

Next, the multi-nozzle simultaneous drive state and the multi-nozzle continuous drive state will be explained once again with reference to FIGS.
9 is a schematic diagram of each nozzle 8 of the head 100 of FIGS. 1 to 4 as viewed from the transport path side of the print paper PA. As shown in the figure, the head 100 of this embodiment has three rows of nozzles 8 in a staggered arrangement. In this type of head 100, adjacent pressure chambers (151, 152, 153, etc.) share actuators (161, 162, etc.), so that ink droplets cannot be ejected simultaneously from adjacent nozzles 8. For this reason, the pressure chambers 15 corresponding to each nozzle 8 are divided into a (3n+1)th group, a (3n+2)th group, and a (3n+3)th group, and are sequentially driven in a time-division manner. For example, in FIG. 9, when the print paper PA is transported in the direction of the arrow, in order to print a horizontal line, the pressure chambers 15 corresponding to the nozzles 8 with the symbol a are driven first, followed by the pressure chambers 15 corresponding to the nozzles 8 with the symbol c, followed by the pressure chambers 15 corresponding to the nozzles 8 with the symbol b. 9, the nozzle 8 with the symbol b is the nozzle of interest, and the pressure chamber 15 corresponds to the pressure chamber 152 in Fig. 4. The nozzles 8 with the symbols a and c are nozzles adjacent to the nozzle of interest, and the pressure chamber 15 corresponds to the pressure chambers 151 and 153 in Fig. 4.

Figure 10 is an explanatory diagram of the multi-nozzle continuous drive state. The multi-nozzle continuous drive state refers to a state in which ink is ejected in a time-division manner from nozzles belonging to at least two groups. For example, as shown in FIG. 10(a), to obtain a print image of two horizontal lines, the actuators are driven so that the required amount of ink droplets are ejected from nozzles 8 in the time order of symbols a, c, and b, as shown in FIG. 10(b). In this case, adjacent actuators are all driven with a time lag. This type of drive mode is called multi-nozzle continuous drive. Here, continuous drive means that nozzles 8 that are consecutive in the spatial direction (the direction in which the nozzles are arranged) are driven.

Figure 11 is an explanatory diagram of the multiple nozzle simultaneous drive state. The multiple nozzle simultaneous drive state refers to a state in which ink is ejected from nozzles belonging to one of the groups, and ink is not ejected from nozzles belonging to the other groups. For example, to obtain a print image in which multiple horizontally aligned dots are arranged in multiple rows as shown in Figure 11(a), the actuator is driven so that the required amount of ink is ejected from nozzles belonging to the same group when the group is divided into the (3n+1)th group, the (3n+2)th group, and the (3n+3)th group, as shown in Figure 11(b). This type of drive mode is called multiple nozzle simultaneous drive.

Simultaneous driving of multiple nozzles is the simplest way of controlling the behavior of the pressure chambers 15. In other words, the actuators belonging to the same group are driven uniformly in the direction in which the nozzles are arranged, so the pressure chambers 15 of all channels behave uniformly.

On the other hand, multi-nozzle continuous drive exhibits complex behavior in the time domain. This is because when ink droplets are ejected from an adjacent channel, the history of driving one wall of the actuator of that channel, which is shared with the adjacent channel, affects the operation of the actuator of that channel.

In addition, in the continuous nozzle drive state, the direction of the history when the actuator operates is different for the first drop and the second drop and onwards. This difference in direction is the cause of the large drop in the ejection speed for the first drop. To understand the reason for this, next we will explain the hysteresis characteristics of PZT (lead zirconate titanate), which is used as piezoelectric members 1 and 2.

A PZT test piece is used to explain this hysteresis characteristic. The test piece is a rectangular parallelepiped with a height of 10 mm, a width of 3 mm, and a thickness of 0.2 mm. This test piece is polarized in the height direction, and a voltage with the waveform shown in Figure 12 is applied in the thickness direction. The voltage was set to 60 V, since the thickness of the test piece is approximately 2.3 times the thickness of the partition wall of the head 100.

When the voltage waveform shown in Figure 12 was applied and the charge P1 [μC/cm2] injected into the test piece and the displacement d [nm] of the test piece were measured, the results shown in Figure 13 were obtained. That is, when the actuator had a history in the same direction, a voltage change of 60 [V] caused a displacement of 60 [nm] in the test piece. In contrast, when the actuator had a history in the opposite direction, a voltage change of 60 [V] caused a displacement of 80 [nm] in the test piece. In other words, by having a history in the opposite direction, the displacement increases to 133% compared to when it had a history in the same direction. In this way, the displacement of the test piece is smaller when it has a history in the same direction than when it has a history in the opposite direction. Therefore, it is thought that the ejection speed drops significantly in the first drop in the multi-nozzle continuous drive state with a history in the same direction.

Figure 14 shows an example of the ejection speed [m/s] when only one drop or two to five drops are ejected consecutively by the multi-drop method without adding an auxiliary pulse waveform in each of the multiple nozzle simultaneous drive state and the multiple nozzle consecutive drive state. In Figure 14, the value on the vertical axis of the point plotted corresponding to the value "1" on the horizontal axis is the ejection speed when only one drop is ejected. The value on the vertical axis of the point plotted corresponding to the value "2" on the horizontal axis is the ejection speed of the second drop when two drops are ejected consecutively. The value on the vertical axis of the point plotted corresponding to the value "3" on the horizontal axis is the ejection speed of the third drop when three drops are ejected consecutively. The value on the vertical axis of the point plotted corresponding to the value "4" on the horizontal axis is the ejection speed of the fourth drop when four drops are ejected consecutively. The value on the vertical axis of the point plotted corresponding to the value "5" on the horizontal axis is the ejection speed of the fifth drop when five drops are ejected consecutively. In FIG. 14, the dashed line graph shows the ejection speed [m/s] when multiple nozzles are simultaneously driven. The dashed line graph shows the ejection speed [m/s] when multiple nozzles are continuously driven.

When an auxiliary pulse is not applied, as shown in FIG. 14, in the multi-nozzle simultaneous drive state and the multi-nozzle continuous drive state, the ejection speed when only one drop is ejected and when the first drop is ejected when two or more drops are ejected in succession is slower than the ejection speed of the final drop when two or more drops are ejected in succession. In particular, in the multi-nozzle continuous drive state, the ejection speed when only one drop is ejected is extremely slow, and stable print quality cannot be obtained.

This problem can be solved by applying an auxiliary pulse to the actuator prior to the drive pulse for ejecting ink. Next, we will explain the auxiliary pulse.

An auxiliary pulse is a pulse that does not eject ink. If an auxiliary pulse is applied to the actuator just before driving the actuator for ejection, and the actuator is driven to a degree that does not eject ink, it will affect the subsequent ejection. The effects include differences in amplitude that depend on the actuator's history and residual vibrations that remain just before driving. In particular, when ejecting ink droplets using the multi-drop method, the auxiliary pulse has a large effect on the ejection speed of the first and second drops. In order to obtain good ejection characteristics, it is necessary to be able to appropriately control both the differences in amplitude that depend on the actuator's history and the residual vibrations just before driving for ejection. Therefore, we will first explain an auxiliary pulse that resets the history and leaves no residual vibrations thereafter. Next, we will explain an auxiliary pulse that is modified to intentionally leave a desired residual vibration just before driving.

[First embodiment]
FIG. 15 is an explanatory diagram of the auxiliary pulse Pa according to the first embodiment. In FIG. 15, the solid line "driving voltage" is a waveform representing the voltage of the auxiliary pulse Pa. The dashed and dotted line "pressure" is a waveform representing the change in pressure generated in the pressure chamber 15 by the auxiliary pulse Pa. The dashed and dotted line "flow velocity" is a waveform representing the change in the flow velocity of ink flowing into the nozzle 8 by the auxiliary pulse Pa. The dashed line "meniscus" is a waveform representing the change in the position of the ink surface by the auxiliary pulse Pa. The horizontal axis represents the passage of time (s). The vertical axis represents the driving voltage, pressure, flow velocity, waveform, and meniscus size, and the numerical values are normalized.

The auxiliary pulse Pa applies a voltage from 0V, which is the steady state, to contract the pressure chamber 15, maintains the voltage for a certain period of time, and then gradually reduces the voltage to return to 0V. The auxiliary pulse Pa suddenly contracts the pressure chamber 15 at its leading edge and maintains the contracted state. At this time, the pressure generated in the pressure chamber 15 increases sharply due to the contraction. Then, while the contracted state is maintained, the pressure decreases and turns to negative pressure, eventually reaching the maximum negative pressure. The maximum negative pressure is reached after 1/2 the pressure vibration period T has elapsed from the leading edge of the auxiliary pulse Pa. After that, the pressure generated in the pressure chamber 15 increases again and turns to positive pressure. If the contracted state of the pressure chamber 15 were maintained as it is, the pressure generated in the pressure chamber 15 would reach its maximum value when the time of the pressure vibration period T has elapsed from the leading edge of the auxiliary pulse Pa, and the residual vibration would continue. However, if the pressure chamber 15, which was in a contracted state, is gradually restored after the pressure generated in the pressure chamber 15 turns to a positive pressure, the residual vibration can be stopped while the actuator has a history of contraction.

The auxiliary pulse Pa causes the pressure chamber 15 to return in two stages at its rear edge. The first stage return operation adjusts the magnitude of vibration so that the pressure generated in the pressure chamber 15 due to the pressure change caused by the subsequent second stage return operation becomes zero. The second stage return operation is performed when the ink flow rate in the pressure chamber 15 becomes zero. The time from the start of the contraction operation at its leading edge to the end of the second stage return operation at its trailing edge of such an auxiliary pulse Pa is greater than the pressure vibration period T of the pressure chamber 15, for example 4.4 μs.

The auxiliary pulse Pa having such a configuration is applied to the actuator prior to the drive pulse for ejecting ink. By doing so, the residual vibration of the pressure chamber 15 is cancelled in a state in which the actuator always has a history of contraction. Then, after the actuator always has a history of contraction and after the residual vibration of the pressure chamber 15 has been cancelled, a drive pulse for ejecting ink is given to the actuator. In other words, regardless of the print contents from the previous drive, the history of the actuator's drive direction before ink ejection is always aligned in one direction.

In this way, the auxiliary pulse Pa in the first embodiment is a signal that first resets the actuator's history and leaves no residual vibration thereafter. In other words, the auxiliary pulse Pa fixes the actuator's history in one direction, eliminating amplitude differences that depend on the history, and also stops the residual vibration before the next ejection.

Figure 16 shows the results of a simulation in which ink droplets are ejected by the multi-drop method using the drive pulse (PRD waveform) of Figure 8 and the auxiliary pulse Pa of Figure 15. The simulation was performed using an LCR equivalent circuit (not shown) that simulates an inkjet head. Figure 16 shows an example in which the auxiliary pulse Pa is applied once, and then the drive pulse is applied three times.

In FIG. 16, the solid line "drive voltage" is a waveform that represents the voltage change of the auxiliary pulse Pa and drive pulse. The drive pulse includes an expansion pulse Pd and a contraction pulse Pp. The dashed and dotted line "pressure" is a waveform that represents the change in pressure generated within the pressure chamber 15. The dashed and dotted line "flow velocity" is a waveform that represents the change in the flow velocity of the ink flowing into the nozzle 8. The dashed line "meniscus" is a waveform that represents the change in the position of the ink liquid surface. In FIG. 16, the horizontal axis represents the passage of time (s), and the vertical axis represents the normalized magnitude of each item.

As shown in FIG. 16, by applying an auxiliary pulse Pa prior to the drive pulse for ejecting ink, the history of the actuator drive direction before ink ejection becomes the contraction direction. In addition, the residual vibration of the pressure chamber 15 is canceled. Thus, ink ejected continuously by the multi-drop method is ejected at a constant ejection speed in the simulation.

However, in reality, the operation does not exactly follow the simulation, and there are cases where it is desired to adjust the subsequent behavior by leaving residual vibration before ejection. Next, as a variation of the auxiliary pulse Pa, we will explain the auxiliary pulse Pe, which intentionally leaves residual vibration after resetting the history.

Figure 17 is an explanatory diagram of the auxiliary pulse Pe. Note that in Figure 17, the vertical axis, horizontal axis, solid line "driving voltage", dashed line "pressure", dashed line "flow velocity", and dashed line "meniscus" are the same as those explained in Figure 15, so explanations will be omitted here.

Like the auxiliary pulse Pa, the auxiliary pulse Pe also applies a voltage from 0V, which is the steady state, to contract the pressure chamber 15, and after maintaining the voltage for a certain period of time, the voltage is gradually reduced back to 0V. The auxiliary pulse Pe is a modified version of the auxiliary pulse Pa that intentionally leaves residual vibration. Residual vibration can be left by shifting the timing of the stepwise return operation of the auxiliary pulse Pa. The auxiliary pulse Pe intentionally leaves residual vibration by delaying the timing of the first return stage.

By delaying the timing of the first stage of return, the pressure vibration in the pressure chamber 15 is reduced by the subsequent second stage return operation, but does not become zero. In other words, the pressure vibration in the pressure chamber 15 remains. In this way, the output of the drive pulse begins while the pressure vibration in the pressure chamber 15 remains. By doing so, the ejection speed of the first or second drop can be adjusted.

Figure 18 shows the simulation results when the ejection speed of the first drop is increased, and Figure 19 shows the simulation results when the ejection speed of the second drop is increased. Figures 18 and 19 show an example in which the auxiliary pulse Pe is applied once, and then the drive pulse is applied three times.

The difference between the simulation in FIG. 18 and the simulation in FIG. 19 is the waiting time Ha from the end of the second stage return operation of the auxiliary pulse Pe until the leading edge of the expansion pulse Pd of the drive pulse. In the simulation in FIG. 18, the waiting time Ha is shorter than in the simulation in FIG. 19. By shortening the waiting time Ha, when the pressure in the pressure chamber 15 is negative, the pressure chamber 15 of the ejection channel ch.x is suddenly expanded by the leading edge of the expansion pulse Pd. In this way, the negative pressure generated in the pressure chamber 15 is larger than when the auxiliary pulse Pa is applied. Therefore, the positive pressure generated in the pressure chamber 15 when the pressure chamber 15 is suddenly returned to the expanded state by the trailing edge of the expansion pulse Pd is larger than when the auxiliary pulse Pa is applied. As a result, the ejection speed of the first drop is faster.

On the other hand, in the simulation of FIG. 19, the waiting time Ha is longer than in the simulation of FIG. 18. By lengthening the waiting time Ha, when the pressure in the pressure chamber 15 is positive, the pressure chamber 15 of the ejection channel ch.x is suddenly expanded at the leading edge of the expansion pulse Pd. In this way, the negative pressure generated in the pressure chamber 15 is smaller than when the auxiliary pulse Pa is applied. Therefore, the positive pressure generated in the pressure chamber 15 when the pressure chamber 15 is suddenly returned to the expanded state at the trailing edge of the expansion pulse Pd is smaller than when the auxiliary pulse Pa is applied. As a result, the ejection speed of the second drop is faster than the ejection speed of the first drop.

For example, for an inkjet head in which the ejection speed of the first drop is slow, the auxiliary pulse Pe that produces the simulation result in FIG. 18 is effective. For an inkjet head in which the ejection speed of the second drop is slow, the auxiliary pulse Pe that produces the simulation result in FIG. 19 is effective.

In this way, by applying the auxiliary pulse Pa or auxiliary pulse Pe, which does not eject ink, to the actuator prior to the drive pulse for ejecting ink, it is possible to achieve the effect of stabilizing the ejection speed of multi-drops regardless of the drive state, such as the multi-nozzle continuous drive state or the multi-nozzle simultaneous drive state. As a result, it is possible to provide an inkjet head 100 capable of high-quality printing.

In addition, the auxiliary pulse Pa or auxiliary pulse Pe is composed of a single pulse signal. Therefore, the number of times it is driven is smaller than when an auxiliary pulse including an expansion pulse and a contraction pulse is output. As a result, there is no risk of increased heat generation.

[Second embodiment]
Fig. 20 is an explanatory diagram of the auxiliary pulse Pb according to Example 2. In Fig. 20, the vertical axis, horizontal axis, solid line "driving voltage", dashed line "pressure", dashed line "flow velocity", and dashed line "meniscus" are the same as those explained in Fig. 15, so the explanation here will be omitted.

The auxiliary pulse Pb has a different leading edge compared to the auxiliary pulse Pa. That is, the auxiliary pulse Pb causes the pressure chamber 15 to contract stepwise in two stages at its leading edge. The auxiliary pulse Pb applies a voltage from 0V, which is the steady state, causing the pressure chamber 15 to contract stepwise, and after maintaining the voltage for a certain period of time, the voltage is reduced stepwise and returned to 0V. The first stage contraction operation causes the pressure chamber 15 to contract at its leading edge, and the contracted state is maintained. At this time, the pressure generated within the pressure chamber 15 increases due to the contraction, but while the contracted state is maintained, the pressure decreases to zero.

When the pressure generated in the pressure chamber 15 becomes zero, a second-stage contraction operation is performed. This contraction operation causes the pressure generated in the pressure chamber 15 to increase again. Then, while the second-stage contraction state is maintained, the pressure decreases and turns to negative pressure, eventually reaching the maximum negative pressure. The maximum negative pressure is reached after 1/2 the pressure vibration period T has elapsed from the second-stage contraction of the auxiliary pulse Pa. After that, the pressure generated in the pressure chamber 15 increases again and turns to positive pressure.

When the pressure that has turned positive becomes zero, the auxiliary pulse Pb, like the auxiliary pulse Pa, returns the pressure chamber 15 in two stages. The first stage return operation adjusts the magnitude of vibration so that the pressure generated in the pressure chamber 15 due to the pressure change caused by the subsequent second stage return operation becomes zero. The second stage return operation is performed when the ink flow rate in the pressure chamber 15 becomes zero. With this type of auxiliary pulse Pb, the time from the start of the first stage contraction operation, which is its leading edge, to the end of the second stage return operation, which is its trailing edge, is greater than the pressure vibration period T of the pressure chamber 15, for example 4.8 μs.

The auxiliary pulse Pb configured in this way is applied to the actuator prior to the drive pulse for ejecting ink. By doing so, the residual vibration of the pressure chamber 15 is cancelled while the actuator always has a history of contraction. Then, after the actuator always has a history of contraction and after the residual vibration of the pressure chamber 15 has been cancelled, a drive pulse for ejecting ink is given to the actuator. In other words, regardless of the print content from the previous drive, the history of the actuator's drive direction before ink ejection is always aligned in one direction.

In this way, the auxiliary pulse Pb in the second embodiment is also a signal that first resets the actuator's history and leaves no residual vibration thereafter. In other words, the auxiliary pulse Pb fixes the actuator's history in one direction, eliminating amplitude differences that depend on the history, and stopping the residual vibration before the next ejection.

Figure 21 shows the results of a simulation in which ink droplets are ejected by the multi-drop method using the drive pulse (PRD waveform) of Figure 8 and the auxiliary pulse Pb of Figure 20. The simulation was performed using an LCR equivalent circuit (not shown) that simulates an inkjet head. Figure 21 shows an example in which the auxiliary pulse Pb is applied once, and then the drive pulse is applied three times.

In FIG. 21, the solid line "drive voltage" is a waveform that represents the voltage change of the auxiliary pulse Pb and the drive pulse. The drive pulse includes an expansion pulse Pd and a contraction pulse Pp. The dashed and dotted line "pressure" is a waveform that represents the change in pressure generated within the pressure chamber 15. The dashed and dotted line "flow velocity" is a waveform that represents the change in the flow velocity of the ink flowing into the nozzle 8. The dashed line "meniscus" is a waveform that represents the change in the position of the ink liquid surface. In FIG. 21, the horizontal axis represents the passage of time (s), and the vertical axis represents the normalized magnitude of each item.

As shown in FIG. 21, by applying an auxiliary pulse Pb prior to the drive pulse for ejecting ink, the history of the actuator drive direction before ink ejection becomes the contraction direction. In addition, the residual vibration of the pressure chamber 15 is canceled. Thus, ink ejected continuously by the multi-drop method is ejected at a constant ejection speed in the simulation.

However, in reality, the operation does not exactly follow the simulation, and there are cases where it is desired to adjust the subsequent behavior by leaving residual vibration before ejection. Next, as a variation of the auxiliary pulse Pb, we will explain the auxiliary pulse Pf, which intentionally leaves residual vibration after resetting the history.

Figure 22 is an explanatory diagram of the auxiliary pulse Pf. Note that in Figure 22, the vertical axis, horizontal axis, solid line "drive voltage", dashed line "pressure", dashed line "flow velocity", and dashed line "meniscus" are the same as those explained in Figure 15, so explanations will be omitted here.

The auxiliary pulse Pf is a modified version of the auxiliary pulse Pb that intentionally leaves residual vibration. As with the auxiliary pulse Pe, the auxiliary pulse Pf can also leave residual vibration by shifting the timing of the stepwise return operation. The auxiliary pulse Pf applies a voltage from 0V, which is the steady state, causing the pressure chamber 15 to contract stepwise, and after maintaining the voltage for a certain period of time, the voltage is gradually reduced and returned to 0V. The auxiliary pulse Pf intentionally leaves residual vibration by delaying the timing of the first return step.

By delaying the timing of the first stage of return, the pressure vibration in the pressure chamber 15 is reduced by the subsequent second stage return operation, but does not become zero. In other words, the pressure vibration in the pressure chamber 15 remains. In this way, the output of the drive pulse begins while the pressure vibration in the pressure chamber 15 remains. By doing so, the ejection speed of the first or second drop can be adjusted.

Figure 23 shows the simulation results when the ejection speed of the first drop is increased, and Figure 24 shows the simulation results when the ejection speed of the second drop is increased. Figures 23 and 24 show an example in which the auxiliary pulse Pf is applied once, and then the drive pulse is applied three times.

The difference between the simulation in FIG. 23 and the simulation in FIG. 24 is the waiting time Hb from the end of the second stage return operation of the auxiliary pulse Pf until the leading edge of the expansion pulse Pd of the drive pulse. In the simulation in FIG. 23, the waiting time Hb is shorter than in the simulation in FIG. 24. By shortening the waiting time Ha, when the pressure in the pressure chamber 15 is negative, the pressure chamber 15 of the ejection channel ch.x is suddenly expanded at the leading edge of the expansion pulse Pd. In this way, the negative pressure generated in the pressure chamber 15 is larger than when the auxiliary pulse Pb is applied. Therefore, the positive pressure generated in the pressure chamber 15 when the pressure chamber 15 is suddenly returned to the expanded state at the trailing edge of the expansion pulse Pd is larger than when the auxiliary pulse Pb is applied. As a result, the ejection speed of the first drop is faster.

On the other hand, in the simulation of FIG. 24, the waiting time Hb is longer than in the simulation of FIG. 23. By lengthening the waiting time Ha, when the pressure in the pressure chamber 15 is positive, the pressure chamber 15 of the ejection channel ch.x is suddenly expanded at the leading edge of the expansion pulse Pd. In this way, the negative pressure generated in the pressure chamber 15 is smaller than when the auxiliary pulse Pb is applied. Therefore, the positive pressure generated in the pressure chamber 15 when the pressure chamber 15 is suddenly returned to the expanded state at the trailing edge of the expansion pulse Pd is smaller than when the auxiliary pulse Pb is applied. As a result, the ejection speed of the second drop is faster than the ejection speed of the first drop.

For example, for an inkjet head in which the ejection speed of the first drop is slow, the auxiliary pulse Pf that produces the simulation result in FIG. 23 is effective. For an inkjet head in which the ejection speed of the second drop is slow, the auxiliary pulse Pf that produces the simulation result in FIG. 24 is effective.

In this way, by applying the auxiliary pulse Pb or auxiliary pulse Pf, which does not eject ink, to the actuator prior to the drive pulse for ejecting ink, it is possible to achieve the effect of stabilizing the ejection speed of multi-drops regardless of the drive state, such as the multi-nozzle continuous drive state or the multi-nozzle simultaneous drive state. As a result, it is possible to provide an inkjet head 100 capable of high-quality printing.

In addition, the auxiliary pulse Pb or the auxiliary pulse Pf is composed of a single pulse signal. Therefore, the number of times it is driven is smaller than when an auxiliary pulse including an expansion pulse and a contraction pulse is output. As a result, there is no risk of increased heat generation.

In addition, the auxiliary pulse Pb or auxiliary pulse Pf causes the pressure chamber 15 to contract stepwise at its leading edge. By appropriately adjusting the time difference of the stepwise contraction, it is possible to reduce the maximum pressure of the pressure chamber 15 while keeping the maximum displacement of the pressure chamber 15 the same as in the first embodiment.

[Third Example]
Fig. 25 is an explanatory diagram of the auxiliary pulse Pc according to Example 3. In Fig. 25 as well, the vertical axis, horizontal axis, solid line "driving voltage", dashed line "pressure", dashed line "flow velocity", and dashed line "meniscus" are the same as those explained in Fig. 15, so explanations thereof will be omitted here.

The auxiliary pulse Pc has different leading and trailing edges compared to the auxiliary pulse Pa or the auxiliary pulse Pb. That is, the auxiliary pulse Pc gradually contracts the pressure chamber 15 at its leading edge. The auxiliary pulse Pc also gradually restores the pressure chamber 15 at its trailing edge. The auxiliary pulse Pc gradually applies a voltage from 0V, which is the steady state, to gradually contract the pressure chamber 15, and after maintaining the voltage for a certain period of time, gradually reduces the voltage back to 0V.

When the pressure chamber 15 is gradually contracted at the leading edge of the auxiliary pulse Pc, the pressure generated in the pressure chamber 15 increases due to the contraction, but reaches a maximum positive pressure during the contraction and then decreases. Then, while the contracted state of the pressure chamber 15 is maintained, the pressure further decreases and turns to negative pressure, eventually reaching the maximum negative pressure. After that, when the pressure chamber 15 is gradually restored, the pressure generated in the pressure chamber 15 turns to positive pressure during the restoration, but then decreases, and at the point when the ink flow rate in the pressure chamber 15 becomes zero, that is, when the pressure chamber 15 is restored, the pressure generated in the pressure chamber 15 becomes zero. Such an auxiliary pulse Pc also has a time F from the start of the contraction operation, which is its leading edge, to the end of the restoration operation, which is its trailing edge, which is greater than the pressure vibration period T of the pressure chamber 15, for example 6.0 μs.

Figure 26 shows the results of a simulation in which ink droplets are ejected by the multi-drop method using the drive pulse (PRD waveform) of Figure 8 and the auxiliary pulse Pc of Figure 25. The simulation was performed using an LCR equivalent circuit (not shown) that simulates an inkjet head. Figure 26 shows an example in which the auxiliary pulse Pc is applied once, and then the drive pulse is applied three times.

In FIG. 26, the solid line "drive voltage" is a waveform that represents the voltage change of the auxiliary pulse Pc and drive pulse. The drive pulse includes an expansion pulse Pd and a contraction pulse Pp. The dashed and dotted line "pressure" is a waveform that represents the change in pressure generated within the pressure chamber 15. The dashed and dotted line "flow velocity" is a waveform that represents the change in the flow velocity of the ink flowing into the nozzle 8. The dashed line "meniscus" is a waveform that represents the change in the position of the ink liquid surface. In FIG. 26, the horizontal axis represents the passage of time (s), and the vertical axis represents the normalized magnitude of each item.

As shown in FIG. 26, by applying an auxiliary pulse Pc prior to the drive pulse for ejecting ink, the history of the actuator drive direction before ink ejection becomes the contraction direction. In addition, residual vibration of the pressure chamber 15 is canceled. Thus, ink is ejected continuously by the multi-drop method at a constant ejection speed.

Even when the auxiliary pulse Pc having such a configuration is applied to the actuator prior to the drive pulse for ejecting ink, it is possible to obtain the same effect as when the auxiliary pulse Pa of the first embodiment or the auxiliary pulse Pb of the second embodiment is applied.
Moreover, compared to the auxiliary pulse Pb, the auxiliary pulse Pc has the advantage of being able to further reduce the maximum pressure while maintaining the maximum displacement of the pressure chamber 15 at a predetermined value.

In this way, the change in pressure that occurs within the pressure chamber 15 decreases in the order of auxiliary pulse Pa, auxiliary pulse Pb, and auxiliary pulse Pc. And, in that order, ejection due to the auxiliary pulses is less likely to occur. However, for ink that has thixotropy, there is an advantage in that the effect of reducing the viscosity of the ink during ejection is greater in the order of auxiliary pulse Pa, auxiliary pulse Pb, and auxiliary pulse Pc.

In addition, by adjusting the waveform of the trailing edge of the auxiliary pulse Pc, it is possible to intentionally leave residual vibrations, as with the auxiliary pulse Pe or auxiliary pulse Pf, and adjust the ejection speed of the first or second drop.

Figure 27 is an explanatory diagram of an auxiliary pulse Pg that intentionally leaves residual vibration after resetting the history as a modified example of the auxiliary pulse Pc. Note that in Figure 27, the vertical axis, horizontal axis, solid line "driving voltage", dashed line "pressure", dashed line "flow velocity", and dashed line "meniscus" are the same as those explained in Figure 15, so explanations will be omitted here.

At its trailing edge, the auxiliary pulse Pc gradually returns the pressure chamber 15. At its trailing edge, the auxiliary pulse Pg, like the auxiliary pulses Pe and Pf, sharply returns the pressure chamber 15, leaving behind a pressure vibration in the pressure chamber 15. While this pressure vibration remains, the output of the drive pulse begins. By doing so, it is possible to adjust the ejection speed of the first or second drop. The auxiliary pulse Pg gradually increases the voltage from 0V, which is the steady state, gradually contracting the pressure chamber 15, and after maintaining the voltage for a certain period of time, the voltage is reduced back to 0V.

Figure 28 shows the simulation results when the ejection speed of the first drop is increased, and Figure 29 shows the simulation results when the ejection speed of the second drop is increased. Figures 28 and 29 show an example in which the auxiliary pulse Pg is applied once, and then the drive pulse is applied three times.

The difference between the simulation in FIG. 28 and the simulation in FIG. 29 is the waiting time from when the gradual return operation of the auxiliary pulse Pg ends until the leading edge of the expansion pulse Pd of the drive pulse. By changing the waiting time, it is possible to increase the ejection speed of the first drop or the ejection speed of the second drop.

The above provides examples of auxiliary pulses that do not eject ink and are applied to the actuator prior to the drive pulse for ejecting ink, including auxiliary pulse Pa, auxiliary pulse Pb, auxiliary pulse Pc, auxiliary pulse Pe, and auxiliary pulse Pf, but the auxiliary pulses are not limited to these.

For example, when the pressure chamber 15 is contracted at its leading edge, it may be a stepwise contraction like auxiliary pulse Pb, and when the pressure chamber 15 is returned at its trailing edge, it may be a gradual return like auxiliary pulse Pc. Alternatively, when the pressure chamber 15 is contracted at its leading edge, it may be a gradual contraction like auxiliary pulse Pc, and when the pressure chamber 15 is returned at its trailing edge, it may be a gradual return like auxiliary pulse Pa or auxiliary pulse Pb.

When the pressure chamber 15 in the contracted state is gradually restored by the auxiliary pulse Pa or auxiliary pulse Pb, the number of stages is not limited to two stages. The pressure chamber 15 may be restored in three or more stages.

When the auxiliary pulse Pb causes the pressure chamber 15 to contract stepwise, the number of steps is not limited to two. The pressure chamber 15 may be contracted in three or more steps.

The time from the start to the end of the auxiliary pulse Pa, auxiliary pulse Pb, auxiliary pulse Pc, auxiliary pulse Pe, and auxiliary pulse Pf is set to be equal to or greater than the pressure vibration period T of the pressure chamber 15. In this regard, as long as the value is close to the pressure vibration period T of the pressure chamber 15, it does not necessarily have to be equal to or greater than the pressure vibration period T.

Although several other 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 implemented 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 invention and its equivalents described in the claims.
In addition, the claims as originally filed of this application are set forth below.
[C1]
A pressure chamber that contains ink;
a nozzle plate having a nozzle communicating with the pressure chamber;
an actuator provided in correspondence with the pressure chamber and configured to change the volume of the pressure chamber; and a drive circuit configured to drive the actuator;
Equipped with
The drive circuit includes:
an inkjet head in which, prior to a drive pulse for ejecting ink, an auxiliary pulse that does not eject ink is applied to the actuator, which drives the pressure chamber in a direction to contract, thereby increasing the pressure in the pressure chamber, and then maintains the pressure chamber in a contracted state until the pressure decreases and becomes negative pressure, then increases again and becomes positive pressure, and then a pulse that returns the pressure chamber to its original state is applied to the actuator.
[C2]
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 and displacing a volume of the pressure chamber;
A drive circuit for driving the actuator;
Equipped with
The drive circuit includes:
An inkjet head that provides the actuator with a pulse that acts as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, drives the pressure chamber in a direction to contract, maintains the contracted state, and then returns to its original state, the time from the start to the end of the pulse being equal to or longer than the pressure vibration period of the pressure chamber.
[C3]
The inkjet head described in C1 or C2, wherein the drive pulse is a pulse that draws in ink by expanding the pressure chamber, and after the pressure of the ink in the pressure chamber changes from negative pressure to positive pressure, returns the pressure chamber from the expanded state to further increase the pressure and ejects ink, and then contracts and expands the pressure chamber to cancel residual vibration.
[C4]
The inkjet head according to any one of C1 to C3, wherein the auxiliary pulse contracts the pressure chamber stepwise in a plurality of times at a leading edge thereof.
[C5]
The inkjet head according to any one of C1 to C3, wherein the auxiliary pulse gradually contracts the pressure chamber at a leading edge thereof.
[C6]
The inkjet head according to any one of C1 to C3, wherein the auxiliary pulse causes the pressure chamber to return from the contracted state stepwise in a plurality of times at a trailing edge thereof.
[C7]
The inkjet head according to any one of C1 to C6, wherein the auxiliary pulse reduces residual vibration of the pressure chamber at its trailing edge.
[C8]
The inkjet head according to any one of C1 to C6, wherein the auxiliary pulse starts outputting the drive pulse at its trailing edge while residual vibration remains in the pressure chamber.
[C9]
The inkjet head according to any one of C1 to C8, wherein adjacent pressure chambers share the actuator.

4...electrode, 7...nozzle plate, 8...nozzle, 15, 151, 152, 153...pressure chamber, 16, 161, 162...partition wall, 100...inkjet head, 101...head drive circuit, Pa, Pb, Pc, Pe, Pf, Pg...auxiliary pulse, Pd...expansion pulse, Pp...contraction pulse.

Claims (10)

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 and displacing a volume of the pressure chamber;
A drive circuit for driving the actuator;
Equipped with
The drive circuit includes:
a pulse is applied to the actuator as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, which drives the pressure chamber in a direction to contract, thereby increasing the pressure in the pressure chamber, and then maintains the contracted state of the pressure chamber until the pressure decreases and turns to negative pressure, then increases again and turns to positive pressure, and then returns the pressure chamber to its original state ;
The auxiliary pulse causes the pressure chamber to return from the contracted state stepwise in a plurality of times at a trailing edge of the auxiliary pulse . 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 and displacing a volume of the pressure chamber;
A drive circuit for driving the actuator;
Equipped with
The drive circuit includes:
a pulse is applied to the actuator as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, which drives the pressure chamber in a direction to contract, thereby increasing the pressure in the pressure chamber, and then maintains the contracted state of the pressure chamber until the pressure decreases and turns to negative pressure, then increases again and turns to positive pressure, and then returns the pressure chamber to its original state ;
The auxiliary pulse causes the pressure chamber to gradually contract at a leading edge of the auxiliary pulse . 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 and displacing a volume of the pressure chamber;
A drive circuit for driving the actuator;
Equipped with
The drive circuit includes:
a pulse is applied to the actuator as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, which drives the pressure chamber in a direction to contract, thereby increasing the pressure in the pressure chamber, and then maintains the contracted state of the pressure chamber until the pressure decreases and turns to negative pressure, then increases again and turns to positive pressure, and then returns the pressure chamber to its original state ;
The auxiliary pulse causes the pressure chamber to contract stepwise in a plurality of times at a leading edge of the auxiliary pulse . 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 and displacing a volume of the pressure chamber;
A drive circuit for driving the actuator;
Equipped with
The drive circuit includes:
a pulse is applied to the actuator as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, the pulse being a pulse that drives the pressure chamber in a direction to contract and returns to its original state after maintaining the contracted state, the pulse having a time period from start to finish that is equal to or longer than the pressure vibration period of the pressure chamber ;
The auxiliary pulse causes the pressure chamber to return from the contracted state stepwise in a plurality of times at a trailing edge of the auxiliary pulse . 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 and displacing a volume of the pressure chamber;
A drive circuit for driving the actuator;
Equipped with
The drive circuit includes:
a pulse is applied to the actuator as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, the pulse being a pulse that drives the pressure chamber in a direction to contract and returns to its original state after maintaining the contracted state, the pulse having a time period from start to finish that is equal to or longer than the pressure vibration period of the pressure chamber ;
The auxiliary pulse causes the pressure chamber to gradually contract at a leading edge of the auxiliary pulse . 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 and displacing a volume of the pressure chamber;
A drive circuit for driving the actuator;
Equipped with
The drive circuit includes:
a pulse is applied to the actuator as an auxiliary pulse that does not eject ink prior to a drive pulse for ejecting ink, the pulse being a pulse that drives the pressure chamber in a direction to contract and returns to its original state after maintaining the contracted state, the pulse having a time period from start to finish that is equal to or longer than the pressure vibration period of the pressure chamber ;
The auxiliary pulse causes the pressure chamber to contract stepwise in a plurality of times at a leading edge of the auxiliary pulse . 7. An inkjet head according to claim 1, wherein the drive pulse is a pulse that expands the pressure chamber to draw in ink, and then returns the pressure chamber from the expanded state after the pressure of the ink in the pressure chamber has changed from negative pressure to positive pressure, thereby further increasing the pressure and ejecting ink, and then contracts and expands the pressure chamber to cancel residual vibration. The ink jet head according to claim 1 , wherein the auxiliary pulse reduces residual vibration of the pressure chamber at a trailing edge of the auxiliary pulse . 8. The ink jet head according to claim 1 , wherein the auxiliary pulse starts outputting the drive pulse at a trailing edge of the auxiliary pulse while residual vibration remains in the pressure chamber. The ink jet head according to claim 1 , wherein adjacent pressure chambers share the actuator.