JP2008023865A - Droplet ejection apparatus and droplet discharge method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To discharge fine droplets while the generation of satellite droplets is preventing. <P>SOLUTION: A drive signal 75 having first pull signal S1, push signal S2, second pull signal S3 and third pull signal S4 is given to a piezoelectric element which changes a volume of a pressure chamber. When the resonance frequency of the pressure chamber is represented by Tc, each interval of the start time t1 of the S1, the start time t2 of the S2, the start time t3 of the S3 and the start time t4 of the S4 is Tc/2. Also, a ratio of the amplitude A1 of the S1, the amplitude A2 of the S2 of the S2 and the amplitude A3 of the S3 is determined so that the pressure change of the liquid in the pressure chamber from the t1 to the t4 having a predetermined amplitude becomes a sinusoidal waveform having a predetermined amplitude, and the amplitude A4 of the S4 is determined so as to brake the vibration of a meniscus. More specifically, A1:A2:A3:A4=e<SP>α π/2</SP>:(e<SP>α π/2</SP>-e<SP>-α π/2</SP>):(e<SP>α π/2</SP>-e<SP>-α π/2</SP>):e<SP>-α π/2</SP>is made. Here, the α of a system constituted by filling a discharge element with the liquid is an attenuation coefficient, and e is the base of a natural logarithm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a droplet discharge device and a droplet discharge method capable of discharging minute droplets while preventing the generation of satellite droplets.

  A nozzle that discharges droplets, a pressure chamber that communicates with the nozzle, a liquid supply port that communicates with the pressure chamber, a piezoelectric element that changes the volume of the pressure chamber, and a drive unit that provides a drive signal to the piezoelectric element. There is known a liquid droplet ejection apparatus provided.

  The drive signal given to the piezoelectric element includes a pull signal for expanding the pressure chamber (sometimes referred to as “Pull”) and a push signal for contracting the pressure chamber (also referred to as “Push”). It is comprised including.

In general, a pull-push-pull waveform is used when discharging a minute droplet. Japanese Patent Application Laid-Open No. H10-228667 describes a method in which droplets are ejected using a Push-Pull-Push-Push-Pull waveform. Japanese Patent Application Laid-Open No. H10-228667 describes a method in which droplets are ejected using a Pull-Push-Pull-Pull waveform.
Japanese Patent No. 3711443 (particularly FIG. 3) JP 2004-142448 A (particularly FIG. 5)

  When ejecting micro droplets, the Pull-Push-Pull waveform has generally been used, but if you try to eject highly viscous ink using this waveform, it will cause a long tail and become unstable. There is a problem that small satellite droplets are generated. For example, it has been difficult to discharge a liquid having a high viscosity of 20 cP or more as a fine droplet of 1 pl or less without satellite droplets.

  FIG. 12 shows a variation 96 of the pressure P (ink generation pressure) generated in the ink in the pressure chamber due to the drive signal having a Pull-Push-Pull waveform being applied to the piezoelectric element. In FIG. 12, time t = 0 is the start time of the first pull signal, time t = (1/2) · Tc is the start time of the push signal, and time t = Tc is the start time of the second pull signal. ing. From time t = 0 to time t = (3/2) · Tc, the amplitude of the ink generation pressure P increases and ink droplets are ejected at a high speed, while after time t = (3/2) · Tc. A vibration with a large amplitude remains.

  Referring to the schematic diagram, as shown in FIG. 13A, an ink droplet 94 having a diameter larger than the diameter of the nozzle 51 is ejected from the nozzle 51, but a long tail 92 is drawn. Due to such a long tail 92, satellite drops 95 are inevitably generated as shown in FIG.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a droplet discharge device and a droplet discharge method capable of discharging minute droplets while preventing generation of satellite droplets.

  In order to achieve the above object, the invention according to claim 1 includes a nozzle that discharges droplets, a pressure chamber that communicates with the nozzle, a liquid supply port that communicates with the pressure chamber, and a volume of the pressure chamber. A discharge element including a piezoelectric element to be changed; and a driving unit that applies a driving signal to the piezoelectric element. The driving signal expands the pressure chamber to make the meniscus of the nozzle concave. A second signal element that contracts the pressure chamber to make the meniscus of the nozzle convex, and a third that generates a droplet from the convex meniscus of the nozzle by expanding the pressure chamber. And a third signal element that expands the pressure chamber to brake residual vibration of the meniscus of the nozzle, and the driving means is configured by filling the discharge element with liquid. System When the resonance period is Tc, the start time t1 of the first signal element, the start time t2 of the second signal element, the start time t3 of the third signal element, and the start time of the fourth signal element The intervals t4 | t1-t2 |, | t2-t3 |, and | t3-t4 | are all Tc / 2, and the pressure fluctuation of the liquid in the pressure chamber has a constant amplitude from t1 to t4. The drive signal in which the ratio of the amplitude A1 of the first signal element, the amplitude A2 of the second signal element, and the amplitude A3 of each of the third signal elements is determined so as to have a sine wave shape is the piezoelectric signal. Provided is a droplet discharge device characterized by being applied to an element.

  According to the present invention, the pressure fluctuation of the liquid in the pressure chamber from the start time t1 of the first pull signal to the start time t4 of the third pull signal is formed into a sine wave shape having a constant amplitude. Even without applying excessive pressure to the liquid, that is, droplets can be generated from the meniscus at a lower discharge speed compared to the conventional method, preventing the occurrence of satellite droplets by preventing the occurrence of long tails from the droplets. It is possible to discharge minute droplets while preventing them. Further, since the residual vibration of the meniscus is damped by the fourth signal element, even if the discharge speed is low, the droplets surely fly without being pulled back to the meniscus.

According to a second aspect of the present invention, in the first aspect of the present invention, the amplitude A1 of the first signal element, the amplitude A2 of the second signal element, the amplitude A3 of the third signal element, and the first The ratio A1: A2: A3: A4 of the amplitude A4 of the signal element 4 is expressed as an index using the damping coefficient α of the system configured by filling the ejection element with liquid and the base e of the natural logarithm. , E ^{α · π / 2} : (e ^{α · π / 2} − e ^{−α · π / 2} ): (e ^{α · π / 2} − e ^{−α · π / 2} ): e ^{−α · π / 2} There is provided a droplet discharge device characterized by the above.

  Here, the attenuation coefficient α is obtained when a pulse waveform (pulse voltage) is applied to a piezoelectric element in a system configured by filling a discharge element including a nozzle, a pressure chamber, a liquid supply port, and a piezoelectric element. Further, it is the natural logarithm of the amplitude ratio of the damping free vibration waveform of the pressure generated in the liquid in the pressure chamber.

  According to a third aspect of the present invention, there is provided a discharge element comprising a nozzle that discharges droplets, a pressure chamber that communicates with the nozzle, a liquid supply port that communicates with the pressure chamber, and a piezoelectric element that changes the volume of the pressure chamber. A droplet discharge method for discharging droplets from the nozzles using the first and second driving signals applied to the piezoelectric element causes the pressure chamber to expand to make the meniscus of the nozzles concave. A second signal element that contracts the pressure chamber to make the meniscus of the nozzle convex, and a third that generates a droplet from the convex meniscus of the nozzle by expanding the pressure chamber. A resonance period of a system configured to include a signal element and a third signal element that expands the pressure chamber to brake residual vibration of the meniscus of the nozzle and is configured by filling the discharge element 54 with liquid. With Tc Intervals of the first signal element start time t1, the second signal element start time t2, the third signal element start time t3, and the fourth signal element start time t4. t1-t2 |, | t2-t3 |, and | t3-t4 | are all Tc / 2, and the pressure variation of the liquid in the pressure chamber has a constant amplitude from t1 to t4. The drive signal in which the ratio of the amplitude A1 of the first signal element, the amplitude A2 of the second signal element and the amplitude A3 of each of the third signal elements is determined is applied to the piezoelectric element. A droplet discharge method is provided.

According to a fourth aspect of the present invention, in the third aspect of the present invention, the amplitude A1 of the first signal element, the amplitude A2 of the second signal element, the amplitude A3 of the third signal element, and the second The ratio A1: A2: A3: A4 of the amplitude A4 of the signal element 4 is expressed as an index using the damping coefficient α of the system configured by filling the ejection element with liquid and the base e of the natural logarithm. , E ^{α · π / 2} : (e ^{α · π / 2} − e ^{−α · π / 2} ): (e ^{α · π / 2} − e ^{−α · π / 2} ): e ^{−α · π / 2} There is provided a droplet discharge method characterized by the above.

  According to the present invention, it is possible to discharge minute droplets while preventing generation of satellite droplets.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a block diagram showing the overall configuration of an image forming apparatus 10 as an example of a droplet discharge apparatus according to the present invention.

  1, the image forming apparatus 10 mainly includes a droplet discharge head 50, a communication interface 110, a system controller 112, memories 114 and 152, a transport unit 116, a transport driver 118, a liquid supply unit 142, a liquid supply control unit 144, It includes an ink identification information acquisition unit 146, a print control unit 150, and a head driver 154 (driving means).

  The droplet discharge head 50 has a number of nozzles arranged two-dimensionally and discharges ink droplets. The image forming apparatus 10 of this example includes a total of four droplet discharge heads, one for each ink color of black (K), cyan (C), magenta (M), and yellow (Y).

  The communication interface 110 receives image data transmitted from the host computer 300. As the communication interface 110, a wired or wireless interface such as USB (Universal Serial Bus) or IEEE1394 can be applied. Image data taken into the image forming apparatus 10 by the communication interface 110 is temporarily stored in the first memory 114 for storing the image data.

  The system controller 112 includes a microcomputer and its peripheral circuits, and controls the entire image forming apparatus 10 according to a predetermined program. That is, the system controller 112 controls each unit such as the communication interface 110, the conveyance driver 118, and the print control unit 150.

  The transport unit 116 includes a roller (not shown) for transporting a medium to be ejected such as paper, and a motor (not shown) for supplying power to the roller. The medium to be ejected and the droplet ejection head 50 are relatively moved by the conveyance unit 116. The transport driver 118 includes a circuit that drives the transport unit 116 in accordance with an instruction from the system controller 112.

  The liquid supply unit 142 stores an ink tank (not shown) for storing ink, a pipe line (not shown) from the tank to the droplet discharge head 50, and drops ink in the tank through the pipe line to the droplet discharge head 50. It includes a pump (not shown) for feeding liquid. For example, a replaceable cartridge type ink tank is used.

  The liquid supply control unit 144 includes a microcomputer and its peripheral circuits, and controls the supply of ink to the droplet discharge head 50 using the liquid supply unit 142 according to a predetermined program.

  The ink identification information acquisition unit 146 acquires ink identification information for identifying the type of ink supplied to the droplet discharge head 50. There are various ways of acquiring the ink identification information. For example, an aspect of reading the ink identification information printed as a barcode on the ink tank of the liquid supply unit 142, an IC tag attached to the ink tank of the liquid supply unit 142 ( There is a mode in which ink identification information stored in advance in “RFID: Radio Frequency Identification”) is received wirelessly. Ink identification information may be received from the host computer 300 using the communication interface 110.

  The print control unit 150 includes a microcomputer and its peripheral circuits, and controls the liquid supply control unit 144, the ink identification information acquisition unit 146, and the head driver 154 according to a predetermined program. The print control unit 150 forms dots on the medium to be ejected by the droplet ejection head 50 ejecting (droplet ejection) toward the medium to be ejected based on the image data input to the image forming apparatus 10. Data (dot data) required for In other words, the print controller 150 generates dot ejection dot data from the image data in the first memory 114 under the control of the system controller 112, and supplies the generated dot data to the head driver 154. The print controller 150 is accompanied by a second memory 152, and dot data and the like are temporarily stored in the second memory 152 during image processing in the print controller 150.

  The head driver 154 generates an ejection drive signal for the droplet ejection head 50 based on the dot data (actually the dot data stored in the second memory 152) given from the print control unit 150. To do. The drive signal output from the head driver 154 is applied to the droplet discharge head 50 (specifically, a piezoelectric element 58 in FIG. 3 described later), whereby an ink droplet is directed from the droplet discharge head 50 toward the discharge target medium. Is discharged.

  Further, the print control unit 150 selects the waveform of the drive signal generated by the head driver 154 based on the ink identification information acquired by the ink identification information acquisition unit 146. Specifically, the second memory 152 stores in advance table information indicating the correspondence between the ink identification information and the waveform of the drive signal (drive waveform). The print controller 150 stores the second memory 152 in the second memory 152. The drive waveform corresponding to the ink identification information acquired by the ink identification information acquisition unit 146 is selected from the plurality of drive waveforms registered in the table information 152. The drive waveform selected by the print control unit 150 is instructed to the head driver 154.

  In FIG. 1, the system controller 112, the liquid supply control unit 144, and the print control unit 150 are illustrated separately, but may be configured by a single microcomputer.

  FIG. 2 is a plan perspective view showing an overall configuration of an example of the droplet discharge head 50 of FIG. 1, and FIG. 3 is a cross-sectional view taken along line 3-3 of FIG.

  A head 50 shown as an example in FIG. 2 is a so-called full-line head, and is indicated by a direction (indicated by an arrow M in the drawing) orthogonal to the transport direction of the medium 16 (sub-scanning direction indicated by an arrow S in the drawing). In the main scanning direction), a plurality of nozzles 51 for ejecting ink toward the ejection medium 16 are two-dimensionally arranged over a length corresponding to the width Wm of the ejection medium 16. .

  The droplet discharge head 50 includes a nozzle 51, a pressure chamber 52 that communicates with the nozzle 51, a liquid supply port 53 that communicates with the pressure chamber 52, and a plurality of ejection elements 54 including a piezoelectric element (58 in FIG. 3) described later. The main scanning direction M and the main scanning direction M are arranged along two oblique directions that form a predetermined acute angle θ (0 degree <θ <90 degrees). In FIG. 2, only a part of the ejection elements 54 is illustrated for convenience of illustration.

  Specifically, the nozzles 51 are arranged at a constant pitch d in an oblique direction that forms a predetermined acute angle θ with respect to the main scanning direction M, and thus, “on a straight line along the main scanning direction M” d × cos θ ”can be handled equivalently.

  FIG. 3 shows a cross-sectional view of one ejection element 54 constituting the head 50 along the line 3-3 in FIG.

  As shown in FIG. 3, each pressure chamber 52 is further in communication with a common liquid chamber 55 via a liquid supply port 53. The common liquid chamber 55 communicates with an ink tank that is a liquid supply source (not shown), and ink supplied from the ink tank is distributed and supplied to each pressure chamber 52 via the common liquid chamber 55.

  A piezoelectric body 58a is disposed on the diaphragm 56 constituting the top surface of the pressure chamber 52, and an individual electrode 57 is disposed on the piezoelectric body 58a. The diaphragm 56 is grounded and functions as a common electrode. The diaphragm 56, the individual electrode 57, and the piezoelectric body 58a constitute a piezoelectric element 58 as means for generating droplet ejection pressure on the ink in the pressure chamber 52.

  When the drive signal generated by the head driver 154 of FIG. 1 is applied to the piezoelectric element 58, that is, when a predetermined drive voltage is applied to the individual electrode 57 of the piezoelectric element 58, the piezoelectric body 58a is deformed and pressure is applied. The volume of the chamber 52 changes, and ink is ejected from the nozzle 51 by the change in the pressure in the pressure chamber 52 accompanying this. Accompanying the ink ejection, new ink is supplied from the common liquid chamber 55 to the pressure chamber 52 through the liquid supply port 53.

  FIG. 2 shows an example in which a plurality of nozzles 51 are two-dimensionally arranged as a structure that can form a high-resolution image on the ejection target medium 16 at high speed. The structure in which the plurality of nozzles 51 are two-dimensionally arranged is not particularly limited, and a structure in which the plurality of nozzles 51 are one-dimensionally arranged may be used. Further, the ejection element 54 constituting the droplet ejection head is an example, and is not particularly limited to such a case. For example, instead of disposing the common liquid chamber 55 below the pressure chamber 52 (that is, the nozzle surface 510 side relative to the pressure chamber 52), the common liquid is disposed above the pressure chamber 52 (that is, opposite to the nozzle surface 510). The chamber 55 may be disposed.

  The nozzle 51, the pressure chamber 52, the liquid supply port 53, and the piezoelectric element 58 shown in FIG.

  FIG. 4 is a waveform diagram showing a drive signal 75 generated by the head driver 154 of FIG. 1 and applied to one piezoelectric element 58 of FIG. 2, and the vertical axis indicates the voltage V (drive voltage) of the drive signal 75. The horizontal axis is time t.

  In FIG. 4, a drive signal 75 is a waveform of one unit for ejecting one ink droplet from one nozzle 51. In the case where ink droplets are continuously ejected from one nozzle 51, the drive signal 75 shown in FIG. 4 is repeatedly given to one piezoelectric element 58.

  The drive signal 75 includes a first pull signal S1 (first signal element) having a rising waveform rising from the initial voltage V0 to the first voltage V1 (> V0), and the first voltage V1 to the second voltage. A push signal S2 (second signal element) having a falling waveform falling to V2 (<V1) and a second pull having a rising waveform rising from the second voltage V2 to the third voltage V3 (> V2) A signal S3 (third signal element), a third pull signal S4 (fourth signal element) having a rising waveform rising from the third voltage V3 to the fourth voltage V4 (> V3), and a fourth It is configured to include a return signal S5 (fifth signal element) having a waveform descending from the voltage V4 to the initial voltage V0 with a gentle gradient. In other words, the drive signal 75 includes a first pull signal S1 having an amplitude A1 (= | V0−V1 |), a push signal S2 having an amplitude A2 (= | V1−V2 |), and an amplitude A3 (= | Second pull signal S3 having V2-V3 |), third pull signal S4 having amplitude A4 (= | V3-V4 |), and amplitude A5 (= | V4-V0 | = A1-A2 + A3 + A4). And a return signal S5.

  When the resonance period of the system configured by filling the ejection element 54 with liquid is represented by Tc and the start time t1 of the first pull signal S1 is “0”, the start time t2 of the push signal S2 is (1 / 2) · Tc, the start time t3 of the second pull signal S3 is Tc, and the start time t4 of the third pull signal S4 is (3/2) · Tc. In other words, intervals t1, t2, t3, and t4 | t1-t2 |, | t2-t3 |, and | t3-t4 | are all set to Tc / 2.

  In order to provide the first pull signal S1, the push signal S2, the second pull signal S3, and the third pull signal S3 to the piezoelectric element 58 at the interval Tc / 2, the first pull signal S1, the push signal S2, Subsequent to the second pull signal S3, predetermined periods d1, d2, and d3 for holding the driving voltage V constant are provided. Further, following the third pull signal S3, a period d4 in which the drive voltage V is held constant is provided in order to ensure reliable flight of the ink droplets.

  The first pull signal S1 is a signal element that expands the pressure chamber 52 and pulls the meniscus so that the meniscus of the nozzle 51 has a concave shape.

  The push signal S2 is a signal element that presses the meniscus so that the pressure chamber 52 contracts and the meniscus of the nozzle 51 has a convex shape.

  The second pull signal S3 is a signal element that expands the pressure chamber 52 and pulls the meniscus of the nozzle 51 so that ink droplets are formed from the convex meniscus.

  The third pull signal S4 is a signal element that expands the pressure chamber 52 and brakes the residual vibration of the meniscus of the nozzle 51. In other words, the third pull signal S4 indicates that when the ink droplet is about to be separated from the meniscus and immediately after the ink droplet is separated from the meniscus, the meniscus becomes convex again and the ink droplet is pulled back to the meniscus. prevent.

  The return signal S5 is a signal element that returns the drive voltage to the initial voltage V0 immediately before application of the first pull signal S1. The gradient of the voltage change of the return signal S5 is made smaller than the gradient of the voltage change of the push signal S2 to such an extent that no droplet is ejected from the nozzle 51.

The ratio A1: A2: A3: A4 of the amplitude A1 of the first pull signal S1, the amplitude A2 of the push signal S2, the amplitude A3 of the second pull signal S3, and the amplitude A4 of the third pull signal S4 is attenuated. Using the coefficient α and the base e of the natural logarithm, the following equation 1 is shown.
[Equation 1]
A1: A2: A3: A4 = e ^{α · π / 2} : (e ^{α · π / 2} −e ^{−α · π / 2} ) ^: (e ^{α · π / 2} −e ^{−α · π / 2} ): e ^{-Α ・ π / 2}
Here, α is a pulse waveform (pulse voltage) applied to the piezoelectric element 58 in a system in which the ejection element 54 including the nozzle 51, the pressure chamber 52, the liquid supply port 53, and the piezoelectric element 58 is filled with ink. Is the natural logarithm of the amplitude ratio of the damped free vibration waveform of the pressure generated in the ink in the pressure chamber 52.

  For example, when α = 0.3, A1 = 7.2 [V], A2 = 4.4 [V], A3 = 4.4 [V], and A4 = 2.8 [V].

  FIG. 5 is a circuit diagram showing an equivalent circuit 80 in which a system configured by filling one ejection element 54 constituting the droplet ejection head 50 shown in FIG. 3 with ink is modeled as an acoustic circuit.

  In FIG. 5, the first circuit element 81 corresponds to the piezoelectric element 58 of FIG. 3, and is formed by connecting a compliance Cp, an inertance Lp, and a resistance Rp in series. The second circuit element 82 corresponds to the pressure chamber 52 of FIG. 3, and is formed by connecting a resistance Rs and an inertance Ls in series. The third circuit element 83 corresponds to the liquid supply port 53 of FIG. 3 and includes a compliance Cc. The fourth circuit element 84 corresponds to the nozzle 51 of FIG. 3 and is formed by connecting an inertance Ln, a resistance Rn, and a compliance Cn in series.

  The input terminal 85 is disposed at one end of the first circuit element 81 corresponding to the piezoelectric element 58 and is applied with a driving voltage V. In other words, the drive signal V (t) given from the head driver 154 of FIG. 1 to the piezoelectric element 58 is given to the input terminal 85.

  The output terminal 86 is disposed at the other end opposite to the input terminal 85 of the first circuit element 81 corresponding to the piezoelectric element 58, and the second circuit element 82 corresponding to the pressure chamber 52 and the liquid supply port 53. It is connected to a corresponding third circuit element 83 and a fourth circuit element 84 corresponding to the nozzle 51, and a response voltage P with respect to the drive voltage V applied to the input terminal 85 is output. In other words, a response corresponding to the pressure generated in the ink in the pressure chamber 52 (internal pressure in the pressure chamber 52) when the drive signal V (t) is given from the head driver 154 in FIG. 1 to the piezoelectric element 58 in FIG. The signal P (t) is output to the output terminal 86. Here, t indicates time. Hereinafter, the response voltage P may be referred to as “ink generation pressure”.

  Specifically, the attenuation coefficient α of the equation 1 is expressed by the following equation 2.

Here, Ln, Ls, Cp, Cc, Rn, and Rs are values of elements constituting an equivalent circuit of the ejection element 54 filled with liquid. Specifically, Ln is the inertance of the nozzle 51, Ls is the inertance of the liquid supply port 53 (squeezing), Cp is the compliance of the piezoelectric element 58 (actuator), Cc is the compliance of the pressure chamber 52, Rn is the resistance of the nozzle 51, Rs Is the resistance of the liquid supply port 53 (squeezing).

  When the drive signal 75 of FIG. 4 is input to the input terminal 85 of the equivalent circuit 80, the response signal 76 of FIG. 6 is output to the output terminal 86 of the equivalent circuit 80. In FIG. 6, the response signal 76 is represented by normalizing the central value corresponding to the atmospheric pressure as “0” and the maximum value as “1.0”.

  In FIG. 6, time t = 0 is the start time t1 of the first pull signal S1. The pressure chamber 52 is expanded by the first pull signal S1, and the ink generation pressure P reaches the maximum value (1.0) at time t = (1/4) · Tc. Time t = (1/2) · Tc is the start time t2 of the push signal S2. The pressure chamber 52 contracts by the push signal S2, and the ink generation pressure P becomes the minimum value (−1.0) at time t = (3/4) · Tc. Time t = Tc is the start time t3 of the second pull signal S3. The pressure chamber 52 is expanded by the second pull signal S3, and the ink generation pressure P reaches the maximum value (1.0) at time t = (5/4) · Tc. Time t = (3/2) · Tc is the start time t4 of the third pull signal S4.

  The response signal 76, that is, the fluctuation P (t) of the ink generation pressure in the pressure chamber 52, is constant amplitude As and resonance period from the start time t1 of the first pull signal S1 to the start time t4 of the third pull signal S4. A sinusoidal waveform 761 having Tc is obtained. Further, P (t) becomes a steady waveform 762 whose fluctuation is maintained within the allowable range after the start time t4 of the third pull signal S4.

  FIG. 7A and FIG. 7B schematically show how the droplet changes after t4.

  FIG. 7A shows a state immediately after t 4, and a tail 32 is connected to the flying ink droplet 34, and this tail 32 is connected to the meniscus 30 of the nozzle 51. Since the flying speed of the ink droplet 34 is small, the tail 32 of the ink droplet 34 is short. Thereafter, as shown in FIG. 7B, the tail 32 of the ink droplet 34 is cut, and the ink droplet 34 is completely separated from the meniscus 30 of the nozzle 51. Here, since the tail 32 of the ink droplet 34 is short, almost no satellite droplet is generated. Further, after t4, the meniscus of the nozzle 51 is maintained in a concave shape, so that the ink droplet 34 is not pulled back by the meniscus of the nozzle 51.

  Next, an example of a method for calculating the amplitudes A1, A2, A3, and A4 of the four signal elements S1, S2, S3, and S4 constituting the drive signal 75 in FIG. 4 will be described in detail.

  When a step-like signal element (here, a vertical rising waveform or a vertical falling waveform) is input to the piezoelectric element 58, the fluctuation of the pressure generated in the ink in the pressure chamber 52 (internal pressure in the pressure chamber 52) is expressed by the following number. 3 shows the response signal Pi (t).

[Equation 3]
Pi (t) = Gi × e ^{(−α × 2π · fc · t)} sin (2π · fc · t)
Here, i is an index of a signal element, Gi is a gain, e is a base of natural logarithm, α is an attenuation coefficient of a system configured by filling the ejection element 54 with liquid, and fc is filled with the liquid. The resonance frequency of the system, t, is the time (elapsed time) when the application time of the stepped signal element is set to “0”. Note that fc = 1 / Tc, where Tc is the resonance period of the system configured by filling the ejection element 54 with liquid.

  In the following, a signal consisting of a vertical rising waveform simulating the first pull signal S1 in FIG. 4, a signal consisting of a vertical falling waveform simulating the push signal S2, and a vertical rising waveform simulating the second pull signal S3 are shown. Signals and vertical rising waveforms that simulate the third pull signal S4 are referred to as a first pseudo signal SS1, a second pseudo signal SS2, a third pseudo signal SS3, and a fourth pseudo signal SS4, respectively.

  Here, the first pseudo signal SS1 has the same amplitude A1 as the first pull signal S1, the second pseudo signal SS2 has the same amplitude A2 as the push signal S2, and the third pseudo signal SS3 is the first pseudo signal SS3. The second pull signal S3 has the same amplitude A3, and the fourth pseudo signal SS4 has the same amplitude A4 as the third pull signal S4. Further, only the ink generation pressure when only the first pseudo signal SS1 is applied to the piezoelectric element 58, the ink generation pressure when only the second pseudo signal SS2 is applied to the piezoelectric element 58, and only the third pseudo signal SS3. The ink generation pressure when applied to the piezoelectric element 58 and the ink generation pressure when only the fourth pseudo signal SS4 is applied to the piezoelectric element 58 are P1, P2, P3, and P4, respectively.

  First, as shown in FIG. 8A, it is assumed that the first pseudo signal SS1 having a vertical rising waveform is input to the piezoelectric element 58 at time t = 0, and the response signal P ( t) = P1 (t) is shown in FIG. Note that the attenuation coefficient α = 0.3.

  Here, as shown in FIG. 8B, G1 that satisfies | P | = | P1 | = 1 at time t = (1/4) · Tc is obtained. Then, G1 is expressed by the following equation 4.

[Equation 4]
G1 = e ^{α · π / 2}
Subsequently, as shown in FIG. 9A, the second pseudo signal SS2 having a vertical falling waveform is input to the piezoelectric element 58 at time t = (1/2) · Tc, and Equation 3 is used. FIG. 9B shows the response signal P (t) = P1 (t) + P2 (t) calculated in this way.

  Here, as shown in FIG. 9B, G2 is obtained such that | P | = | P1 + P2 | = 1 at time t = (3/4) · Tc. Then, G2 is expressed by the following formula 5.

[Equation 5]
G2 = (e− ^{π / 2α−} eπ ^{/ 2α} )
Subsequently, as shown in FIG. 10A, it is assumed that a third pseudo signal SS3 having a vertically rising waveform is input to the piezoelectric element 58 at time t = Tc, and the response signal P ( t) = P1 (t) + P2 (t) + P3 (t) is shown in FIG.

  Here, as shown in FIG. 10B, G3 is obtained so that | P | = | P1 + P2 + P3 | = 1 at time t = (5/4) · Tc. Then, G3 is expressed by the following formula 6.

[Equation 6]
G3 = (e [ ^{pi] / 2 [} alpha] ^{-e- [ pi] / 2 [ alpha]} )
Finally, as shown in FIG. 11A, it is assumed that a fourth pseudo signal SS4 having a vertical rising waveform is input to the piezoelectric element 58 at time t = (2/3) · Tc, FIG. 11B shows the calculated response signal P (t) = P1 (t) + P2 (t) + P3 (t) + P4 (t).

  Here, as shown in FIG. 11B, G4 is determined so that P = P1 + P2 + P3 + P4 = 0 at time t ≧ (3/2) · Tc. Then, G4 is expressed by the following formula 7.

[Equation 7]
G4 = e ^{(−π / 2α)}
By the way, since A1: A2: A3: A4 = G1: G2: G3: G4, the above-described Equation 1 indicating A1: A2: A3: A4 is obtained from Equation 4, Equation 5, Equation 6, and Equation 7.

  Unlike the pseudo signals SS1, SS2, SS3, and SS4, the signal elements S1, S2, S3, and S4 that make up the actual drive signal 75 shown in FIG. 4 have gradients to prevent undershoot and overshoot. I have it. Therefore, an actual response signal, that is, 76 in FIG. 6 is confirmed by simulation using the equivalent circuit 80 of FIG.

  The ratio Es / As between the amplitude As (= 1.0) of the sine waveform 761 from time t1 to t4 and the allowable value Es of the steady waveform 762 after time t4 is 0.5 or less.

An experiment was performed using an experimental machine to which the present invention was applied. The experimental conditions are as follows.
<Conditions>
Nozzle diameter 18μm
Nozzle length 18μm
Resonance period 3.4μs
Ink viscosity 20cP
Surface tension 30mN / m
As a result of the experiment, as shown in FIG. 7B, it was confirmed that minute ink droplets can be ejected without generating satellite droplets.

  As described above, in the image forming apparatus 10 of the present embodiment, when ink is ejected, the drive signal 75 shown in FIG. 4 is given to the piezoelectric element 58 of FIG. 3 by the head driver 154 of FIG. The Specifically, first, the pressure chamber 52 in FIG. 3 is expanded by the first pull signal S1 in FIG. 4, and the meniscus of the nozzle 51 in FIG. 3 is pulled to make the meniscus concave. Next, the pressure chamber 52 is contracted by the push signal S2 of FIG. 4 and the meniscus of the nozzle 51 is pushed to make the meniscus convex. Next, the pressure chamber 52 is expanded by the second pull signal S3 in FIG. 4 and the meniscus of the nozzle 51 is pulled to make the meniscus concave. The pressure fluctuation P (t) generated in the ink in the pressure chamber 52 due to the first pull signal S1, the push signal S2, and the second pull signal S3 is from t1 to t4 as shown in FIG. Up to this point, a sine waveform 761 having a constant amplitude As is obtained, and the ink in the pressure chamber 52 is in a resonance state. As described above, the pressure variation of the ink in the pressure chamber 52 from the time t1 to the time t4 is set to the sine waveform 761 having the constant amplitude As, so that the pressure in the pressure chamber 52 is not applied more than necessary. It can improve and prevent the generation of satellite drops due to long tailing. Next, the internal pressure of the pressure chamber 52 is established in a steady state by the third pull signal S4 in FIG. 4 to brake the residual vibration of the meniscus of the nozzle 51, and the meniscus of the nozzle 51 is maintained in a concave shape. As a result, the ink droplet 34 surely flies without being pulled back to the meniscus 30.

  Although the case where the liquid is ink has been described as an example, the present invention is not limited to the case where the liquid is ink, and may be applied when discharging other liquid.

  As mentioned above, although embodiment of this invention was described in detail, this invention is not limited to the example demonstrated in this specification and the example shown in drawing, In the range which does not deviate from the summary of this invention, various Of course, design changes and improvements may be made.

1 is a block diagram illustrating an overall configuration of an image forming apparatus as an example of a droplet discharge device according to the present invention. FIG. 2 is a plan perspective view showing an overall configuration of an example of a droplet discharge head. FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2. It is a wave form diagram which shows a drive signal. It is a circuit diagram showing an equivalent circuit of one ejection element constituting the droplet ejection head. It is a figure which shows the fluctuation | variation of a response signal when the drive signal shown in FIG. 4 is input into the equivalent circuit shown in FIG. (A) is a schematic diagram of a droplet having a short tail connected to the meniscus, and (B) is a schematic diagram showing a droplet flying without generating a satellite droplet. (A) is a waveform diagram of a pseudo signal corresponding to the first pull signal, and (B) is a waveform diagram of a response signal corresponding to fluctuations in the internal pressure of the pressure chamber. (A) is a waveform diagram of a pseudo signal corresponding to a push signal, and (B) is a waveform diagram of a response signal corresponding to fluctuations in the internal pressure of the pressure chamber. (A) is a waveform diagram of a pseudo signal corresponding to the second pull signal, and (B) is a waveform diagram of a response signal corresponding to fluctuations in the internal pressure of the pressure chamber. (A) is a waveform diagram of a pseudo signal corresponding to the third pull signal, and (B) is a waveform diagram of a response signal corresponding to fluctuations in the internal pressure of the pressure chamber. It is a figure which shows the fluctuation | variation of the internal pressure of a pressure chamber in the conventional image forming apparatus. (A) is a schematic diagram of a droplet having a long tail connected to a meniscus in a conventional image forming apparatus, and (B) is a schematic diagram illustrating a droplet flying with satellite droplets in the conventional image forming apparatus.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 50 ... Droplet discharge head, 51 ... Nozzle, 52 ... Pressure chamber, 53 ... Liquid supply port, 54 ... Discharge element, 55 ... Common liquid chamber, 58 ... Piezoelectric element, 75 ... Drive signal, 80 ... Equivalent circuit, 85 ... Equivalent circuit input terminal, 86 ... Equivalent circuit output terminal, 112 ... System controller, 114, 152 ... Memory, 146 ... Ink identification information acquisition unit, 150 ... Print control unit, 154 ... Head driver (driving means), S1 ... First pull signal (first signal element), S2... Push signal (second signal element), S3... Second pull signal (third signal element), S4. S5 ... return signal, Tc ... resonance period, .alpha .... attenuation coefficient.

Claims (4)

A nozzle that discharges droplets, a pressure chamber that communicates with the nozzle, a liquid supply port that communicates with the pressure chamber, a discharge element that changes the volume of the pressure chamber, and a drive for the piezoelectric element Drive means for applying a signal,
The drive signal includes a first signal element that expands the pressure chamber to make the meniscus of the nozzle concave, and a second signal element that contracts the pressure chamber to make the meniscus of the nozzle convex. A third signal element that expands the pressure chamber to generate droplets from the convex meniscus of the nozzle, and a fourth signal element that expands the pressure chamber and brakes residual vibration of the meniscus of the nozzle. And comprising
The drive means has a start time t1 of the first signal element, a start time t2 of the second signal element, and Tc when a resonance period of a system configured by filling the discharge element with liquid is Tc. Each interval | t1-t2 |, | t2-t3 |, | t3-t4 | between the start time t3 of the third signal element and the start time t4 of the fourth signal element is set to Tc / 2, and The amplitude A1 of the first signal element, the amplitude A2 of the second signal element, and the third signal element so that the pressure fluctuation of the liquid in the pressure chamber has a constant amplitude from t1 to t4. A liquid droplet ejection apparatus, wherein the drive signal in which a ratio of amplitude A3 of each signal element is determined is applied to the piezoelectric element. The ratio A1: A2: A3: A4 of the amplitude A1 of the first signal element, the amplitude A2 of the second signal element, the amplitude A3 of the third signal element and the amplitude A4 of the fourth signal element is: When expressed by an exponent using a damping coefficient α of a system configured by filling the ejection element with a liquid and a base e of a natural logarithm, e ^{α · π / 2} : (e ^{α · π / 2} −e ^{− 2. The} droplet discharge device according to claim 1, wherein [ ^{alpha] [pi] / 2} ) :( e [ ^{alpha]. [pi] / 2} -e- [ ^{alpha] [pi] / 2} ): e- [ ^{alpha]. [pi] / 2} . Using a discharge element comprising a nozzle that discharges a droplet, a pressure chamber that communicates with the nozzle, a liquid supply port that communicates with the pressure chamber, and a piezoelectric element that changes the volume of the pressure chamber, the droplets from the nozzle A droplet discharge method for discharging
The drive signal to be applied to the piezoelectric element includes a first signal element that expands the pressure chamber to make the meniscus of the nozzle concave, and a contraction of the pressure chamber to make the meniscus of the nozzle convex. A second signal element that expands the pressure chamber to generate droplets from the convex meniscus of the nozzle, and a residual vibration of the meniscus of the nozzle that expands the pressure chamber. A fourth signal element for braking,
When the resonance period of the system configured by filling the discharge element with liquid is Tc, the start time t1 of the first signal element, the start time t2 of the second signal element, and the third signal element The intervals | t1-t2 |, | t2-t3 |, | t3-t4 | of the start time t3 of the fourth signal element and the start time t4 of the fourth signal element are all set to Tc / 2, and from t1 to t4 The amplitude A1 of the first signal element, the amplitude A2 of the second signal element, and the amplitude of each of the third signal elements so that the pressure fluctuation of the liquid in the pressure chamber has a sinusoidal shape having a constant amplitude. A droplet discharge method, wherein the drive signal having a determined A3 ratio is applied to the piezoelectric element. The ratio A1: A2: A3: A4 of the amplitude A1 of the first signal element, the amplitude A2 of the second signal element, the amplitude A3 of the third signal element and the amplitude A4 of the fourth signal element is: When expressed by an exponent using a damping coefficient α of a system configured by filling the ejection element with a liquid and a base e of a natural logarithm, e ^{α · π / 2} : (e ^{α · π / 2} −e ^{− The} droplet discharge method according to claim 3, wherein ^{α · π / 2} ) :( e ^{α · π / 2 −e−α · π / 2} ): e− ^{α · π / 2} .

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