JP2015013415A - Recording head drive circuit, recording head drive method, and image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve waveform difference due to wiring impedance difference of head drive voltage of a drive circuit.SOLUTION: A recording head drive circuit comprises: a piezoelectric element for compressing an ink in a compression chamber communicating with a nozzle of a recording head and discharging an ink droplet; a switching element for charging and discharging the piezoelectric element for each nozzle; and a capacitive element connected in parallel to the piezoelectric element of which an impedance of wiring from a power supply part to a drive circuit for each piezoelectric element is the highest, and supplying current during drive of the piezoelectric element.

Description

  The present invention relates to a recording head driving circuit, a recording head driving method, and an image forming apparatus.

  An ink jet recording apparatus used as an image recording apparatus or an image forming apparatus such as a printer, a facsimile machine, a copying apparatus, or a plotter is equipped with an ink jet head as a recording head having a nozzle, a pressurized liquid chamber, and pressure generating means. It is.

  The nozzle is a member that ejects ink droplets. The pressurized liquid chamber communicates with the nozzle and is also referred to as a discharge chamber, a pressure chamber, a pressurized chamber, a liquid chamber, an ink flow path, a flow path, and the like. The pressure generating means is a member that pressurizes the ink in the pressurized liquid chamber.

Ink jet heads use electromechanical transducers such as piezoelectric elements as pressure generating means to pressurize the ink in the pressurized fluid chamber, and deform the diaphragm that forms the walls of the ink channels to change the volume inside the ink channels There is a piezoelectric element type that discharges droplets.
This piezoelectric element type ink-jet head ejects ink droplets by applying a drive signal to a piezo element.
Some head drive circuits are composed of switching elements, which control charging / discharging of the piezoelectric elements by turning on / off the switching elements at a predetermined timing and apply a rectangular wave signal as a drive signal. Are known.

  Here, since the electrical wiring of the piezoelectric element of the inkjet head is a fine pattern having a smaller thickness and width than the electrical wiring used in the drive circuit, the wiring resistance becomes high. For example, when the width is 1 mm and the thickness is 0.07 mm, it is 0.0024Ω per 10 mm length, whereas when the width is 0.1 mm and the thickness is 0.07 mm, it is 0.024Ω per 10 mm length.

Therefore, a voltage drop occurs due to the drive current on the wiring for driving the head, and the waveform shape of the signal applied to each CH (channel) of the inkjet head, that is, the dullness is different.
The voltage drop varies depending on the number of driven CHs.
Since the magnitude of the voltage drop differs between driving a small number of CHs in the head and driving a large number of CHs, focusing on one CH, the driving waveform rises as the number of driving CHs in the head increases. / Falling edge is dull. For this reason, a discharge speed becomes slow and discharge amount becomes small.

  The magnitude of the voltage drop varies depending on the current path length. The voltage drop becomes larger as the current path length is longer, and the potential on the wiring is different between each CH. For this reason, as the distance from the power feeding unit becomes farther, the rise / fall of the drive waveform becomes dull, the discharge speed becomes slow, and the discharge amount becomes small.

  In this way, due to the voltage drop of the wiring, the ejection characteristics of each CH fluctuate depending on the number of drive CHs, and there is a difference in the ejection characteristics between the CHs, resulting in printing unevenness and image quality degradation. It was.

For this reason, various proposals have been made to solve such problems (see, for example, Patent Documents 1 to 3).
Patent Document 1 relates to “a capacitive load driving circuit, a capacitive load driving method, and an apparatus using the same”. The invention described in the document 1 aims to efficiently recover and reuse the energy stored in the capacitive load. In the literature 1, as a capacitive load driving circuit and a capacitive load driving method, a capacitive load, that is, a capacitor and a switch element are connected to a current path of a piezoelectric element, and the capacitive element is controlled by controlling the switch element. The structure which regenerates the energy of is disclosed.

  An object of the invention described in Patent Document 2 is to uniform discharge variations due to differences in wiring resistance and capacitance for each CH. The literature 2 discloses a wiring connection method of piezoelectric elements, and discloses a configuration in which the time constant of a piezoelectric element having a high time constant is lowered by connecting to a plurality of drive circuits.

  An object of the invention described in Patent Document 3 is to eliminate a difference in ejection characteristics between nozzles. The same document 3 discloses a head driving method and a driving circuit configuration, in which the nozzle of the head is divided into a plurality of blocks and different driving waveforms are applied to each block so that the ejection characteristics of each block are equal. Is disclosed.

  However, in the inventions described in Patent Documents 1 and 3, there is a difference in the amount of voltage drop due to the wiring impedance difference of the head drive voltage of the drive circuit. In the invention described in Patent Document 2, although the voltage drop amount is taken into account due to the wiring impedance difference of the head driving voltage of the driving circuit, the waveform generated by the wiring impedance difference between the power feeding unit and the driving circuit of each CH. Differences are not taken into account. In addition, the invention described in Patent Document 2 uses a transistor and a resistor to improve the waveform dullness, so that the apparatus becomes complicated and the cost increases accordingly.

  Accordingly, an object of the present invention is to improve the waveform difference due to the wiring impedance difference of the head drive voltage of the drive circuit.

  In order to solve the above-mentioned problems, the invention described in claim 1 is directed to a piezoelectric element that pressurizes ink in a pressurizing chamber that communicates with a nozzle of a recording head to eject droplets, and the piezoelectric element is filled for each nozzle. A switching element for discharging, and a capacitive element connected in parallel to the piezoelectric element having the highest impedance of the wiring from the power feeding unit to the driving circuit of each piezoelectric element, and supplying a current when the piezoelectric element is driven. Features.

  According to the present invention, the waveform difference due to the wiring impedance difference of the head drive voltage of the drive circuit can be improved.

1 is a conceptual diagram according to an embodiment of an image forming apparatus. FIG. 2 is a plan view of the image forming apparatus illustrated in FIG. 1. 2 is a diagram showing a nozzle surface of a head array 2 and one nozzle row used in the image forming apparatus according to the present embodiment. FIG. 2 is a block diagram illustrating an outline of a main body control unit of the image forming apparatus according to the present exemplary embodiment. FIG. FIG. 4 is a cross-sectional view along the longitudinal direction of the liquid chamber of the recording head. FIG. 6 is a cross-sectional view taken along the lateral direction of the liquid chamber of the recording head. FIG. 3 is a plan view of a main part of the recording head. It is an example of the circuit diagram of the drive circuit by the switching element using an inverter. (a) shows the basic operation at the time of discharging of the inverter as the switching element of the drive circuit, and (b) is a diagram showing the basic operation at the time of charging of the inverter as the switching element of the driving circuit. It is the figure which showed the relationship between the pulse signal produced | generated in a pulse production | generation part, and the applied voltage of a piezoelectric element. FIG. 5 is a diagram illustrating a configuration of a head driving circuit and data transfer during a printing operation. (a) is an example of a block diagram of a pulse generation circuit, and (b) is a detailed block diagram within a broken line frame shown in (a). It is an example of the timing chart of the pulse signal produced | generated by a pulse generation circuit. It is explanatory drawing about the data stored in the register | resistor of a pulse generation circuit. It is an example of the timing chart which shows operation | movement of a pulse generation circuit. It is a figure showing the detail of pulse generation data. It is an example of the circuit diagram around a piezoelectric element showing the wiring resistance on the conventional piezoelectric element charge path. FIG. 18 is a diagram illustrating a difference in voltage applied to piezoelectric elements of CH close to the main body power supply unit and CH far from the main body power supply unit in FIG. It is a circuit diagram of the periphery of a piezoelectric element showing the wiring resistance on the piezoelectric element charging path of the present embodiment. It is a figure showing the difference of the voltage applied to the piezoelectric element of CH near from the main body electric power feeding part of this embodiment, and CH far from the main body electric power feeding part.

<Embodiment>
Hereinafter, an embodiment will be described.
[Constitution]
FIG. 1 is a conceptual diagram according to an embodiment of an image forming apparatus. FIG. 2 is a plan view of the image forming apparatus shown in FIG.
The image forming apparatus of the present embodiment is a line type ink jet printer having the configuration shown in the drawing. The paper 4 that is a recording medium is conveyed by the conveyance belt 6, and ink is ejected from the FC head array 2 or the Bk head array 3 to the conveyed paper 4 to form an image.
The image forming apparatus of the present embodiment has two print modes, an FC (full color) print mode and a Bk (black) print mode. The head arrays 2 and 3 are provided for each printing mode. Each of the head arrays 2 and 3 performs a printing operation in each print mode, and the FC head array 2 corresponds to the FC print mode, and the Bk head array 3 corresponds to the Bk print mode. The conveyance roller 5, the conveyance belt 6, and a motor (not shown) constitute conveyance means. The conveying means conveys the paper 4 to the nozzle side of the FC head array 2 and the Bk head array 3 as recording heads.

  Each of the head arrays 2 and 3 can independently move in a direction orthogonal to the upper surface of the transport belt 6. In addition, the image forming apparatus includes a capping unit 1 on a conveyance belt 6. The capping unit 1 is movable in the horizontal direction with respect to the upper surface of the conveyor belt 6. The capping unit 1 is located immediately below the head arrays 2 and 3 that are not used during the printing operation, and each head constituting the head arrays 2 and 3 is capped by the capping unit 1.

  The head arrays 2 and 3 used for printing are fixed at a position where the gap between the conveyor belt 6 and the array surface is about 1 mm. In printing, the paper 4 is conveyed so as to be orthogonal to the head arrays 2 and 3, and when the paper 4 passes under the head arrays 2 and 3, ink droplets from each head constituting the head arrays 2 and 3. Is carried out by discharging.

  Each of the head arrays 2 and 3 is configured by arranging a plurality of droplet discharge heads in an array in the main scanning direction. The FC head array 2 is composed of a head having nozzle rows of C (cyan), M (magenta), Y (yellow), and K (black) colors. All of the Bk array 3 is composed of Bk nozzle rows, and has a nozzle arrangement configuration capable of printing with higher resolution than FC.

FIG. 3 is a diagram showing the nozzle surface and one nozzle row of the head array 2 used in the image forming apparatus according to the present embodiment.
The head arrays 2 and 3 in the image forming apparatus according to the present embodiment are configured by a head including four nozzle rows as illustrated.
The heads constituting the head arrays 2 and 3 are arranged in a staggered manner as shown in the figure.
The heads constituting the FC array 2 eject ink of different colors (CMYK) in each nozzle row in one head.
The heads constituting the Bk array 3 eject Bk ink from all nozzle rows.
The droplet discharge heads constituting the head arrays 2 and 3 include, for example, four nozzle arrays, and the head arrays 2 and 3 constitute the paper 4 conveyance direction and the head arrays 2 and 3 in the image forming apparatus of the present invention. The nozzle rows of each droplet discharge head are mounted in the orthogonal direction.

  In the FC mode, printing is performed using one nozzle row for each color. On the other hand, in the Bk print mode, printing is performed using a monochrome print 4-nozzle array, so that the print resolution is four times that of the FC print mode.

FIG. 4 is a block diagram illustrating an outline of a main body control unit of the image forming apparatus according to the present embodiment.
The main body control unit includes a CPU (Central Processing Unit) 12, a ROM (Read Only Memory) 14, a RAM (Random Access Memory) 16, a nonvolatile memory (not shown), and an ASIC (Application Specific Integrated Circuit). Circuit) 15.

  The CPU 12 is a circuit that controls the entire image forming apparatus. The ROM 14 is a circuit for storing a program executed by the CPU 12 and other fixed data. The RAM 16 is a circuit that temporarily stores image data and the like. The nonvolatile memory is a rewritable storage medium for holding data even when the power of the image forming apparatus is shut off. The ASIC 15 is a circuit that processes various signal processing and rearrangement on image data, and other input / output signals for controlling the entire image forming apparatus.

The main body control unit includes a host I / F (Inter Face) 11, a head control unit 20, a transport motor driving unit 21, an I / O (Input Output) 17, and the like.
The host I / F 11 is a circuit for transmitting and receiving data and signals to and from the host side.
The head controller 20 is a circuit including data transfer means for driving and controlling the recording head 22.
The transport motor driving unit 21 is a circuit that drives the transport motor 27.
The I / O 17 is used to input detection pulses from the linear encoder 26 and the wheel encoder 29, a detection signal from the temperature sensor 24 that detects the ambient temperature, image data read by the scanner 18, and detection signals from various other sensors. Circuit.

An operation panel 13 for inputting and displaying information necessary for the image forming apparatus is connected to the main body control unit.
The main body control unit receives print data from the host side of an information processing device such as a personal computer, an image reading device such as an image scanner, an imaging device such as a digital camera, etc., via a cable or a network at the host I / F 11 .
The CPU 12 of the main body control unit reads and analyzes the print data in the reception buffer included in the host I / F 11, performs the necessary image processing and data rearrangement processing, etc. in the ASIC 15, and controls the head drive of this image data Transfer from the unit 20 to the recording head 22.
The dot pattern data for image output may be generated by storing font data in the ROM 14, for example. The host side printer driver develops the image data into bitmap data and transfers it to the image forming apparatus. May be.

The head controller transfers the above-described image data as serial data. At the same time, a transfer clock, a latch signal, and an operation clock for driving the pulse generation circuit in the drive circuit necessary for transferring the image data and confirming the transfer are input to the drive circuit 23 of each head of the head arrays 2 and 3. To do.
Since the viscosity of ink changes depending on the temperature, it is necessary to change the pulse data to be transferred depending on the environmental temperature. Based on the temperature data in the vicinity of the recording head 22 input from the temperature sensor 24 to the main body control unit, the corresponding print pulse data is selected by the head control unit 20 and transferred to the drive circuit 23.

The head controller 20 also includes a nonvolatile memory that stores pulse data for driving and controlling the recording head 22 and nozzle rank data indicating ejection characteristics for each nozzle of the recording head 22 to be controlled. The head controller 20 transfers the pulse data to the drive circuit at a timing immediately before the printing operation such as when a print job is received.
The nozzle rank data is obtained by reading a chart for determining the discharge variation of the printed droplet discharge head with a scanner, determining the discharge characteristics with the ASIC 15, and converting it as rank data.

  The drive circuit 23 selects the pulse signal generated by the pulse generation circuit 32 based on the image data of the recording head 22 input serially and inputs it to the inverter 34 connected to each piezoelectric element of the recording head 22 Thus, the piezoelectric element as the actuator is driven.

  The transport motor control unit 21 calculates a control value based on a target value given from the CPU 12 side and a speed detection value obtained by sampling a detection pulse from the wheel encoder 29, and transports the control value via the internal motor drive unit 21. The motor 27 is driven.

In addition to the drive data of each piezoelectric element, the drive circuit includes a register for selecting which pattern of heating pulse to apply to each nozzle, and each nozzle has a pulse pattern corresponding to the data in this memory. Applied.
The data in this register is transferred from the head controller 20 based on the information of the temperature sensor 24 provided in the recording head 22, and is rewritten every time the temperature is detected.
The conveyor belt 28 corresponds to the conveyor belt 6 in FIG.

5 is a cross-sectional view of the recording head along the longitudinal direction of the liquid chamber, FIG. 6 is a cross-sectional view of the recording head along the lateral direction of the liquid chamber, and FIG. 7 is a plan view of the main part of the recording head. .
The piezoelectric element in FIGS. 5 to 7 is a first piezoelectric element for ejecting ink.
The recording head 22 includes a flow channel plate 41 formed of a single crystal silicon substrate, a vibration plate 42 bonded to the lower surface of the flow channel plate 41, and a nozzle plate 43 bonded to the upper surface of the flow channel plate 41. In the recording head 22, an ink supply port 49 is provided in a pressure chamber 46, which is an ink flow path in which the nozzle 45 communicates via a nozzle communication path 45 a, and a common liquid chamber 48 for supplying ink to the pressure chamber 46. An ink supply path 47 serving as a fluid resistance portion communicating with each other is formed.

  As an electromechanical conversion element that is a pressure generating means (actuator means) for pressurizing ink in the pressurizing chamber 46 corresponding to each pressurizing chamber 46 on the outer surface side (the side opposite to the liquid chamber) of the vibration plate 42. The laminated piezoelectric element 52 is bonded, and the piezoelectric element 52 is bonded to the base substrate 53. Further, between the piezoelectric elements 52, support columns 54 are provided corresponding to the partition walls 41a between the pressurizing chambers 46, 46. Here, the piezoelectric element member is divided into comb teeth by performing slit processing by half-cut dicing, and the piezoelectric element 52 and the column portion 54 are formed for each one. The column portion 54 has the same configuration as that of the piezoelectric element 61, but is simply a column because no drive voltage is applied.

The outer periphery of the diaphragm 42 is joined to the frame member 44 with an adhesive 50 including a gap material.
The frame member 44 is formed with a recess serving as the common liquid chamber 48 and an ink supply hole (not shown) for supplying ink to the common liquid chamber 48 from the outside. The frame member 44 is formed of, for example, an epoxy resin or polyphenylene sulfite by injection molding.

  The channel plate 41 is formed by, for example, subjecting a single crystal silicon substrate having a crystal plane orientation (110) to anisotropic etching using an alkaline etching solution such as an aqueous potassium hydroxide solution (KOH), so that the nozzle communication path 45a, the pressure chamber 46, a recess or a hole to be the ink supply path 47 is formed. However, the substrate is not limited to a single crystal silicon substrate, and other stainless steel substrates, photosensitive resins, and the like can be used.

The vibration plate 42 is formed from a nickel metal plate, and is produced by, for example, an electroforming method, which is a kind of electroforming method. Other metal plates, resin plates, or a joining member between a metal and a resin plate, etc. Can also be used.
The diaphragm 42 forms a diaphragm portion 55 as a thin portion for facilitating deformation and an island-like convex portion 56 as a thick portion for joining with the piezoelectric element 52 in a portion corresponding to the pressurizing chamber 46. . At the same time, a thick portion 57 is also formed at a portion corresponding to the column portion 54 and a joint portion with the frame member 44, the flat surface side is adhesively joined to the flow path plate 41, and the island-like convex portion 56 is connected to the piezoelectric element. Adhesive bonded to 52. Further, the thick portion 57 is joined to the support portion 54 and the frame member 44 with an adhesive 50. Here, the diaphragm 42 is formed by nickel electroforming having a two-layer structure.

The nozzle plate 43 forms a nozzle 45 corresponding to each pressurizing chamber 46 and is bonded to the flow path plate 41 with an adhesive.
The nozzle plate 43 may be made of a metal such as stainless steel or nickel, a combination of a metal and a resin such as a polyimide resin film, silicon, or a combination thereof. Here, it is formed of a Ni plating film or the like by electroforming. Further, the inner shape or the inner shape of the nozzle 43 is formed in a horn shape, a substantially cylindrical shape, or a substantially frustum shape.

  On the surface in the ejection direction or the ejection surface as the nozzle surface of the nozzle plate 43, a water repellent treatment layer having a water repellent surface treatment (not shown) is provided. Water-repellent treatment layers include PTFE-Ni eutectoid plating, fluororesin electrodeposition coating, evaporative fluororesin, evaporative coating layers such as fluorinated pitch, and baking after solvent application of silicon-based resin / fluorinated resin. And a water-repellent treatment film selected according to the physical properties of the ink. As a result, the ink droplet shape and flight characteristics are stabilized, and high-quality image quality can be obtained. PTFE-Ni is an abbreviation for Poly Tetra Fluoro Ethlene-Ni (polytetrafluoroethylene-nickel).

  The piezoelectric element 52 is formed by alternately laminating a single layer of lead zirconate titanate (PZT) piezoelectric element 61 and an internal electrode layer 62 made of silver and palladium (AgPd). The internal electrodes 62 are alternately electrically connected to the individual electrode 63 and the common electrode 64 which are external electrodes as end face electrodes on the end face. The pressurizing chamber 46 is contracted and expanded by expansion and contraction of the piezoelectric element 52 whose piezoelectric constant is d33. The piezoelectric element 52 expands when a drive signal is applied and is charged, and contracts in the opposite direction when the charge charged in the piezoelectric element 52 is discharged.

  The end face electrode on one end face of the piezoelectric element member is divided by dicing by half-cut to be an individual electrode 63, and the end face electrode on the other end face is conducted by all the piezoelectric elements 52 without being divided by a restriction by notching or the like. A common electrode 64 is formed.

  An FPC (Flexible Print Circuit) cable 65 is connected to the individual electrode 63 of the piezoelectric element 52 by solder bonding, ACF (Anisotropic Conduction Film) bonding or wire bonding in order to give a drive signal. A drive circuit (driver IC) for selectively applying a drive waveform to each piezoelectric element 52 is connected to the FPC cable 65. The common electrode 64 is connected to the ground (GND) electrode of the FPC cable 65 by providing an electrode layer at the end of the piezoelectric element and turning it around.

For example, when the recording head 22 applies a pulse voltage to the piezoelectric element 52 in accordance with a recording signal to charge / discharge the piezoelectric element 52, the piezoelectric element 52 is displaced in the stacking direction and applied via the diaphragm 42. Ink in the pressure chamber 46 is pressurized, and ink is ejected from the nozzle 45.
As the ink droplet ejection ends, the ink pressure in the pressurizing chamber 46 decreases, and a negative pressure is generated in the pressurizing chamber 46 due to the inertia of the ink flow and the discharge process of the drive pulse, and the process proceeds to the ink filling process. To do. At this time, the ink supplied from an ink tank (not shown) flows into the common liquid chamber 48 and is filled from the common liquid chamber 47 through the ink supply port 49 through the fluid resistance portion 47 into the pressurizing chamber 46. The fluid resistance portion 47 is effective in damping the residual pressure vibration after ejection, but is resistant to the maximum filling or refill due to surface tension.
By appropriately selecting the fluid resistance value of the fluid resistance unit 47, the balance between the attenuation of the residual pressure and the refill time can be achieved, and the time until the transition to the next ink droplet ejection operation, that is, the drive cycle can be shortened.

FIG. 8 is an example of a circuit diagram of a drive circuit using a switching element using an inverter.
As shown in the figure, the inverter 34 of the drive circuit of the present embodiment is provided with a CMOS (Complementary Metal-Oxide Semiconductor) inverter 34 as one inverter 34 for each first piezoelectric element. The piezoelectric element is charged and discharged by controlling the CMOS inverter.
A head drive voltage VH is applied to the power feeding unit of the inverter 34, and the head drive voltage VH is a drive voltage of the piezoelectric element. The output is connected to the individual electrodes of the piezoelectric element, and the piezoelectric element is charged and discharged according to the input pulse signal.

  The left end in the figure is a connection part with the main body side, and the wiring CH from the main body side becomes longer as the right CH is. In the present embodiment, the second piezoelectric element 520 as a capacitive element that does not discharge ink near the CH far from the right main body side in the drawing is connected in parallel. That is, the recording head includes a capacitive element 520 that is connected in parallel to the piezoelectric element having the highest impedance of the wiring from the power feeding unit to the driving circuit of each piezoelectric element and supplies a current when the piezoelectric element is driven. The capacitive element 520 has the same structure as the piezoelectric element.

  The electrode of the second piezoelectric element 520 is connected to the power supply wiring of the first piezoelectric element 52 and the GND wiring on the main body side, and is always charged to the head driving voltage VH.

[Operation]
9A shows a basic operation at the time of discharging of the inverter 34 as a switching element of the driving circuit, and FIG. 9B shows a basic operation at the time of charging of the inverter 34 as a switching element of the driving circuit. It is. FIG. 10 is a diagram showing the relationship between the pulse signal generated by the pulse generator and the voltage applied to the piezoelectric element.

A pulse signal is input to the gate portion of each MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) constituting the inverter 34. This pulse signal is a signal output from the level shifter, and is a signal obtained by boosting and logically inverting the voltage level of the pulse signal generated by the pulse shaping circuit as the pulse generator.
When the level of the input pulse signal is low as shown in FIG. 9B, the Pch MOSFET is turned on, the Nch MOSFET is turned off, and the piezoelectric element and the power feeding unit are connected via the Pch MOSFET. Short-circuited and the piezoelectric element is charged to the level of the head drive voltage VH.

  As shown in FIG. 9A, when the input pulse signal is at Hi level, the Pch MOSFET is turned OFF, the Nch MOSFET is turned ON, and the piezoelectric element and the ground (GND) are short-circuited through the Nch MOSFET. Therefore, the piezoelectric element is discharged and the potential becomes the ground level (0 V).

Each MOSFET constituting the inverter 34 has an on-resistance, and a time constant is determined by the on-resistance and the capacitance of the piezoelectric element, and the rise and fall times of the potential of the piezoelectric element are Tr and Tf. That is, as shown in FIG. 10, even if the pulse signal is a rectangular wave, the voltage applied to the piezoelectric element only rises by 0.9 VH after the lapse of Tr, and further time is required until saturation. On the contrary, when the pulse signal falls, it decreases exponentially until it reaches 0V, and Tf is required.
For example, if the capacitance of the piezoelectric element is 1000 pf and the on-resistance of each MOSFET is 200Ω, the rise / fall times Tr and Tf are about 450 ns.
Like the potential fluctuation of the piezoelectric element, the piezoelectric element expands and contracts, whereby the meniscus on the nozzle surface fluctuates and ink droplets are ejected.
As described above, the image forming apparatus of the present embodiment performs a printing operation by repeatedly charging and discharging the piezoelectric elements.

The effect will be further described.
The potential of each CH drive circuit at the start of charging is expressed by equations (1) and (2).
In the following, the mathematical expressions (1) and (2) ignore the charging current to the nCH and the charging current from the second piezoelectric element to the 1CH from the small power feeding unit. The correspondence diagram is shown in FIG.
When driving only 2CH of the first CH and the nth CH in FIG.
Vh_1 = VH−I × R (1)
Vh_n = VH′−I × R (2)
I: Driving current or charging current per 1CH of the first piezoelectric element
R: CH wiring resistance
VH: Feeding part voltage VH *: Second piezoelectric element potential Since the second piezoelectric element is discharged when the first piezoelectric element is charged, VH ′ changes its voltage as shown by the following formula (3), By increasing the capacitance of the piezoelectric element, the voltage drop due to discharge can be reduced.

I '(t): Discharge current of the second piezoelectric element As described above, by providing the second piezoelectric element, the voltage drop of Vh_n far from the power feeding part can be reduced, so the rise of the drive waveform between CH Can be reduced.

  Since the viscosity of the ink changes depending on the ambient temperature, the head drive voltage VH is supplied to the inverter 34 by changing the drive voltage VH based on the temperature data detected by the temperature sensor as in the case of pulse data transfer. To do.

FIG. 11 is a diagram illustrating the configuration of the head driving circuit and data transfer during the printing operation.
The head drive circuit 39 of the present embodiment includes a shift register 38, a register 37 as a latch circuit, a pulse generation circuit 32 as a drive waveform generation unit, a selector 36, a level shifter 35, and an inverter 34.

The shift register 38 is a circuit for inputting a transfer clock and serial print data or a nozzle rank of a recording head for driving from the head control unit 31.
The register 37 is a circuit that latches each resist value of the shift register 38 with a latch signal.
The pulse generation circuit 32 as a drive waveform generation unit is a circuit that generates a pulse signal for controlling the inverter 34.
The selector 36 is a circuit that selects a generated pulse signal input according to print data.
The level shifter 35 is a circuit that converts the logic level voltage signal of the drive signal output from the selector 36 to a voltage level at which the inverter 34 can operate, inverts it, and outputs it.
The inverter 34 is a circuit that is connected to each piezoelectric element 52 of the recording head and charges and discharges the piezoelectric element 52.

During the printing operation, serial print data is input to the shift register 38, and a latch signal is input to the register for storing print data at each ejection timing, and the print data is latched.
The pulse generation circuit 32 includes a counter and a register in which generated pulse data is stored, and outputs a pulse waveform of a drop type (large, medium, small, fine drive). That is, the register stores pulse data corresponding to the size of the ink droplet.

  During the printing operation, the transfer clock and the print data are serially transferred from the head controller 31 to the shift register 38. Further, the data in the shift register 38 is latched into the register 37 by the latch signal, and pulse generation is started by the pulse generation circuit 32, and droplets are ejected from each nozzle.

  In this embodiment, since a register for storing pulse data is provided in the pulse generation circuit 32 in the drive circuit, it is not necessary to transfer drive waveform data as a pulse for each ejection during printing. Further, the head drive can be performed only by transferring the print data to the head, and the number of wirings can be reduced as compared with the prior art.

Prior to the printing operation, pulse generation data corresponding to the ejection characteristics of each nozzle is serially transferred to the pulse generation circuit 32.
This pulse data is data stored in the nonvolatile memory 40 provided in the head control unit 31 of the main body (not shown).
The pulse generation circuit 32 includes a register for storing pulse data, and the pulse data is transferred to the transfer pulse generation circuit 32 (not shown) immediately before the printing operation. In the present embodiment, the register in the pulse generation circuit 32 has only a shift register, and pulse data is only transferred serially.

  FIG. 12 (a) is an example of a block diagram of a pulse generation circuit, and FIG. 12 (b) is a detailed block diagram within a broken line frame shown in FIG. 12 (a). FIG. 13 is an example of a timing chart of a pulse signal generated by the pulse generation circuit. FIG. 14 is an explanatory diagram of data stored in the register of the pulse generation circuit. FIG. 15 is an example of a timing chart showing the operation of the pulse generation circuit. FIG. 16 is a diagram showing details of the pulse generation data.

12 (a) includes a counter 74, registers 70, 71, 72, 73, comparators 75, 76, 77, 78, and D-type flip-flops 79, 80, 81, 82. And having.
The counter 74 is a circuit that counts drive clocks.
The registers 70, 71, 72, and 73 are circuits that store pulse data.
Comparators 75, 76, 77, and 78 are circuits that detect a match between the count value of the counter 74 and the pulse data.
The flip-flops 79, 80, 81, and 82 are circuits that invert the output level and generate a pulse signal using the coincidence detection pulse output from the comparators 75, 76, 77, and 78 as a trigger.
The register 70 includes ten registers 70-1 to 70-10 as shown in FIG. 12 (b), and the outputs of the registers 70-1 to 70-10 are ten comparators 75-1 to 75-. 10 is input to one input terminal, and the counter output is input to the other input terminal of each of the comparators 75-1 to 75-10. The outputs of the comparators 75-1 to 75-10 are input to the input terminals of 75-1 to 75-10, and the outputs of 75-1 to 75-10 are input to the clock input terminal of the flip-flop 79.
Similarly to the register 70, the other registers 71 to 73 are configured to include comparators corresponding to the number of registers and D-type flip-flops.

A circuit shown in the figure is provided for each nozzle, and one of the pulse signals generated according to the print data is applied to the piezoelectric element of each nozzle.
The pulse data is numerical data for switching the level of the pulse signal, and is used in comparison with the count value of the counter 74 in this embodiment.
As an example, as shown in FIG. 16, when the count value of the counter 74 matches the pulse data, the level of the generated pulse signal is switched, that is, inverted. Since the counter of the pulse generation circuit 12 has 11 bits, the data is handled as binary data. That is, the pulse data is fixed numerical data. The comparator compares the count value and the pulse output is inverted when the count value matches the data.

The amount of data required to generate one pulse signal is
Number of constituent pulses of generated pulse signal × 2 (rising / falling) × counter bit.
In this embodiment, the counter bit is 11, and the number of pulses of the generated pulse signal is as follows: (a) Large droplet signal is composed of 5 pulses, (b) Medium droplet signal is composed of 3 pulses, (c) Small droplet signal is composed of 1 pulse (D) The fine drive signal has a single pulse configuration. The pulse generation circuit 32 is provided with a register for storing necessary data.

  The comparators 75 to 78 receive the pulse data necessary for the generated pulse signal and the count value of the counter, respectively. The comparators 75 to 78 are set to 1 when the input count value and the pulse data match. The coincidence detection pulse is output for the number of clocks.

The output inversion flip-flops 79 to 82 of the pulse data generation circuit 32 are provided for each number of pulse signals to be generated. The outputs of the comparators 75 to 78 are input to the flip-flops 79 to 82, and the comparison outputs corresponding to the generated pulse signals are combined into one by OR logic as shown in FIG. 12 (b). Yes.
As a result, the coincidence detection pulses are input from the comparators 75 to 78 to the flip-flops 79 to 82 at the level inversion timing of the pulse signals to be generated, and pulse signals are generated by inverting the output using this input as a trigger. can do.

An operation clock and a latch signal are input to the counter of the pulse generation circuit 32. The operation clock is for counter counting, and the level inversion timing of the generated pulse signal is synchronized with the operation clock, and the generated pulse can be generated with higher resolution as the frequency of the operation clock becomes higher.
The latch signal is connected to the reset terminal of the counter 74, and the count value is reset every time the latch signal is input, and pulse signal generation is started.
Although not shown, all the registers in the figure are cascade-connected and operate as shift registers. Therefore, the pulse data is transferred as serial data from the head control 31 unit.

<Comparative example>
FIG. 17 is an example of a circuit diagram around a piezoelectric element, showing wiring resistance on a conventional piezoelectric element charging path. FIG. 18 is a diagram illustrating a difference in voltage applied to the piezoelectric elements of CH close to the main body power supply unit and CH far from the main body power supply unit in FIG.

  The resistance shown in FIG. 17 represents the wiring resistance on the piezoelectric element charging path. The broken line arrows P1 and P2 represent the charging current path, the broken line arrow P2 represents the current path of the first CH nozzle closest to the main body feeding part, and the broken arrow P2 represents the nth current path farthest from the main body feeding part. Represents.

The magnitude of the voltage drop on the electrical wiring can be expressed as V = I * R by Ohm's law.
VH_1 and VH_n in the figure represent the potentials of the connection portions between the respective CHs and the power supply wirings during head charging.
For example, when only the first CH and the second CH 2CH in the figure are driven, the respective potentials are expressed by the following equations (4) and (5).
Vh_1 = VH-2 * I * R (4)
Vh_n = VH-2 * I * R-I * (n-1) * R (5)
(I: drive current per channel, R: wiring resistance between channels)
Thus, depending on the position of the CH, there is a difference between the potentials of the connection parts of each CH and the power supply wiring when the head is driven, and the difference is similarly generated in the waveform of the applied voltage. Further, the difference in potential increases as the number of driving channels increases.
Although not shown in the figure, wiring resistance also exists in the GND wiring of the inverter, so that a similar potential difference occurs on GND during discharging.

FIG. 18 shows the waveform of the voltage applied to each electric element during multi-channel driving, where the solid line is the first CH and the broken line is the n-th CH.
Since a voltage drop occurs on the electrical wiring as described above when driving a plurality of channels, a difference occurs in the rising / falling of the waveform.
Tf_1, Tf_n, 1st CH, nth CH waveform voltage fall time, Tr_1, Tr_n represent rise time, and the nth waveform where a large voltage drop occurs is much duller than the 1st CH with the smallest voltage drop. And Tf / Tr becomes longer.

  The magnitude of the voltage drop of each CH varies depending on the number of CHs to be driven, and the waveform bluntness also varies. In the case of a small number of CH drives, the voltage drop is small, so there is not much difference in Tf / Tr, but in the case of driving a large number of CHs, a large difference in Tr / Tf occurs as shown in the figure, which is the discharge characteristic. It appears as a difference.

<Modification>
FIG. 19 is a circuit diagram around the piezoelectric element, showing the wiring resistance on the piezoelectric element charging path of the present embodiment. FIG. 20 is a diagram illustrating a difference in voltage applied to the piezoelectric elements of the CH close to the main body power supply unit and the CH far from the main body power supply unit according to the present embodiment.

  Arrows P3 and P4 represent charging current paths, a broken line arrow P3 indicates the current path of the first CH nozzle closest to the main body power supply unit, and a broken arrow P4 indicates the current path of the nth CH nozzle farthest from the main body power supply unit. Become. Actually, the charging current of each CH is supplied from both sides of the main body side power supply section and the second piezoelectric element, but since one of the currents is very small, it is not shown here.

  As shown in the figure, in the present embodiment, the CH near the main body power supply unit is charged mainly by the current from the main body side, and the CH far from the main body supply unit is charged mainly by the current from the second piezoelectric element side.

  As shown in the figure, since the power feeding portions are dispersed, the peak value of the charging current flowing through the electric wiring is reduced, and thus the voltage drop of VH_1 on the wiring is reduced. In addition, the voltage drop of VH_1 on the wiring of the n-th CH portion far from the conventional power feeding unit is short because the second piezoelectric element serving as a new power feeding unit is provided in the vicinity. The descent is smaller than before.

  FIG. 20 shows the applied waveform of each piezoelectric element when a large number of CHs are driven. The solid line is for the first CH, and the broken line is for the nth CH.

  In the present embodiment, by providing the second piezoelectric element in the vicinity of the nth CH, a voltage drop occurs on the electrical wiring, so that a difference occurs in the rising / falling of the waveform. However, as shown in FIG. 19, since the charging current is distributed to the main body feeding portion and the second piezoelectric element, the voltage drop in the feeding wiring path is reduced, and the difference in the rise time Tr is reduced as compared with the conventional technique.

1 Capping unit 2 FC head array 3 Bk head array 4 Paper 5 Transport roller 6, 28 Transport belt 11 Host I / F
12 CPU
13 Operation panel 14 ROM
15 ASIC
16 RAM
17 I / O
DESCRIPTION OF SYMBOLS 18 Scanner 19, 24 Temperature sensor 20 Head control part 21 Conveyance motor control part 22 Recording head 23 Drive circuit 26 Encoder 27 Conveyance motor 29 Wheel encoder 31 Head control part 32 Pulse generation circuit 34 Inverter 35 Level shifter 36 Selector 37 Register 38 Shift register 39 Head drive circuit 40 Non-volatile memory 41 Flow path plate 42 Vibration plate 43 Nozzle plate 45 Nozzle 45a Nozzle communication path 46 Pressurization chamber 47 Ink supply path 48 Common liquid chamber 49 Ink supply port 50 Adhesives 52, 61, 520 Piezoelectric element 53 Base substrate 54 Supporting part 55 Diaphragm part 56 Island-like convex part 57 Compact part 62 Internal electrode layer 63 Individual electrode 64 Common electrode 65 FPC cable 70, 71, 72, 73 Register 74 Counter 75, 76, 7 , 78 comparators 79,80,81,82 flip-flop 83 selector

JP 2003-285441 A JP 2012-131089 A JP 2005-088436 A

Claims (8)

A piezoelectric element that pressurizes ink in a pressurizing chamber that communicates with the nozzles of the recording head to eject droplets;
A switching element for charging and discharging the piezoelectric element for each nozzle;
A capacitive element connected in parallel to the piezoelectric element having the highest impedance of the wiring from the power feeding unit to the drive circuit of each piezoelectric element, and supplying a current when driving the piezoelectric element;
A drive circuit for a recording head, comprising:   2. The recording head drive circuit according to claim 1, wherein the capacitance of the capacitive element is larger than the capacitance of the piezoelectric element.   3. The recording head drive circuit according to claim 1, wherein the capacitive element has a structure identical to that of the piezoelectric element and includes a plurality of piezoelectric elements having no pressurizing chamber. The switching element is
A counter that counts the drive clock; and
A register for storing pulse data corresponding to the size of the droplet;
A comparator that detects a match between the count value of the counter and the pulse data;
A flip-flop that inverts an output level triggered by a coincidence detection pulse from the comparator and generates a pulse signal to be applied to the piezoelectric element;
The recording head drive circuit according to claim 1, further comprising:   By charging / discharging the piezoelectric element for each nozzle of the recording head, when the ink in the pressurizing chamber communicating with the nozzle of the recording head is pressurized and droplets are ejected, the power supply unit to the drive circuit of each piezoelectric element A recording head driving method, comprising: supplying a current from a capacitive element to a piezoelectric element having the highest wiring impedance when driving the piezoelectric element. A piezoelectric element that pressurizes ink in a pressurizing chamber that communicates with the nozzles of the recording head to discharge droplets, a switching element that charges and discharges the piezoelectric element for each nozzle, and a drive unit to a drive circuit of each piezoelectric element A recording head having a capacitive element connected in parallel to a piezoelectric element having the highest wiring impedance and supplying a current when the piezoelectric element is driven;
Conveying means for conveying paper to the nozzle side of the recording head;
Control means for controlling the drive of the recording head;
An image forming apparatus comprising the image forming apparatus.   The image forming apparatus according to claim 6, wherein a capacitance of the capacitive element is larger than a capacitance of the piezoelectric element.   The image forming apparatus according to claim 6, wherein the capacitive element is configured by a plurality of piezoelectric elements having the same structure as the piezoelectric element and having no pressurizing chamber.

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