JPH06297709A - Ink jet recording apparatus - Google Patents

Abstract

PURPOSE:To prevent the fatigue destruction of a piezoelectric element, in a apparatus wherein the voltage applied to a piezoelectric element is changed to expand and contract the volume of an ink chamber to emit ink from a nozzle, by always applying strain to the piezoelectric element in a definite direction at a time of the expansion/contraction of the ink chamber. CONSTITUTION:An ink chamber 2 is allowed to communicate with a nozzle 1 emitting ink. A pressure wall 2p changing the volume of the ink chamber 2 is arranged to the ink chamber 2 and ink is supplied from a supply passage 3 and a piezoelectric element 4 is arranged. The volume of the ink chamber 2 is expanded and contracted by the change of the voltage applied to the piezoelectric element 4 to emit ink from the nozzle 1. In this case, the common electrode terminal tc of the piezoelectric element 4 is connected to a charging and discharging device 5 and a segment electrode terminal ts is connected to a switching device 7. A bias voltage applying device 6 is provided to the charging and discharging device 5 and, in such a state that bias voltage is applied, the piezoelectric element 4 is charged and discharged to expand and contract the ink chamber 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ink jet recording apparatus for recording characters and images by ejecting ink on a paper surface or a recording medium based on data.

[0002]

2. Description of the Related Art Ink jet recording apparatuses are widely used for recording data such as characters and images by connecting to a computer or the like because they are small in size, have low noise, and are relatively inexpensive in terms of apparatus cost and running cost. ing. However, in the case where high-speed recording is required due to the slow printing speed, it is necessary to rely on an expensive, complicated, and large-sized electrophotographic recording apparatus. In order to increase the recording speed of the ink jet recording apparatus to a speed comparable to that of the electrophotographic method, the repetition response frequency of ink ejection per nozzle of the ink jet recording apparatus should be improved and the number of nozzles of the recording apparatus should be increased. It is required to increase.

In the current ink jet recording apparatus, the repetition response frequency of ink ejection per nozzle is 3 to 10 kHz.
To some extent, if this repetition frequency is experimentally raised and driven, ink can be ejected from each nozzle, but the amount of ink ejected from each nozzle and the ejection ink speed will vary greatly, and further drive repetition will occur. When the frequency is increased, some nozzles can eject ink, but the remaining nozzles cannot eject ink. That is, if the ejection repetition frequency is increased in the current inkjet recording apparatus, the dispersion of ink ejection characteristics between nozzles becomes large and ejection failure occurs, which makes the recording apparatus unpractical.

Further, when the ejection repetition frequency is experimentally increased in the current ink jet recording apparatus, compression of the piezoelectric element,
The fatigue life due to repeated stretching shortens the breaking life remarkably, which also impairs the practicality of the recording apparatus. Further, in order to realize high-speed recording comparable to the electrophotographic system, it is necessary to increase the ink ejection repetition frequency and simultaneously increase the number of ink nozzles from the current 48 to 64 to at least 100 or more.
However, in the inkjet recording apparatus, if even one nozzle has a defective ejection, it becomes defective as a recording apparatus. Therefore, increasing the number of nozzles in the current inkjet method remarkably increases the defective rate of the apparatus, resulting in a practical reliability. It was difficult to get sex.

That is, the conventional ink jet recording apparatus has problems in that the ink ejection characteristics are varied, the life is short due to the fatigue breakdown of the piezoelectric element, and the defective occurrence rate of the piezoelectric element is too high due to the increase in speed.

16 and 17 show cross sections of an example of the structure of an ink ejection portion of a conventional ink jet recording apparatus. In FIG. 16, the ink filled in the ink flow path including the ink chamber 2 communicating with the nozzle 1 and the ink supply path 3 is transferred to the piezoelectric element 4 connected to the pressure wall 2 p provided in a part of the ink chamber 2. By applying a voltage by the electrodes 4e on both surfaces, the piezoelectric element 4 is distorted as a change in the aspect ratio, and the ink chamber 2
Is applied to eject ink from the nozzle 1.
In FIG. 17, the ink flow path including the ink chamber 2 communicating with the nozzle 1 and the ink supply path 3 is filled with ink, and the pressure wall 2p provided in a part of the ink chamber 2 has electrodes 4e on both sides.
The piezoelectric element 4 is formed by laminating the piezoelectric ceramics having the above and the elastic plate 4t, the voltage is applied between the electrodes to bend the piezoelectric element 4, pressurize the ink chamber 2, and eject the ink from the nozzle 1. . FIG. 1 showing a conventional example
6 and 17 are common to the ink jet recording apparatus using the piezoelectric element in that the ink chamber is expanded or contracted by the voltage applied to the piezoelectric element.

FIG. 18 is a graph showing a drive voltage waveform applied to a piezoelectric element and distortion of the piezoelectric element in a conventional example of an ink jet recording apparatus using a piezoelectric element. In the figure, the solid line V is the applied voltage waveform, and the broken line P is the distortion of the piezoelectric element. The horizontal axis of the graph represents time, and the vertical axis represents voltage and strain. Since the voltage waveform V in the figure is a voltage value and the strain P of the piezoelectric element is a displacement value, the unit of the vertical axis of the graph is different, but in this figure it is overlaid for the sake of explanation. When the voltage in the same direction as the polarization voltage of the piezoelectric element is applied in the upper direction of the graph, the strain of the piezoelectric element at that time is the direction in which the distance between the electrodes extends. The state of the graph horizontal axis 0 is the state of zero strain, and the downward direction is the strain in the contracting direction.

FIG. 18 (a) shows a driving method called pushing, which has a structure in which the ink chamber is contracted by applying a voltage to the piezoelectric element to charge the piezoelectric element. Is applied to rapidly charge the piezoelectric element to distort it and reduce the volume of the ink chamber, thereby ejecting ink from the nozzle. After the ink is ejected, the piezoelectric element is discharged as indicated by the symbol R in the figure to set the strain of the piezoelectric element to 0 and the ink chamber is returned to the state before the ejection. In the structure of FIG. 16A, when voltage is applied to the piezoelectric element in the same direction as during polarization, the piezoelectric element is distorted in the direction in which the distance between the electrode surfaces 4e expands. Therefore, one electrode surface is bonded to the wall surface of the ink chamber. The ink chamber can be compressed because it is present. Similarly, when the piezoelectric element having the structure of FIG. 17A is driven by the driving method of FIG. 18A, a voltage is applied to the piezoelectric element to rapidly charge the piezoelectric element 4 as indicated by reference numeral I. Expands in the thickness direction and at the same time contracts in the length direction, a difference in strain from the elastic plate 4t attached to the piezoelectric element occurs, and the vibrating plate warps in the direction of pressurizing the ink chamber to eject ink. After the ink is ejected, the piezoelectric element is discharged as indicated by a symbol R in the figure, so that the distortion of the piezoelectric element is set to 0 and the ink chamber is returned to the state before the ejection.

FIG. 18 (b) shows a driving method called pulling-out, which has a structure in which the ink chamber is expanded by charging the piezoelectric element, and a voltage is applied to the piezoelectric element as indicated by symbol R in the drawing in preparation for ink ejection. Is applied to charge and distort the piezoelectric element to expand the ink chamber, and at the timing of ink ejection, the charged piezoelectric element is rapidly discharged as indicated by reference numeral I in the drawing to reduce the distortion of the vibrator. It is a method of pressurizing the ink chamber and ejecting ink by releasing it to the state. FIG.
In the structure of (b), the piezoelectric element is distorted in the direction in which the ink chamber 2 is expanded by the application of voltage, so that ink can be ejected by the driving method of FIG. Similarly, in the structure of FIG. 17B, the warp of the piezoelectric element occurs in the direction in which the ink chamber 2 is expanded by the application of voltage, so that the ink can be ejected by the driving method of FIG.

As described in US Pat. No. 4,471,363, an ink jet recording apparatus using a conventional piezoelectric element has an example shown in FIG. 18 or an example in which its timing is improved, and a combination of them is used as a piezoelectric element. There is an example in which the voltage applied to is reversible. However, if the recording speed is increased by these conventional methods, there is a problem that the piezoelectric element becomes impractical due to fatigue failure and the life of the piezoelectric element is remarkably shortened. FIG. 19 is a sectional view showing a broken portion of a piezoelectric element in an ink jet recording apparatus using a conventional piezoelectric element.
In the figure, reference numeral 4 is piezoelectric ceramics, and reference numeral 4e is an electrode for applying a voltage to the piezoelectric ceramics. FIG. 19
(A), (b) and (c) are examples of piezoelectric elements that expand or contract the ink chamber by strain displacement in the thickness direction. FIG. 19
(D), (e), and (f) of the piezoelectric element that expands or contracts the ink chamber by converting strain in the thickness and spreading direction of the piezoelectric ceramics 4 into bending displacement by attaching the elastic plate 4t to the piezoelectric element. Here is an example. When these piezoelectric elements are driven for ink ejection for a long time by the conventional method shown in FIG. 18, a crack is generated at a portion illustrated by reference numeral K in FIG. 19, and the driving operation of the piezoelectric element is not performed. The characteristics are significantly impaired, and ink cannot be ejected. FIG. 19
19A and 19D are examples in which a crack K is generated near the joint between the piezoelectric ceramics 4 and the electrode 4e, and FIG.
The cracks indicated by K in (b) and (e) are examples generated near the surface of the piezoelectric ceramics, and the cracks indicated by K in FIGS. 19 (c) and (f) occurred inside the piezoelectric ceramics. Here is an example. In all cases, it is considered that fatigue fracture occurred due to repeated stretching of the piezoelectric element in the portion where the composition of the joint changed and the portion where stress such as minute defects or foreign matters was likely to concentrate. In addition, when the recording speed is increased in the ink jet recording apparatus using the conventional piezoelectric element, the fatigue breakdown of the piezoelectric element becomes apparent, and the element-to-element variation of the electromechanical conversion characteristics of the piezoelectric element becomes significant, which is not practical. There were also problems.

[0011]

Therefore, the problem to be solved by the present invention is to prevent the fatigue destruction of the piezoelectric element and to prevent the piezoelectric element from being broken, which is an object to increase the recording speed of the ink jet recording apparatus using the piezoelectric element. This is to minimize the inter-element variation in the electromechanical conversion characteristics of the elements.

[0012]

In order to solve such a problem, in the present invention, a nozzle for ejecting ink, an ink chamber communicating with the nozzle, and an ink provided in a part of the ink chamber A pressure wall that changes the volume of the chamber, a supply path that supplies ink to the ink chamber, and a piezoelectric element that changes its shape by the application of a voltage are provided, and the pressure wall and the piezoelectric element are mechanically connected. In the ink jet recording apparatus that expands and contracts the volume of the ink chamber by changing the voltage applied to the piezoelectric element, or contracts and expands and ejects ink from the nozzle, when expanding or contracting the pressure chamber, or Before and after that, a mechanism for constantly applying a unidirectional strain to the piezoelectric element was provided.

Further, a nozzle for ejecting ink, an ink chamber communicating with the nozzle, a pressure wall for changing the volume of the ink chamber provided in a part of the ink chamber, and ink are supplied to the ink chamber. A supply path and a piezoelectric element whose shape is changed by applying a voltage are provided, the pressure wall and the piezoelectric element are mechanically connected, and the volume of the ink chamber is changed by a change in the voltage applied to the piezoelectric element. In an ink jet recording apparatus that expands and contracts, or contracts and expands and ejects ink from the nozzles, the electric field applied to the voltage application unit of the piezoelectric element is in the same direction as during polarization and is always 50 kV / m or more. A mechanism driven by electric field strength was provided.

[0014]

The piezoelectric element is always charged or discharged, or the distortion of the piezoelectric element that occurs before or after the discharge is 0 or does not repeat both compression and expansion beyond 0, and the piezoelectric element is always strained in one direction. The fatigue failure of the piezoelectric element is significantly reduced due to the operation of the. In addition, since the piezoelectric element is always used in a state where a certain voltage or more is applied, it is possible to avoid a state in which the electromechanical conversion characteristics vary greatly between the elements of the piezoelectric element in the vicinity of voltage 0, so that the characteristic variation between elements is minimized. it can.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments.

A first embodiment using the present invention is shown in FIGS.
This will be described with reference to FIGS. 3, 4, and 5. The present embodiment is an inkjet recording apparatus capable of recording 12 sheets per minute on a recording medium having a large A4 sheet shape. In this embodiment, in order to enable high-speed recording, an ink nozzle, an ink chamber that communicates with the ink nozzle, and a piezoelectric element that pressurizes the ink chamber are used as a set of 640 inkjet elements, and the repetition drive frequency of each element is set to 24 kHz. There is. Also,
The printing density is 600 dots / inch, and it is possible to record with high-definition image quality at high speed.

FIG. 2 is a perspective view showing the external appearance of the components of this embodiment. In the figure, reference numeral 10 is a recording head, reference numeral 11 is a head drive circuit, reference numeral 12 is recording paper, and reference numeral 1 is shown.
3 is a head support mechanism, reference numeral 14 is a head moving mechanism,
In the figure, reference numeral 15 is a print drum, reference numeral 16 is a recording paper clamp mechanism, reference numeral 17 is a drum drive mechanism, and reference numeral 18 is an ink tank. The recording paper 12 is fed from a paper cassette (not shown), wound around the print drum 15, and fixed by the recording paper clamp mechanism 16. The recording head 10 is provided with 640 line-shaped ink jet elements which form a set of a nozzle for ejecting ink, an ink chamber communicating with the nozzle, and a piezoelectric element for pressurizing the ink. The nozzle intervals are 8/600 inch intervals, and each time the print drum 15 makes one rotation, the head moving mechanism 14 moves 1/600 inch in a direction parallel to the print drum axis. The recording head 10 is the print drum 1
It is driven in synchronization with the rotation of 5. Since the repetitive driving frequency of one element of the recording head 10 is 24 kHz, the recording of the print drum is possible within 1 second per rotation, and the recording on the entire surface of the paper is completed by 8 rotations of the print drum. Therefore, the recording apparatus of the present embodiment is capable of high-speed recording by increasing the driving frequency and increasing the number of nozzles of the piezoelectric element, even though it is an inkjet method.

FIG. 1 is a diagram showing the configuration of a nozzle, ink chamber, and piezoelectric element of one element of the recording head of the embodiment shown in FIG. 2 and the configuration of an apparatus for driving the piezoelectric element. Reference numeral 1 in the drawing is a nozzle, reference numeral 2 in the drawing is an ink chamber, and ink is supplied from an ink tank through an ink supply path 3 in the drawing to fill the ink chamber 2. A pressure wall 2p in the drawing is provided in a part of the ink chamber 2 and is joined to the piezoelectric element 4. The piezoelectric element used in this example is called a laminated type piezoelectric element in which a plurality of plate-shaped piezoelectric ceramics are laminated with the film-shaped electrode 4e interposed therebetween, and a voltage is applied between the electrodes that are alternately connected. It is polarized. Figure 15
(A) shows the electrode structure of the laminated piezoelectric element used in this example and the polarization state of each piezoelectric ceramic layer. FIG. 15B shows the most basic piezoelectric element, which has a single layer with electrodes formed on both sides. Here, a voltage is applied to the piezoelectric ceramics 4 by a power source E for polarization so that the upper electrode 4e of the drawing is positive and the lower electrode 4e thereof is negative.
The charges in the piezoelectric element are polarized in the direction indicated by reference numeral 4c in the figure. In this embodiment, the polarization direction in this case is indicated by an arrow 4a from positive to negative, and the polarities of the terminals of the piezoelectric element polarized in this state are described as + on the plus side and − on the minus side. In FIG. 15A showing the laminated type piezoelectric element used in this example, the electrodes 4e are alternately formed in a comb-like shape with the piezoelectric ceramic layers 4 sandwiched therebetween, and are polarized by the power source E. Each piezoelectric layer 4 is polarized by alternately changing directions as shown by an arrow 4a in the figure. In FIG. 1 of the first embodiment, a symbol tc in the figure is a common electrode terminal commonly connected to a plurality of elements, and a symbol ts in the figure is a segment electrode terminal switched in each element. In this embodiment, the piezoelectric element 4 is polarized with the common electrode terminal tc side being negative and the segment electrode terminal ts side being positive. Therefore, when a voltage is applied between the segment electrode terminals ts with the common electrode terminal tc side being negative, the piezoelectric element 4 is distorted in the direction of expanding the distance between the electrodes, and at the same time, it is distorted in the direction orthogonal thereto. In this embodiment, as shown in FIG. 1, the surface orthogonal to the electrode 4e is connected to the thick wall 2p, so that a voltage in the same direction as when polarization is applied between the terminals of the piezoelectric element 4 causes the ink chamber to move. Can be extended. In this embodiment, the common electrode terminal tc of the piezoelectric element 4 is connected to the charging / discharging device 5, and the segment electrode terminal ts is connected to the switching device 7 of each element. The charging / discharging device 5 uses a minus power source of the bias voltage Vb and the bias voltage Vb.
Minus voltage of drive voltage VD added to-(V
b + VD), the potential of the common electrode terminal tc can be controlled by the input of the charge control signal and the discharge control signal to perform the charging operation and the discharging operation of the piezoelectric element 4. Further, the switching device connected to the segment electrode terminal of each piezoelectric element can turn on / off the discharge operation of each piezoelectric element according to the print data. Here, the charging / discharging device 5 is provided with a bias voltage applying device 6, and the piezoelectric element 4 is for applying the bias voltage Vb in the same direction as the polarization voltage to expand and contract the ink chamber for the ink ejection operation. Is charged and discharged.

Next, the ink ejection operation of this embodiment will be described. FIG. 3 is a graph showing the driving voltage between the electrodes of the piezoelectric element shown in FIG. 2 and the distortion of the piezoelectric element at that time. In the present embodiment, as indicated by B in the figure, a recording signal is input while the recording apparatus is powered on and a recording operation is started, and at the same time, a bias voltage Vb in the same direction as the polarization voltage is applied to the piezoelectric element. This causes the piezoelectric element to be in a base strain state during the printing operation. In this embodiment, since the so-called pulling-out operation described in the conventional example is performed, the voltage VD is further applied to the piezoelectric element 4 by the drive voltage generator 5 shown in FIG. Then, the piezoelectric element is charged to cause distortion in the direction of expanding the ink chamber. When the discharge control signal is input to the charging / discharging device at the driving timing, the driving voltage generator performs a circuit operation so as to rapidly discharge the driving voltage VD, and the voltage between the piezoelectric elements is a bias voltage, as shown by reference numeral I in FIG. Discharge to Vb, the strain of the piezoelectric element is released to the base strain state, the ink chamber is compressed, and the ink is ejected from the nozzle. After the ink is ejected, the drive voltage generator gradually applies the voltage VD to the piezoelectric element to expand the ink chamber in preparation for the next ink ejection operation.

In this operation, the piezoelectric element is always in a unidirectional strain state. In the conventional method, as indicated by symbol O in FIG. 18B, it is inevitable that the piezoelectric element is temporarily distorted due to overshoot due to abrupt discharge at the time of ink ejection. The repetition of the above causes a fatigue failure of the piezoelectric element. According to the present invention, the bias voltage Vb in the same direction as when the piezoelectric element is polarized is always applied to the piezoelectric element, so that as shown by reference numeral O in FIG. Since the strain direction of the piezoelectric element does not exceed 0 and does not reverse,
Even if the high-speed repeated operation is performed for a long time, the piezoelectric element does not fatigue damage. The bias voltage Vb of this embodiment is shown in FIG.
, The distance h between adjacent electrodes of the piezoelectric element, the layer thickness H of the voltage applying portion of the piezoelectric element, the maximum value ΔD of the amplitude of the residual vibration actually measured during the discharge operation of the piezoelectric element,
A value (ΔD × h) / specified from the piezoelectric constant d of the piezoelectric element
The voltage was set to about twice as high as (d × H). In this embodiment, ΔD is
In the system having the structure of the piezoelectric element and the ink chamber shown in FIG. 1, the velocity change of the surface of the pressure plate 2p is measured under the condition close to the actual driving condition by using the laser Doppler velocity measuring device,
The result was integrated to obtain the displacement amount of the piezoelectric element, which was obtained as the difference between the maximum displacement at the time of overshoot during the discharge operation of the piezoelectric element and the converged displacement. In this embodiment, the piezoelectric constant d used is d31 which is a physical property value of the piezoelectric element.

FIG. 20 shows a strain displacement state of a general piezoelectric element, which is charged by a constant current from a state in which the potential between electrodes of the piezoelectric element is set to 0 to a certain voltage in the same direction as during polarization. The figure shows the strain displacement of the piezoelectric element when the voltage is held for a certain period of time after the end and then discharged with the same constant current as during charging until the potential between the electrodes becomes zero. The thin line Pi in the figure is an ideal displacement state, and the thick line Pr in the figure is an actual displacement state. When the piezoelectric element is charged and discharged relatively slowly as shown in FIG. 20A to displace it, the overshoot amount of strain displacement of the piezoelectric element is small, and the piezoelectric element is charged at high speed as shown in FIG. 20B. It can be seen that the overshoot of the strain displacement of the piezoelectric element becomes severe when it is attempted to operate by discharging. The maximum value ΔD of the amplitude of the residual vibration during the discharge operation of the piezoelectric element described in the present invention indicates the overshoot amount of the portion indicated by Po in FIG.

FIG. 4 is a graph showing the electromechanical conversion characteristics of a general piezoelectric element, in which the horizontal axis represents the voltage applied to the piezoelectric element and the vertical axis represents the distortion rate. Generally, the strain of the piezoelectric element is not proportional to the applied voltage, and there is a portion where the slope of the graph is gentle from 0 to a certain voltage, and the slope of the graph is almost constant from the middle. According to the present invention, by applying the bias voltage of Vb shown in FIG. 4, the most electric voltage can be obtained by applying the bias voltage of Vb shown in FIG. Since the piezoelectric element is driven in the VD portion having a high mechanical conversion efficiency, there is an advantage that the electromechanical conversion efficiency is excellent. In addition, as a result of investigation according to the present invention, most of the variations in the electromechanical characteristics that cannot be avoided when a plurality of piezoelectric elements are used appear as variations in the electromechanical conversion efficiency near a voltage of 0, and are constant. It was found that the electromechanical conversion efficiency does not vary above a voltage and exhibits a substantially constant value. Furthermore, it has been found that, in most piezoelectric elements, by setting the electric field strength of the driving portion to 50 kV / m or more, the characteristics vary greatly among a plurality of piezoelectric elements, and a region with low electromechanical conversion efficiency can be avoided. The solid line a and the broken lines b, c, and d shown in FIG. 4 are examples of measuring the electromechanical conversion characteristics of a plurality of piezoelectric elements manufactured by the same manufacturing method, but in the range of voltage from 0 to Vb, the graph shape and It can be seen that the slopes of the graph showing the electromechanical conversion rate are almost the same in the range of VD equal to or higher than the voltage Vb, while the slopes are variously different. Therefore, according to the present invention, it is possible to drive the piezoelectric element in a portion having the highest electromechanical conversion efficiency and, in addition, it is possible to ignore variations in the electromechanical conversion efficiency of a plurality of piezoelectric elements during driving. It is also possible to keep the drive characteristics constant.

FIG. 5 shows a drive circuit for the piezoelectric element of this embodiment. A portion surrounded by a broken line indicated by reference numeral CG in the drawing is a charging / discharging circuit for charging and discharging the piezoelectric element,
Symbols C1, C2, ... In the drawing indicate a plurality of piezoelectric elements provided in the respective ink chambers, and the polarities of the applied voltage during polarization are indicated by + and −. In the present embodiment, the terminal tp3 of the charging / discharging circuit CG is commonly connected to the negative electrodes of the plurality of piezoelectric elements for charging and discharging. A plurality of piezoelectric elements C1, C
The electrodes on the positive side of 2, ... Are connected to the respective switching circuits S1, S2, ..., and a control voltage is applied to the respective control terminals tpS1, tpS2 to turn ON / OFF the discharge of each element. Control. Charge / discharge circuit CG is-
It is connected to two negative power sources having different potentials of (Vb + VD) and -Vb, and the charge control terminal tp1 and the discharge control terminal tp2 are set to the potential of 5V in the initial state. In the circuit, when the potential of the charge control terminal tp1 is set to 0V, the portion indicated by the symbol M1 in the figure is turned on and the capacitor C0 is turned on.
Are discharged by a constant current, the transistor Tr1 is turned on, and the piezoelectric elements C1, C2, ... Are charged until the charging / discharging terminal tp3 has a potential of − (Vb + VD). Therefore, the piezoelectric elements C1, C2, ... Are charged to the potential Vb + VD and distorted in the direction of expanding the respective ink chambers. Next, the discharge operation of the piezoelectric element for ejecting ink will be described. Here, the element that ejects ink is C1 and the element that does not eject ink is C2. By returning the charge control terminal tp1 of the charge / discharge circuit CG to the original state of 5V and setting the discharge control terminal tp2 to 0V, the portion M2 in the figure is turned on.
Then, charging of the capacitor C0 is started, the transistor Tr2 is turned on, and the potential of the charging / discharging terminal tp3 rises. By setting the switching terminal tpS1 of the switching circuit connected to the piezoelectric element to be ejected here to 5V, the piezoelectric element C1 is discharged through the switching circuit S1. On the other hand, the piezoelectric element C2 is not discharged even if the potential of tp3 rises by setting the switching terminal tpS2 of the switching circuit S2 connected to the piezoelectric element C2 not to be discharged to 0V and keeping the switching circuit S2 in the OFF state. The piezoelectric element is discharged until the electric potential on the upper side of the capacitor C0 in the figure reaches −Vb which is the electric potential of the bias power source connected through the diode. Therefore, the piezoelectric element C1 is discharged to the voltage Vb. Therefore, if the circuit of this embodiment is used for driving, it is possible to perform the pull-out driving operation while always applying the bias voltage Vb in the same direction as the polarization voltage to the piezoelectric element.

As described above, the first embodiment is 64
Even though each nozzle is repeatedly driven at a maximum of 24 kHz using a large number of 0 elements, the characteristics of the piezoelectric element are uniform and it is possible to eliminate defects such as fatigue damage. High-speed recording is now practically possible.

According to the experiment of this embodiment, the average life due to fatigue failure of the piezoelectric element is extended more than twice as compared with the conventional method, and the characteristic variation among the plurality of elements is reduced to a substantially negligible level. .

A second embodiment of the present invention will be described with reference to FIGS. 6, 7, 8 and 9. FIG. 6 is a perspective view showing the components of the recording apparatus. The recording apparatus records serially on a recording sheet of A4 size with a 120-element recording head at a recording density of 300 dpi. Reference numeral 20 in the drawing denotes a recording head, which is provided with 120 elements of ink jet elements forming a set of nozzles, ink chambers communicating with the nozzles, and piezoelectric elements arranged in the ink chambers. Reference numeral 21 in the drawing denotes a replaceable ink tank, which is connected to the recording head 20 and supplies ink. The recording paper denoted by reference numeral 22 in the figure is supplied to the bottom of the head by a paper feed roller denoted by reference numeral 23 in the figure. The recording head 20 has a structure in which the head moving mechanism 24 serially scans the recording paper 22 along the slide shaft 25.

FIG. 7 shows a piezoelectric element, an ink ejection mechanism, and a piezoelectric element driving mechanism of the recording head. Nozzle 1
Ink is supplied from the ink supply path 3 to the ink chamber 2 communicating with the. A part of the wall surface of the ink chamber is a pressure wall 2p, and a piezoelectric element composed of the piezoelectric ceramics 4 having electrodes 4e on both sides is attached. The pressure wall 2p also uses the function of the elastic plate 4t for converting the strain displacement of the piezoelectric ceramics 4 into the bending displacement. When a voltage in the same direction as the applied voltage at the time of polarization is applied to the electrode 4e, the piezoelectric ceramics 4 causes expansion strain in the thickness direction and at the same time contracts in the spreading direction. The elastic plate warps outward. In this embodiment, the elastic plate 4t
Also serves as the pressure wall 4p of the ink chamber, so that the ink chamber shrinks when a voltage is applied to the piezoelectric element 4 in the same direction as the applied voltage during polarization. In the present embodiment, the positive electrode at the time of polarization of the piezoelectric element 4 is connected as a common electrode terminal tc to the charging / discharging device 5 in common with a plurality of piezoelectric elements, and the negative electrode is a segment electrode terminal ts for each element. Independently connected to the switching device 7. Charge / discharge device 5
Is connected to a power supply of a bias voltage Vb and a power supply of a voltage obtained by adding a drive voltage VD to the bias voltage Vb, and includes a bias voltage applying device 6.

FIG. 8 shows the inter-electrode voltage and the bending displacement of the piezoelectric element of this embodiment. In this embodiment, when the power of the recording apparatus is turned on, a bias voltage Vb in the same direction as the applied voltage at the time of polarization is applied to the piezoelectric element as indicated by reference numeral B in the figure. The driving operation for ejecting ink is performed by the method called push-out described in the conventional example, with the state where this voltage is applied as the base state of the piezoelectric element. That is, as indicated by reference numeral I in the figure,
By applying a voltage VD to the piezoelectric element by a drive signal and rapidly charging the piezoelectric element, the piezoelectric element bends in a direction of pressurizing the ink chamber, and ink is ejected from the nozzle. When the ejection of the ink is completed, the voltage VD is lowered to the voltage Vb to be discharged as indicated by a symbol R in the figure, and the piezoelectric element is returned to the base state before the ink ejection by the voltage Vb, and the ink chamber is opened. Expand. In this embodiment, each piezoelectric element is repeatedly driven at a high speed of up to 12 kHz, even though a plate-shaped piezoelectric element having a relatively large overshoot is used. However, according to the present invention, the same voltage direction as polarization is applied. Since the state in which the bias voltage is applied to the piezoelectric element is the ground state, as shown by the symbol O in the figure, the strain direction of the piezoelectric element exceeds 0 at the time of overshoot and does not reverse and always stays in the same direction. Therefore, fatigue failure of the piezoelectric element is prevented. In addition, since the distortion at the time of overshoot does not become an unstable state around 0, the contraction time of overshoot is faster than that of the conventional method, and since it does not vary between elements, the frequency of high-speed repetitive driving is high. It has become possible to practically increase. In addition, in the present embodiment according to the present invention, the electromechanical conversion efficiency of the piezoelectric element is high as in the first embodiment, and since a constant region can be used between the elements, the electromechanical conversion efficiency can be improved. The effects of improvement and reduction of variation between elements are effectively obtained, and high-speed driving and multi-elementization are practically possible.

FIG. 9 shows a drive circuit for the piezoelectric element of this embodiment. A portion surrounded by a broken line indicated by reference numeral CG in the drawing is a charging / discharging circuit for charging and discharging the piezoelectric element,
Reference numerals C1, C2, ... In the drawing indicate a plurality of piezoelectric elements provided in the respective ink chambers. The polarities of the applied voltage when the piezoelectric element is polarized are indicated by the symbols + and −. In this embodiment, the terminal tp3 of the charging / discharging circuit CG is commonly connected to the plus electrodes of the plurality of piezoelectric elements, and voltage is applied during charging and discharging operations. The negative electrodes of the plurality of piezoelectric elements C1, C2, ... Are connected to the respective switching circuits S1, S2, ..., and a control voltage is applied to the respective control terminals tpS1, tpS2 of each element. Controls charging ON / OFF. The charge / discharge circuit CG is connected to two power sources having different potentials of Vb + VD and Vb, and the charge control terminal tp1 and the discharge control terminal tp2 are set to the potential of 0 V in the initial state. In the circuit, when the potential of the charge control terminal tp1 is set to 5V, the portion indicated by M1 in the figure is turned on and the capacitor C
Charging is started at a constant current of 0, Tr1 is turned on, and the potential of the charging / discharging terminal Tp3 rises to Vb + VD.
Here, if the piezoelectric element that performs the ejection operation is C1, the switching control terminal tpS1 of the switching circuit S1 connected to the negative terminal of the piezoelectric element C1 is set to 5V, and a current flows through the piezoelectric element C1 to be charged to the potential Vb + VD. . On the other hand, if the piezoelectric element that does not discharge is C
2, the switching control circuit tpS of the switching circuit S2 connected to the negative terminal of the piezoelectric element C2.
By setting 2 to 0V, no current flows in the piezoelectric element C2 even if the potential of the charging / discharging terminal tp3 of the charging / discharging circuit CG rises, so that the charging operation is not performed. By the above operation, the piezoelectric element to be ejected is charged and the ink chamber is pressurized to eject the ink. Next, the operation of discharging all the piezoelectric elements to the potential of Vb in order to return the piezoelectric elements to the original state will be described. By returning the charge control terminal tp1 to 0V and setting the discharge control terminal tp2 to 5V, the portion indicated by M2 in the figure is turned on, and the electric charge of the capacitor C0 is discharged by a constant current,
The transistor Tr2 is turned on and the charging / discharging terminal tp
The piezoelectric elements C1, C2, to which the potential of 3 is lowered and connected,
Discharge ... Here, since the capacitor C0 is connected to the bias power source Vb through the diode D, CO
When the voltage of 2 is approximately Vb, Tr2 is in the OFF state, and the potential of the charging / discharging terminal tp3 is approximately Vb, the discharge of the piezoelectric element is stopped. Therefore, if the circuit of this embodiment is used for driving, a bias voltage Vb in the same direction as the polarization voltage is always applied to the piezoelectric element, and the plurality of piezoelectric elements can be pushed and driven.

A third embodiment using the present invention will be described with reference to FIGS. 10, 11, 12 and 13. The present embodiment is a color recording apparatus having a total of four elements, one element each capable of ejecting yellow, magenta, cyan, and black inks. The recording density is 300 dpi, and the recording area is equivalent to A6 to A4 for the purpose of printing television images for home use and recording photographic data. Each element is a thickness vibration type piezoelectric element that expands and contracts the ink chamber and performs a driving operation called pulling. However, by modulating the driving waveform for color recording, the ink ejection amount per stroke is increased. Is capable of being modulated.

FIG. 10 shows an element structure of the color recording apparatus of this embodiment. This embodiment has an element structure of one nozzle for each color in order to realize color recording at a low price. In the conventional method, if the recording apparatus is configured with a small number of elements, the number of driving times per element becomes extremely large with respect to the number of recording media,
Although it was difficult to put into practical use in terms of life, the recording apparatus of this example was realized by using the present invention because the life of the piezoelectric element was remarkably improved. Further, in the present embodiment, since the drive repetition frequency of the element can be increased to 28 kHz by the present invention, a practical recording speed can be achieved even with a small number of recording elements. Reference numeral 34 in the drawing denotes recording sheet 3
3 is a drum around which reference numeral 30 in the drawing is a head having nozzles, ink chambers, and ink ejection elements composed of piezoelectric elements. Each head has one nozzle for each color and a total of four nozzles. Colored ink is supplied. The head 30 is fixed to a carriage denoted by reference numeral 32 in the figure, and the carriage 32 is structured to be driven in the drum axis direction by a screw-shaped head moving shaft 35. After winding the paper on the drum 34, the DC motor 36
The head is rotated at a high speed, and the head moving shaft is moved by 1/300 inch per one rotation of the drum by the stepping motor 37. In this embodiment, a full-color A5 equivalent screen can be printed in about 2 minutes despite the structure that can be manufactured at a very low cost.

FIG. 11 is a diagram showing a one-element ink ejection mechanism and a piezoelectric element driving mechanism of this embodiment. A nozzle indicated by reference numeral 1 in the drawing, an ink chamber 2 communicating with the nozzle, an ink supply path 3 for supplying ink to the ink chamber, and a part of the ink chamber 2 are used as pressure walls 2p and are joined to the piezoelectric element 4. The piezoelectric element 4 is composed of piezoelectric ceramics in which electrodes 4e are formed on both sides, and the electrodes 4e are formed on the surface orthogonal to the surface joined to the pressure wall 2p to vibrate the thickness. The operation of expanding the ink chamber can be performed by applying the voltage of the element. In this embodiment, the charging / discharging device indicated by reference numeral 5 in the drawing drives the piezoelectric element by a so-called so-called ejection method to eject ink.
A bias voltage applying device indicated by reference numeral 6 in the drawing has a structure in which a voltage in the same direction as the polarization voltage of the piezoelectric element is applied to the piezoelectric element when the ink chamber is expanded, contracted, or before or after the expansion. As a result, the driving speed as described in the first and second embodiments can be increased, and fatigue life of the piezoelectric element can be prevented to prolong the life significantly.

Further, in this embodiment, in order to improve the gradation recording property, the driving voltage of the piezoelectric element is modulated in eight steps, and
When the normal ink discharge amount is 100%, the ink discharge amount per time is 90%, 80%, 70%, ..., 30%, and 10%.
It is possible to modulate the ink discharge amount up to 30% at a% interval. That is, when ejecting 100% of the ink amount, as shown in FIG. 12, the charging voltage is set to VD1 with respect to the bias voltage Vb to distort the piezoelectric element in the direction of expanding the ink chamber, and discharge to Vb at the ejection timing. In the case of 60% and 30% ejection, for example, the piezoelectric element is charged by the voltages shown by VD2 and VD3 in FIG. 12 to reduce the expansion amount of the ink chamber and Vb at the ejection timing.
The required amount of ink is ejected by discharging the ink. When the piezoelectric element is driven by modulating the voltage as in this embodiment by the conventional method and the amount of ink ejection is attempted to be modulated, the electromechanical conversion efficiency varies near the voltage 0 of the piezoelectric element, and the driving voltage is modulated. However, it was impossible to quantitatively control the discharge amount. According to the present invention, since a voltage higher than the voltage Vb is constantly applied to the piezoelectric element for driving, the piezoelectric element can be driven in the region VD where the electromechanical conversion efficiency is constant as shown in the graph of FIG. Since the efficiency is not constant and the characteristics near distortion 0 are not affected, the ejection amount can be modulated according to the design value.

FIG. 13 shows a drive circuit for the piezoelectric element of this embodiment. The charging / discharging circuit CG has the same structure as that of the second embodiment described above, and the piezoelectric element C has a charge control terminal tp1.
Is charged by applying a control voltage of 5 V to the discharge control terminal and discharged by applying a control voltage of 5 V to the discharge control terminal tp2. Further, since the bias voltage is determined in the portion indicated by BC in the figure, the charging / discharging of the piezoelectric element C is always the bias voltage Vb.
This is done. Further, as shown in FIG. 12, in order to modulate the charge potential, in this embodiment, the charge control signal applied to the charge control terminal tp1 is pulse-width modulated in accordance with the density of the print data to modulate the charge time and change the charge amount. Have control. In the conventional method of charging from 0V, the charging potential is not proportional to the charging time even if the charging time is controlled, and the charging potential becomes different from the design value due to the characteristic variation of the piezoelectric element. In this embodiment, the charging potential of the piezoelectric element can be regulated according to the design value by the charging time.

FIG. 14 is a sectional view showing the fourth and fifth embodiments of the present invention, which is an example in which a structural material for holding the piezoelectric element in a compressed state is provided. In FIG. 14A, the piezoelectric element 4 having the electrodes 4e on both sides is held in a compressed state by the pressing member 8 via the spacer 8s. In this embodiment, the compression member 8 is made of zirconia ceramics, and the spacers 8s are made of resin. The compression member 8 sandwiches the piezoelectric element 4 and is then given a pressure in a direction to compress the piezoelectric element 4 by an adjusting screw (not shown). ing. FIG. 14B shows the nozzle 1 and the ink chamber 2.
In this embodiment, the ink flow path including the ink supply path 3 and the pressure wall 2p provided in a part of the ink chamber 2 and the piezoelectric element 4 connected to the pressure wall 2p are used. It is pre-compressed in the vertical direction of the figure by the compression member 8 made of an alloy. In all the examples shown in FIG. 14, the piezoelectric element is structurally in a compressed state. By doing so, even if the bias voltage is not applied during driving, the strain of the piezoelectric element does not exceed 0 and does not reverse due to overshoot during discharging of the piezoelectric element, and it is possible to prevent fatigue breakdown of the piezoelectric element.

As described above, the present invention as described in the present embodiment has the same effect in the so-called push driving method and pulling driving method. The piezoelectric element described in this embodiment ejects ink by utilizing piezoelectric ceramics, or displacement of the laminated piezoelectric ceramics in the thickness direction, or converts strain in the thickness direction into bending displacement to generate ink. Although the discharge method is used, the present invention has the same effect even if the displacement of the piezoelectric element is a shear displacement. In addition, in each of the present embodiments, the piezoelectric element has a structure that displaces the wall surface of the ink chamber, but the present invention also applies to a system in which the piezoelectric element is provided in the ink chamber and pressure is directly applied to the ink. Have an effect. Furthermore, in the present embodiment, the method of defining the discharge potential of the charge / discharge circuit is used as the method of applying the bias voltage to the piezoelectric element, but the present invention limits the discharge time of the piezoelectric element and holds the potential of the piezoelectric element. The same effect can be obtained regardless of the bias voltage application method.

[0037]

According to the present invention, the piezoelectric element is always driven in a strain state in the same direction without exceeding 0 and being inverted. Therefore, fatigue failure of the piezoelectric element is prevented and the life is remarkably improved. Further, since the piezoelectric element is driven and operated in a portion having high electromechanical conversion efficiency, high speed and high efficiency driving operation is possible. Further, when a plurality of piezoelectric elements are used, the driving operation is performed not in the vicinity of strain 0 where the variation of electromechanical conversion characteristics is severe but in a region of uniform characteristics, so that characteristic variations between elements can be substantially reduced with a simple structure. . Therefore, high-speed repetitive driving can be substantially performed using a large number of piezoelectric elements, and a high-speed inkjet recording device can be realized.

Further, according to the present invention, in addition to the effect described in the embodiment, by keeping the piezoelectric element always in a distorted state, the rigidity of the piezoelectric element is increased and unnecessary vibration of the pressure wall of the ink chamber is suppressed. Therefore, there is also an effect that crosstalk due to propagation vibration between elements can be reduced. Further, since the voltage in the same direction as that at the time of polarization is constantly applied to the piezoelectric element, there is also an effect that the polarization state is stably maintained even during long-term repeated driving.

[Brief description of drawings]

FIG. 1 is a sectional view and a configuration diagram of an apparatus showing a first embodiment of the present invention.

FIG. 2 is a perspective view of an apparatus showing a first embodiment of the present invention.

FIG. 3 is a diagram showing drive waveforms and displacement according to the first embodiment of the present invention.

FIG. 4 is a diagram showing electromechanical conversion characteristics of a piezoelectric element.

FIG. 5 is a circuit diagram of a first embodiment of the present invention.

FIG. 6 is a perspective view of an apparatus showing a second embodiment of the present invention.

FIG. 7 is a sectional view and a configuration diagram of an apparatus showing a second embodiment of the present invention.

FIG. 8 is a diagram showing a drive waveform and a displacement according to a second embodiment of the present invention.

FIG. 9 is a circuit diagram of an apparatus showing a second embodiment of the present invention.

FIG. 10 is a perspective view of an apparatus showing a third embodiment of the present invention.

FIG. 11 is a sectional view and a configuration diagram of an apparatus showing a third embodiment of the present invention.

FIG. 12 is a diagram showing drive waveforms and displacement according to the third embodiment of the present invention.

FIG. 13 is a circuit diagram of a third embodiment of the present invention.

FIG. 14 is a sectional view showing the fourth and fifth embodiments of the present invention.

FIG. 15 is a diagram showing the structure and polarization direction of a piezoelectric element.

FIG. 16 is a sectional view of a device showing a conventional example.

FIG. 17 is a sectional view of an apparatus showing a conventional example.

FIG. 18 is a diagram showing a drive waveform and displacement of a conventional example.

FIG. 19 is a cross-sectional view showing a destruction example of a conventional piezoelectric element.

FIG. 20 is a diagram showing driving displacement of a piezoelectric element.

[Explanation of symbols]

 1: Nozzle 2: Ink chamber 2p: Pressure wall 3: Ink supply path 4: Piezoelectric element 5: Charge / discharge device 6: Bias voltage application device 7: Switching device 8: Compression member

Claims (5)

[Claims] 1. A nozzle for ejecting ink, an ink chamber communicating with the nozzle, a pressure wall for changing the volume of the ink chamber provided in a part of the ink chamber, and supplying ink to the ink chamber. A supply path and a piezoelectric element whose shape is changed by applying a voltage are provided, the pressure wall and the piezoelectric element are mechanically connected, and the volume of the ink chamber is changed by a change in the voltage applied to the piezoelectric element. In an inkjet recording apparatus that expands and contracts, or contracts and expands and ejects ink from the nozzles, a mechanism that constantly applies unidirectional strain to the piezoelectric element when the pressure chamber is expanded, contracted, or before or after expansion. An inkjet recording apparatus comprising: 2. A mechanism for applying a bias voltage in the same direction as the polarization voltage of the piezoelectric element to the piezoelectric element when the pressure chamber is expanded, contracted, or before or after the expansion. Item 2. The inkjet recording device according to item 1. 3. The bias voltage has a maximum value ΔD of an amplitude of residual vibration during a discharge operation of the piezoelectric element, a total thickness H of a voltage applying portion of the piezoelectric element, and a distance between adjacent electrodes of the piezoelectric element. 3. The ink jet recording apparatus according to claim 2, wherein the distance is equal to or more than a value (ΔD × h) / (d × H) defined by the distance h and the piezoelectric constant d. 4. The ink jet recording apparatus according to claim 1, further comprising a structural member that holds the piezoelectric element in a compressed state, or a pressure mechanism. 5. A nozzle for ejecting ink, an ink chamber communicating with the nozzle, a pressure wall for changing the volume of the ink chamber provided in a part of the ink chamber, and supplying ink to the ink chamber. A supply path and a piezoelectric element whose shape is changed by applying a voltage are provided, the pressure wall and the piezoelectric element are mechanically connected, and the volume of the ink chamber is changed by a change in the voltage applied to the piezoelectric element. In an ink jet recording apparatus that expands and contracts, or contracts and expands and ejects ink from the nozzles, the electric field applied to the voltage application unit of the piezoelectric element is in the same direction as during polarization and is always 50 kV / m or more. An inkjet recording apparatus comprising a mechanism driven by electric field strength.