JP3594628B2 - Ink jet recording device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to an ink jet recording apparatus that discharges ink onto paper or a recording medium based on data to record characters, images, and the like.
[0002]
[Prior art]
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 since recording apparatuses are small and have low noise and both apparatus cost and running cost are relatively low. However, when high-speed recording is required due to a low printing speed, an expensive, complicated, and large-sized electrophotographic recording apparatus must be used. In order to increase the recording speed of the ink jet recording apparatus to a speed comparable to that of the electrophotographic method, it is necessary to improve the repetition response frequency of ink ejection per nozzle of the ink jet recording apparatus and to increase the number of nozzles of the recording apparatus. It is required to increase.
[0003]
The repetition response frequency of ink ejection per nozzle of the current inkjet recording apparatus is about 3 to 10 kHz, but when this repetition frequency is experimentally increased and driven, ink ejection from each nozzle is possible. When the variation in the ejection ink amount and the ejection ink speed among the nozzles becomes severe, and when the drive repetition frequency is further increased, ink ejection is possible in some nozzles but not in the other nozzles. That is, if the ejection repetition frequency is increased in the current inkjet recording apparatus, the ink ejection characteristics between the nozzles become more uneven or ejection failure occurs, which makes the ink jet recording apparatus impractical.
[0004]
Furthermore, when the ejection repetition frequency is experimentally increased in the current inkjet recording apparatus, the fatigue life caused by the repeated compression and expansion of the piezoelectric element significantly shortens the destruction life, which also impairs the practicality of the recording apparatus. . Further, in order to realize high-speed printing comparable to the electrophotographic system, the number of ink nozzles must be increased from the current 48 to 64 nozzles to at least 100 or more at the same time as the ink ejection repetition frequency is increased. However, in the ink jet recording apparatus, if an ejection failure occurs even for one nozzle, the recording apparatus becomes defective. Therefore, increasing the number of nozzles in the current ink jet system significantly increases the rejection rate of the apparatus. It was difficult to obtain sex.
[0005]
In other words, the conventional ink jet recording apparatus has problems in that the ink ejection characteristics vary when the speed is increased, the life is short due to the fatigue destruction of the piezoelectric element, and the failure rate of the piezoelectric element is too high.
[0006]
16 and 17 show cross sections of an example of the configuration of an ink ejection section of a conventional ink jet recording apparatus. FIG. 16 shows that the ink filled in the ink flow path including the ink chamber 2 and the ink supply path 3 communicating with the nozzle 1 is transferred to the piezoelectric element 4 connected to the pressure wall 2 p provided in a part of the ink chamber 2. When voltage is applied by the electrodes 4e on both surfaces, distortion is generated in the piezoelectric element 4 as a change in aspect ratio, so that the ink chamber 2 is pressurized and ink is ejected from the nozzle 1. FIG. 17 shows a piezoelectric device in which ink is filled in an ink flow path including an ink chamber 2 and an ink supply path 3 communicating with a nozzle 1, and a pressure wall 2p provided in a part of the ink chamber 2 has electrodes 4e on both surfaces. A piezoelectric element 4 is formed by laminating a ceramic and an elastic plate 4t, and a voltage is applied between the electrodes to bend the piezoelectric element 4, pressurize the ink chamber 2, and discharge ink from the nozzle 1. 16 and 17 shown as conventional examples are common to ink jet recording apparatuses using piezoelectric elements in that the ink chambers are expanded or reduced by a voltage applied to the piezoelectric elements.
[0007]
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, a solid line V indicates a voltage waveform to be applied, and a broken line P indicates distortion of the piezoelectric element. The horizontal axis of the graph represents time, and the vertical axis represents voltage and distortion. The voltage waveform V in the figure is a voltage value, and the strain P of the piezoelectric element is a displacement value. Therefore, the units on the vertical axis of the graph are different, but are overlapped for the sake of explanation in this figure. The upward direction of the graph is a case where a voltage in the same direction as the polarization voltage of the piezoelectric element is applied, and the distortion of the piezoelectric element at that time is a direction in which the distance between the electrodes is extended. The state on the horizontal axis 0 of the graph is the state of zero distortion, and the downward direction is the distortion in the contraction direction.
[0008]
FIG. 18A shows a driving method referred to as a pushing operation, in which the ink chamber is contracted by applying a voltage to the piezoelectric element and charging the same, and a pulse-like voltage is applied to the piezoelectric element as indicated by reference numeral I in the drawing. This is a method in which ink is ejected from a nozzle by rapidly charging and distorting the applied piezoelectric element to reduce the volume of the ink chamber by distortion. After the ink is ejected, the piezoelectric element is discharged as shown by a symbol R in the drawing to set the distortion of the piezoelectric element to 0 and return the ink chamber to the state before the ejection. In the structure shown in FIG. 16A, when a voltage is applied to the piezoelectric element in the same direction as that during polarization, the piezoelectric element is distorted in a direction in which the distance between the electrode surfaces 4e is expanded. So that the ink chamber can be compressed. Similarly, when the piezoelectric element having the structure shown in FIG. 17A is driven by the driving method shown in FIG. 18A, the piezoelectric element 4 is rapidly charged by applying a voltage to the piezoelectric element as indicated by reference numeral I. Expands in the thickness direction and contracts in the length direction at the same time, so that a difference in distortion from the elastic plate 4t attached to the piezoelectric element occurs, and the diaphragm warps in the direction in which the ink chamber is pressurized, thereby discharging ink. After the ink is ejected, the piezoelectric element is discharged to a state of zero by discharging the piezoelectric element as indicated by a symbol R in the drawing, and the ink chamber is returned to the state before the ejection.
[0009]
FIG. 18 (b) shows a driving method called pulling, in which the ink chamber is expanded by charging the piezoelectric element, and a voltage is applied to the piezoelectric element as indicated by a symbol R in the drawing in preparation for ink ejection. The ink chamber is expanded by charging and distorting the piezoelectric element, and the charged piezoelectric element is suddenly discharged at the timing of ink discharge as shown by the symbol I in the figure to restore the distortion of the vibrator to the original state. When the ink chamber is released, the ink chamber is pressurized to discharge ink. In the structure of FIG. 16B, the piezoelectric element is distorted in the direction in which the ink chamber 2 is expanded by the application of a voltage, so that the ink can be ejected by the driving method of FIG. Similarly, in the structure of FIG. 17B, the warpage of the piezoelectric element occurs in the direction in which the ink chamber 2 is expanded by the application of the voltage, so that the ink can be ejected by the driving method of FIG.
[0010]
As described in U.S. Pat. No. 4,471,363 and the like, a conventional ink jet recording apparatus using a piezoelectric element applies an example shown in FIG. 18 or an example in which the timing is improved, and a combination thereof to apply to the piezoelectric element. There are examples in which the voltage can be inverted. However, when the recording speed is increased by the conventional methods, there is a problem that the failure due to the fatigue breakdown of the piezoelectric element and the life of the piezoelectric element are remarkably reduced, which is not practical. FIG. 19 is a cross-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 denotes a piezoelectric ceramic, and reference numeral 4e denotes an electrode for applying a voltage to the piezoelectric ceramic. FIGS. 19A, 19B, and 19C show examples of a piezoelectric element that expands or contracts an ink chamber by strain displacement in the thickness direction. FIGS. 19 (d), (e), and (f) show a piezoelectric element in which the elastic plate 4t is attached to the piezoelectric element to convert the distortion of the piezoelectric ceramic 4 in the thickness and spreading direction into a bending displacement to expand or contract the ink chamber. It is an example of an element. When these piezoelectric elements are driven for long-time ink ejection by the conventional method shown in FIG. 18, cracks occur in the portion illustrated by reference character K in FIG. 19, and the driving operation of the piezoelectric elements is not performed. The characteristics are significantly impaired, making it impossible to eject ink. FIGS. 19A and 19D show an example in which a crack K is generated near the joint between the piezoelectric ceramics 4 and the electrode 4e. Similarly, the cracks indicated by the symbol K in FIGS. 19B and 19E are piezoelectric ceramics. 19 (c) and 19 (f) are cracks generated inside the piezoelectric ceramics. In any case, it is considered that fatigue fracture due to repeated extension of the piezoelectric element occurred in a portion where stress tends to concentrate, such as a change in the composition of the joint, a minute defect, or a foreign matter. In addition, when the recording speed is increased with an ink jet recording apparatus using a conventional piezoelectric element, fatigue breakdown of the piezoelectric element becomes apparent, and the electromechanical conversion characteristics of the piezoelectric element significantly vary among the elements, which is not practical. There were also problems.
[0011]
[Problems to be solved by the invention]
Therefore, the problem to be solved by the present invention is to increase the recording speed of an ink jet recording apparatus using a piezoelectric element. It is to minimize the variation.
[0012]
[Means for Solving the Problems]
In order to solve such a problem, in the present invention, a nozzle that discharges ink, an ink chamber that communicates with the nozzle, and a pressure wall that changes the volume of an ink chamber provided in a part of the ink chamber are provided. A supply path for supplying ink to the ink chamber, and a piezoelectric element whose shape is changed by applying a voltage, wherein the pressure wall and the piezoelectric element are mechanically connected, and a voltage applied to the piezoelectric element is provided. In the on-demand type ink jet recording apparatus for expanding and reducing the volume of the ink chamber, or reducing and expanding the volume of the ink chamber and ejecting ink from the nozzles, at the time of expanding or reducing the pressure chamber or before or after it Also provided with a mechanism for applying a bias voltage in the same direction as the polarization voltage of the piezoelectric element as a mechanism for always applying a one-way strain to the piezoelectric element, The mechanism for applying the bias voltage applies the bias voltage so that the strain width of the piezoelectric element due to the applied bias voltage is larger than the maximum value of the amplitude of the residual vibration of the piezoelectric element. The distortion direction of the piezoelectric element is not reversed by the residual vibration of the piezoelectric element that occurs when the element expands or contracts.
Further, in the ink jet recording apparatus, preferably, the bias voltage is a maximum value ΔD of an amplitude of a residual vibration during a discharging operation of the piezoelectric element, a total thickness H of a voltage applying portion of the piezoelectric element, It is characterized by being equal to or more than a value (ΔD × h) / (d × H) defined by a distance h between adjacent electrodes and a piezoelectric constant d.
In the ink jet recording apparatus, preferably, the bias voltage is at least twice the specified value.
[0013]
Further, in the above-described ink jet recording apparatus, preferably, the ink discharge amount is modulated by changing a maximum voltage applied to the piezoelectric element.
Further, in the ink jet recording apparatus, preferably, the electric field applied to the voltage applying unit of the piezoelectric element is driven in the same direction as the polarization, and is always driven at an electric field strength of 50 kV / m or more.
In addition, an ink jet recording apparatus according to another aspect of the present invention includes a nozzle that discharges ink, an ink chamber that communicates with the nozzle, and a pressure wall that changes the volume of an ink chamber provided in a part of the ink chamber. A supply path for supplying ink to the ink chamber, and a piezoelectric element whose shape is changed by applying a voltage, wherein the pressure wall and the piezoelectric element are mechanically connected, and a voltage applied to the piezoelectric element is provided. In the on-demand type ink jet recording apparatus for expanding and reducing the volume of the ink chamber, or reducing and expanding the volume of the ink chamber and ejecting ink from the nozzles, at the time of expanding or reducing the pressure chamber or before or after it Also, as a mechanism for constantly applying a one-way strain to the piezoelectric element, a structural material for holding the piezoelectric element in a compressed state or a pressing mechanism is provided. Characterized in that the strain direction of the piezoelectric element by the residual vibration of the piezoelectric element occurs during expansion or during reduction of the pressure chamber is prevented from inverting.
[0014]
[Action]
The piezoelectric element is always operated in one-way strain without distortion of the piezoelectric element caused before or after charging or discharging of the piezoelectric element or before and after the state of the piezoelectric element does not repeat the state of compression or expansion beyond the state of zero or zero. Therefore, the fatigue fracture of the piezoelectric element is significantly reduced. In addition, since the piezoelectric element is always used in a state where a constant voltage or more is applied, the variation in electromechanical conversion characteristics between the piezoelectric elements is extremely large. it can.
[0015]
【Example】
Therefore, the details of the present invention will be described below based on the illustrated embodiment.
[0016]
A first embodiment using the present invention will be described with reference to FIGS. 1, 2, 3, 4, and 5. FIG. This embodiment is an ink jet recording apparatus capable of recording 12 sheets per minute on an A4 large sheet recording medium. In the present embodiment, in order to enable high-speed printing, 640 ink jet elements, which are a set of ink nozzles, ink chambers communicating with the ink nozzles, and piezoelectric elements that pressurize the ink chambers, are used, and the repetitive drive frequency of each element is 24 kHz. I have. The printing density is 600 dots / inch, and high-definition image recording is possible at high speed.
[0017]
FIG. 2 is a perspective view showing the appearance of the components of the present embodiment. In FIG. 2, reference numeral 10 denotes a recording head, reference numeral 11 denotes a head drive circuit, reference numeral 12 denotes recording paper, and reference numeral 13 denotes a head support mechanism. Reference numeral 14 denotes a head moving mechanism, reference numeral 15 denotes a printing drum, reference numeral 16 denotes a recording paper clamping mechanism, reference numeral 17 denotes a drum driving mechanism, and reference numeral 18 denotes an ink tank. The recording paper 12 is fed from a paper cassette (not shown), wound around a print drum 15 and fixed by a recording paper clamp mechanism 16. The recording head 10 has 640 linearly arranged ink jet elements, each of which includes a nozzle for discharging ink, an ink chamber communicating with the nozzle, and a piezoelectric element for pressurizing the ink. Each nozzle interval is 8/600 inch, and each time the print drum 15 makes one rotation, it is moved by the head moving mechanism 14 by 1/600 inch in a direction parallel to the print drum axis. The recording head 10 is driven in synchronization with the rotation of the printing drum 15. Since the repetitive drive frequency of one element of the recording head 10 is 24 kHz, the recording can be performed within one second per rotation of the printing drum, and the recording on the entire surface of the paper is completed by eight rotations of the printing drum. Therefore, the recording apparatus of this embodiment is capable of high-speed recording by increasing the driving frequency and increasing the number of nozzles of the piezoelectric element even though the recording apparatus is of the ink jet type.
[0018]
FIG. 1 is a diagram showing a configuration of one nozzle, an ink chamber, and a piezoelectric element of the recording head of the embodiment shown in FIG. 2, and a configuration of an apparatus for driving the piezoelectric element. In the figure, reference numeral 1 denotes a nozzle, and reference numeral 2 denotes an ink chamber. Ink is supplied from an ink tank through an ink supply path indicated by reference numeral 3 in the figure to fill the ink chamber 2. A pressure wall indicated by reference numeral 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 embodiment is called a laminated type piezoelectric element in which a plurality of plate-shaped piezoelectric ceramics are stacked with a film-shaped electrode 4e interposed therebetween, and a voltage is applied between the alternately connected electrodes. Polarized. FIG. 15A shows the electrode structure of the laminated piezoelectric element used in this embodiment and the polarization state of each piezoelectric ceramic layer. FIG. 15B shows the most basic piezoelectric element, which is a single-layer piezoelectric element having electrodes formed on both surfaces. Here, a voltage is applied to the piezoelectric ceramics 4 by a power source E for polarization so that the upper electrode 4e in the figure is positive and the lower electrode 4e is negative. The electric charge in the piezoelectric element is polarized in the direction indicated by reference numeral 4c in the drawing. In this embodiment, the direction of polarization in this case is indicated by an arrow 4a extending from plus to minus, and the polarities of the terminals of the piezoelectric element polarized in this state are described as + on the plus side during polarization and-on the minus side. FIG. 15A showing a laminated type piezoelectric element used in the present embodiment has electrodes 4e alternately formed in a comb-tooth shape with a piezoelectric ceramic layer 4 interposed therebetween. Each piezoelectric layer 4 is alternately polarized, 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 by a plurality of elements, and a symbol ts in the figure is a segment electrode terminal switched by each element. In this embodiment, the piezoelectric element 4 is polarized with the common electrode terminal tc side minus and the segment electrode terminal ts side plus. 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 a direction in which the distance between the electrodes is expanded, and at the same time, reduced in a direction perpendicular thereto. In the present embodiment, as shown in FIG. 1, the surface perpendicular to the electrode 4e is connected to the thick wall 2p. 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 is connected to a negative power supply of the bias voltage Vb and a negative power supply − (Vb + VD) of a voltage obtained by adding the drive voltage VD to the bias voltage Vb. The charging operation and the discharging operation of the piezoelectric element 4 can be performed by controlling the potential of tc. Further, the switching device connected to the segment electrode terminal of each piezoelectric element can turn on / off the discharging operation of each piezoelectric element according to the print data. Here, the charging / discharging device 5 includes a bias voltage applying device 6, and the piezoelectric element 4 is applied with a bias voltage Vb in the same direction as the polarization voltage to expand and contract the ink chamber for the ink discharging operation. Is charged and discharged.
[0019]
Next, the ink discharging operation of the present embodiment will be described. FIG. 3 is a graph showing the drive voltage between the electrodes of the piezoelectric element shown in FIG. 2 and the distortion of the piezoelectric element at that time. In this embodiment, as shown by B in the figure, a bias signal Vb in the same direction as the polarization voltage is applied to the piezoelectric element at the same time as the recording signal is input and the recording operation is started while the power of the recording apparatus is turned on. As a result, the piezoelectric element enters a base distortion state during a printing operation. In this embodiment, since the so-called pulling operation described in the conventional example is performed, as shown by a reference symbol R in FIG. 3, a voltage VD is further applied to the piezoelectric element 4 by the driving voltage generator 5 shown in FIG. To charge the piezoelectric element, causing a 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 generating device performs a circuit operation so as to rapidly discharge the driving voltage VD as shown by reference numeral I in FIG. Vb is discharged, the distortion of the piezoelectric element is released to the base distortion state, the ink chamber is compressed, and ink is ejected from the nozzle. After the ink discharge, the drive voltage generator gradually applies the voltage VD to the piezoelectric element to expand the ink chamber in preparation for the next ink discharge operation.
[0020]
In this operation, the piezoelectric element is always in a one-way distortion state. In the conventional method, as shown by reference symbol O in FIG. 18 (b), it is inevitable that the piezoelectric element is temporarily distorted in the opposite direction due to overshoot due to rapid discharge during ink discharge. This has caused the fatigue breakage of the piezoelectric element. According to the present invention, the bias voltage Vb in the same direction as the polarization of the piezoelectric element is always applied to the piezoelectric element, so that, as indicated by reference numeral O in FIG. Since the distortion direction of the piezoelectric element does not exceed 0 and does not reverse, even if the high-speed repetitive operation is performed for a long time, the piezoelectric element does not break down due to fatigue. The bias voltage Vb of the present embodiment is determined by the distance h between the adjacent electrodes of the piezoelectric element shown in FIG.Total thickness HAnd approximately twice the value (ΔD × h) / (d × H) defined from the actually measured maximum value ΔD of the residual vibration during the discharging operation of the piezoelectric element and the piezoelectric constant d of the piezoelectric element. Voltage. In this embodiment, ΔD is measured by using a laser Doppler velocimeter under a condition close to an actual driving condition in a system having the structure of the piezoelectric element and the ink chamber shown in FIG. The results were integrated to determine the amount of displacement of the piezoelectric element, and the difference was determined as the difference between the maximum displacement during overshoot and the converged displacement during the discharging operation of the piezoelectric element. In this example, the piezoelectric constant d used was d31 which is a physical property value of the piezoelectric element.
[0021]
FIG. 20 shows a strain displacement state of a general piezoelectric element. The piezoelectric element is charged with a constant current from a state in which the potential between the electrodes of the piezoelectric element is set to 0 to a voltage in the same direction as the polarization, and when the charging is completed, the voltage is changed. Is held for a certain period of time, and then the piezoelectric element is discharged with the same constant current as during charging until the potential between the electrodes becomes 0, showing the strain displacement of the piezoelectric element. 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 charging and discharging of the piezoelectric element is performed relatively slowly as shown in FIG. 20A and displaced, the amount of overshoot of the distortion displacement of the piezoelectric element is small, and as shown in FIG. It can be seen that when the discharge operation is performed, the overshoot of the strain displacement of the piezoelectric element becomes severe. The maximum value ΔD of the amplitude of the residual vibration during the discharging operation of the piezoelectric element described in the present invention indicates the amount of overshoot in the portion indicated by Po in FIG.
[0022]
FIG. 4 is a graph showing the electromechanical conversion characteristics of a general piezoelectric element. The horizontal axis indicates the voltage applied to the piezoelectric element, and the vertical axis indicates the distortion factor. In general, the distortion 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 graph shows a substantially constant gradient from the middle. According to the present invention, by applying the bias voltage of Vb shown in FIG. 4, the electric power is most reduced compared to the conventional method of driving the piezoelectric element by changing the voltage from the voltage 0 shown by the reference symbol VDo in the figure. Since the piezoelectric element is driven in the VD portion having good mechanical conversion efficiency, there is an advantage that the electromechanical conversion efficiency is excellent. In addition, as a result of investigations according to the present invention, most of the inevitable variations in electromechanical characteristics between elements when using a plurality of piezoelectric elements appear as variations in electromechanical conversion efficiency near a voltage of 0, and have a certain level. It was found that the electromechanical conversion efficiency did not vary above the voltage and showed a substantially constant value. Further, it has been found that in most of the piezoelectric elements, by setting the electric field strength of the driving portion to 50 kV / m or more, the characteristics greatly vary among a plurality of piezoelectric elements, and a region having 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 in which the electromechanical conversion characteristics of a plurality of piezoelectric elements manufactured by the same manufacturing method are measured. It can be seen that the slopes of the graph showing the electromechanical conversion ratio 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, in addition to being able to drive the portion of the piezoelectric element having the highest electromechanical conversion efficiency, the variation in the electromechanical conversion efficiency of a plurality of piezoelectric elements can be ignored during driving, so that the piezoelectric element can be driven at high speed. And driving characteristics can be kept constant.
[0023]
FIG. 5 shows a driving circuit of the piezoelectric element of this embodiment. A portion surrounded by a broken line indicated by reference numeral CG in the figure is a charge / discharge circuit for charging and discharging the piezoelectric elements, and reference numerals C1, C2,... Represent a plurality of piezoelectric elements provided in 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 charge / discharge circuit CG is commonly connected to the minus electrodes of the plurality of piezoelectric elements to perform charging and discharging. The positive electrodes of the plurality of piezoelectric elements C1, C2, ... are connected to the respective switching circuits S1, S2, ..., and apply a control voltage to the respective control terminals tpS1, tpS2 to apply a control voltage to each of the elements. Controls ON / OFF of discharge. The charge / discharge circuit CG is connected to two negative power supplies having different potentials of-(Vb + VD) and -Vb, and the charge control terminal tp1 and the discharge control terminal tp2 are set to a potential of 5 V in an initial state. In the circuit, by setting the potential of the charge control terminal tp1 to 0 V, the portion indicated by the symbol M1 in the figure is turned on, the capacitor C0 is discharged with a constant current, the transistor Tr1 is turned on, and the charge / discharge terminal tp3 is set to-( (Vb + VD) are charged to the piezoelectric elements C1, C2,... Therefore, the piezoelectric elements C1, C2,... Are charged to the potential Vb + VD and are distorted in a direction to expand the respective ink chambers. Next, a discharging operation of the piezoelectric element for discharging ink will be described. Here, an element that causes ink ejection is C1 and an element that does not eject ink is C2. The charge control terminal tp1 of the charge / discharge circuit CG is returned to the original state of 5V, and the discharge control terminal tp2 is set to 0V, thereby turning on the portion indicated by the symbol M2 in the figure and starting to charge the capacitor C0. The state becomes ON, and the potential of the charge / discharge terminal tp3 increases. Here, by setting the switching terminal tpS1 of the switching circuit connected to the piezoelectric element to be discharged to 5 V, the piezoelectric element C1 is discharged through the switching circuit S1. On the other hand, by setting the switching terminal tpS2 of the switching circuit S2 connected to the non-ejected piezoelectric element C2 to 0 V and setting the switching circuit S2 to the OFF state, the piezoelectric element C2 is not discharged even when the potential of tp3 increases. The piezoelectric element C1 is discharged to the voltage Vb because the piezoelectric element C1 is discharged until the potential on the upper side of the capacitor C0 in the drawing becomes -Vb which is the potential of the bias power supply connected through the diode. Therefore, when the piezoelectric element is driven by using the circuit of this embodiment, it is possible to perform the pulling drive operation while always applying the bias voltage Vb in the same direction as the polarization voltage to the piezoelectric element.
[0024]
As described above, according to the first embodiment of the present invention, the characteristics of the piezoelectric element are uniform and defects such as fatigue fracture are caused despite the fact that each nozzle is repeatedly driven at a maximum of 24 kHz using a large number of nozzles of 640 elements. Can be practically eliminated, and practically high-speed recording has become possible.
[0025]
According to the experiment of the present embodiment, the average life of the piezoelectric element due to fatigue fracture was extended more than twice as compared with the conventional method, and the characteristic variation among a plurality of elements was reduced to a substantially negligible extent.
[0026]
A second embodiment using the present invention will be described with reference to FIGS. 6, 7, 8, and 9. FIG. FIG. 6 is a perspective view showing the components of the printing apparatus. The printing apparatus prints A4 printing paper at a printing density of 300 dpi by serially scanning a printing head of 120 elements. In the drawing, reference numeral 20 denotes a recording head, which includes 120 nozzles, ink chambers communicating with the nozzles, and 120 ink jet elements which constitute one set of piezoelectric elements arranged in each of the ink chambers. In the drawing, reference numeral 21 denotes a replaceable ink tank which is connected to the recording head 20 and supplies ink. The recording paper indicated by reference numeral 22 in the drawing is supplied to a position below the head by a paper feed roller indicated by reference numeral 23 in the drawing. The recording head 20 has a structure in which a head moving mechanism 24 serially scans the recording paper 22 along a slide shaft 25.
[0027]
FIG. 7 shows one piezoelectric element and an ink ejection mechanism of the recording head and a driving mechanism of the piezoelectric element. An ink chamber 2 communicating with the nozzle 1 is supplied with ink from an ink supply path 3. A part of the wall surface of the ink chamber is a pressure wall 2p, and a piezoelectric element composed of a piezoelectric ceramic 4 having electrodes 4e on both surfaces is bonded. The pressure wall 2p also has a function of an elastic plate 4t for converting a strain displacement of the piezoelectric ceramics 4 into a 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 generates expansion strain in the thickness direction and shrinks in the spreading direction, so that the difference in elastic modulus between the elastic plate 4t and the bonded elastic plate 4t. The elastic plate warps outward. In this embodiment, since the elastic plate 4t also serves as the pressure wall 4p of the ink chamber, when a voltage is applied to the piezoelectric element 4 in the same direction as the voltage applied during polarization, the ink chamber is reduced. In this embodiment, the positive electrode during polarization of the piezoelectric element 4 is connected to the charge / discharge device 5 in common with a plurality of piezoelectric elements as a common electrode terminal tc, and the negative electrode is used as a segment electrode terminal ts for each element. And is independently connected to the switching device 7. The charging / discharging device 5 is connected to a power source of a bias voltage Vb and a power source of a voltage obtained by adding a drive voltage VD to the bias voltage Vb, and includes a bias voltage applying device 6.
[0028]
FIG. 8 shows the inter-electrode voltage of the piezoelectric element of this embodiment and the bending displacement thereof. In the present 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 shown by reference numeral B in the drawing. The state in which the voltage is applied is set as the base state of the piezoelectric element, and a driving operation for ejecting ink is performed by a method called pressing described in the conventional example. That is, as indicated by reference numeral I in the drawing, the piezoelectric element bends in the direction of pressurizing the ink chamber by applying the voltage VD to the piezoelectric element in response to the drive signal and rapidly charging the piezoelectric element, thereby ejecting ink from the nozzles. When the ejection of the ink is completed, the voltage VD is reduced to the voltage Vb and discharged as shown by a symbol R in the figure, and the distortion of the piezoelectric element is returned to the base state before the ink ejection at the voltage Vb, and the ink chamber is filled. Expand. Although the present embodiment uses a plate-like piezoelectric element having a relatively large overshoot, each piezoelectric element is repeatedly driven at a high speed of up to 12 kHz. Since the state in which the bias voltage is applied to the piezoelectric element is the base state, the distortion direction of the piezoelectric element at the time of overshoot does not exceed 0 and always stays in the same direction without reversing, as shown by reference numeral O in the drawing. Therefore, fatigue breakage of the piezoelectric element is prevented. In addition, since the distortion at the time of overshoot does not become an unstable state near distortion 0, the contraction time of overshoot is faster than that of the conventional method, and since there is no variation between elements, the frequency of high-speed repetitive driving is reduced. Became practically possible. Further, in the present embodiment according to the present invention, the electromechanical conversion efficiency of the piezoelectric element is high as described in the first embodiment, and a constant area can be used between the elements. The effect of improvement and reduction of variation between elements can be effectively obtained, and high-speed driving and multi-element implementation are practically possible.
[0029]
FIG. 9 shows a driving circuit of the piezoelectric element of this embodiment. A portion surrounded by a broken line indicated by reference numeral CG in the figure is a charge / discharge circuit for charging and discharging the piezoelectric elements, and reference numerals C1, C2,... Represent a plurality of piezoelectric elements provided in respective ink chambers. Is shown. The polarity of the applied voltage during polarization of the piezoelectric element is indicated by + and-. In this embodiment, the terminal tp3 of the charge / discharge circuit CG is commonly connected to the plus electrodes of a plurality of piezoelectric elements, and a voltage is applied during the charge and discharge operations. The negative electrodes of the plurality of piezoelectric elements C1, C2, ... are connected to the respective switching circuits S1, S2, ... and apply a control voltage to the respective control terminals tpS1, tpS2 to apply a control voltage to each of the elements. Controls ON / OFF of charging. The charge / discharge circuit CG is connected to two power supplies having different potentials of Vb + VD and Vb, and the charge control terminal tp1 and the discharge control terminal tp2 are set to a potential of 0 V in an initial state. In the circuit, by setting the potential of the charge control terminal tp1 to 5 V, the portion indicated by M1 in the drawing is turned on, charging of the capacitor C0 is started with a constant current, Tr1 is turned on, and the potential of the charge / discharge terminal Tp3 is Vb + VD. To rise. Here, assuming that the piezoelectric element for performing the discharging operation is C1, by setting the switching control terminal tpS1 of the switching circuit S1 connected to the minus terminal of the piezoelectric element C1 to 5 V, a current flows through the piezoelectric element C1 and is charged to the potential Vb + VD. . On the other hand, assuming that the piezoelectric element that does not perform the ejection operation is C2, by setting the switching control circuit tpS2 of the switching circuit S2 connected to the minus terminal of the piezoelectric element C2 to 0 V, the potential of the charge / discharge terminal tp3 of the charge / discharge circuit CG becomes Even if it rises, no current flows through the piezoelectric element C2, so that the charging operation is not performed. The piezoelectric element to be discharged by the above operation is charged, and the ink chamber is pressurized to discharge ink. Next, an operation of discharging all the piezoelectric elements to the potential Vb in order to return the piezoelectric elements to the original state will be described. By returning the charge control terminal tp1 to 0 V and setting the discharge control terminal tp2 to 5 V, the portion indicated by M2 in the figure is turned on, the electric charge of the capacitor C0 is discharged at a constant current, and the transistor Tr2 is turned on. The electric potential of the charge / discharge terminal tp3 is reduced, and the connected piezoelectric elements C1, C2,. Here, since the capacitor C0 is connected to the bias power supply Vb through the diode D, the Tr2 is turned off when the voltage of the CO is substantially Vb, and the piezoelectric element is turned on when the potential of the charge / discharge terminal tp3 is substantially Vb. Discharge stops. Therefore, when the piezoelectric element is driven by using the circuit of the present embodiment, the bias voltage Vb in the same direction as the polarization voltage is always applied to the piezoelectric element, so that a plurality of piezoelectric elements can be driven by pushing.
[0030]
A third embodiment using the present invention will be described with reference to FIGS. 10, 11, 12, and 13. FIG. This embodiment is a color printing apparatus provided with a total of four elements each of which can discharge yellow, magenta, cyan, and black inks. The recording density is 300 dpi and the recording area is equivalent to A6 to A4, and is intended for 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”. By modulating the driving waveform for color recording, the ink ejection amount per time Can be modulated.
[0031]
FIG. 10 shows an element configuration of the color printing apparatus of this embodiment. This embodiment has an element configuration of one nozzle for each color in order to realize color printing at low cost. In the conventional method, when a printing apparatus is configured with a small number of elements, the number of driving operations per element becomes extremely large with respect to the number of printing media, and it is difficult to put the printing apparatus to practical use in terms of life. Since the life of the piezoelectric element was remarkably improved, the recording apparatus of this embodiment was realized. Further, in the present embodiment, the driving repetition frequency of the element can be increased to 28 kHz by the present invention, so that a practical recording speed can be achieved with a small number of recording elements. Numeral 34 in the figure is a drum around which the recording paper 33 is wound. Numeral 30 in the figure is a head having nozzles, ink chambers, and ink ejection elements composed of piezoelectric elements. Four color inks are supplied from the possible ink cartridges 31. The head 30 is fixed to a carriage denoted by reference numeral 32 in the figure, and the carriage 32 is configured to be driven in a drum axis direction by a screw-shaped head moving shaft 35. After the paper is wound, the drum 34 is rotated at a high speed by a DC motor 36, and the head moving shaft is moved by 1/300 inch per one rotation of the drum by a stepping motor 37. In this embodiment, a full-color A5 equivalent screen can be printed in about 2 minutes despite a structure that can be manufactured at a very low cost.
[0032]
FIG. 11 is a view showing a one-element ink ejection mechanism and a driving mechanism of a piezoelectric element of the present embodiment. A nozzle denoted by reference numeral 1 in the drawing, an ink chamber 2 communicating therewith, an ink supply path 3 for supplying ink to the ink chamber, and a part of the ink chamber 2 are pressure walls 2p and are joined to the piezoelectric element 4. The piezoelectric element 4 is made of piezoelectric ceramics having electrodes 4e formed on both surfaces, and the electrodes 4e are formed on the surface orthogonal to the surface joined to the pressure wall 2p and the thickness is vibrated. The operation of expanding the ink chamber can be performed by applying the voltage of the element. In this embodiment, the piezoelectric element is driven by a charging / discharging device indicated by reference numeral 5 in the drawing to drive the piezoelectric element in a so-called pulling mode, and ink is ejected. A voltage is applied to the piezoelectric element in the same direction as the polarization voltage of the piezoelectric element during expansion, reduction, or before and after the expansion. As a result, it is possible to remarkably prolong the service life by increasing the driving speed and preventing the fatigue breakage of the piezoelectric element as described in the first and second embodiments.
[0033]
Further, in this embodiment, in order to improve the gradation recordability, the drive voltage of the piezoelectric element is modulated in eight steps, and when the normal ink discharge amount is 100%, the ink discharge amount at one time is 90%, 80%. %, 70%,..., 30%, and up to 30% at 10% intervals. That is, when discharging 100% of the ink amount, as shown in FIG. 12, the charging voltage is set to VD1 with respect to the bias voltage Vb, the piezoelectric element is distorted in a direction to expand the ink chamber, and the discharge is performed to Vb at the discharge timing. In the case of 60% and 30% discharge, for example, the piezoelectric element is charged with the voltages shown by VD2 and VD3 in FIG. 12 to reduce the expansion amount of the ink chamber, and is discharged to Vb at the discharge timing, respectively. A certain amount of ink is ejected. In the conventional method, when the voltage is modulated as in the present embodiment to drive the piezoelectric element, and the ink ejection amount is modulated, the electromechanical conversion efficiency fluctuates around the voltage 0 of the piezoelectric element, and the driving voltage is modulated. However, quantitative control of the discharge amount was impossible. According to the present invention, since the piezoelectric element is always driven by applying a voltage equal to or higher than the voltage Vb, the piezoelectric element can be driven in a region VD where the electromechanical conversion efficiency is constant as shown in the graph of FIG. Since the efficiency is not affected by the characteristic near distortion 0 where the efficiency is not constant, the modulation of the amount of ejected ink can be performed according to the design value.
[0034]
FIG. 13 shows a driving circuit of the piezoelectric element of the present embodiment. The charging / discharging circuit CG has the same configuration as that of the above-described embodiment 2, and the piezoelectric element C has a control voltage of 5 V applied to the charge control terminal tp1. Is applied, and is discharged by applying a control voltage of 5 V to the discharge control terminal tp2. In addition, since the bias voltage is determined at the portion indicated by BC in the figure, the charging and discharging of the piezoelectric element C is always performed at the bias voltage Vb or higher. In this embodiment, as shown in FIG. 12, in order to modulate the charge potential, the charge control signal applied to the charge control terminal tp1 is pulse width modulated according to the density of the print data, thereby modulating the charge time and the charge amount. Controlling. In the conventional method of charging from 0 V, even if the charging time is controlled, the charging potential is not proportional to the charging time, and the charging potential becomes a design value due to the characteristic variation of the piezoelectric element. In the present embodiment, the charging potential of the piezoelectric element can be defined by the charging time as a design value.
[0035]
FIG. 14 is a sectional view showing the fourth and fifth embodiments according to the present invention, in which a structural material for holding the piezoelectric element in a compressed state is provided. FIG. 14A shows that a piezoelectric element 4 having electrodes 4e on both surfaces is held in a compressed state by a pressing member 8 via a spacer 8s. In this embodiment, the compression member 8 is made of zirconia-based ceramics, and the spacer 8s is made of resin. After the piezoelectric member 4 is sandwiched between the compression member 8 and the adjustment screw (not shown), a pressure is applied in a direction to compress the piezoelectric element 4. ing. FIG. 14B shows an ink flow path including a nozzle 1, an ink chamber 2, and an ink supply path 3, a pressure wall 2p provided in a part of the ink chamber 2, and a piezoelectric element 4 connected to the pressure wall 2p. In the embodiment, the piezoelectric element 4 is compressed in advance in the vertical direction in the figure by a compression member 8 made of a shape memory alloy. In each of the embodiments shown in FIG. 14, the piezoelectric element is structurally in a compressed state. In this way, even if a bias voltage is not applied during driving, the distortion of the piezoelectric element does not exceed 0 due to overshoot at the time of discharging of the piezoelectric element and does not reverse, so that fatigue damage of the piezoelectric element can be prevented.
[0036]
As described above, the present invention is similarly effective in the so-called push driving method and the pull driving method as described in the present embodiment. Further, the piezoelectric element described in the present embodiment is a method of ejecting ink using displacement in the thickness direction of the piezoelectric ceramics or the laminated piezoelectric ceramics, or converting the distortion in the thickness direction into bending displacement to convert the ink. Although the ejection method is used, the present invention has the same effect even when the displacement of the piezoelectric element is a shear displacement. Further, in each of the embodiments, the piezoelectric element has a structure in which the wall surface of the ink chamber is displaced. However, the present invention is also applicable to a method in which the piezoelectric element is provided in the ink chamber and a pressure is directly applied to the ink. Has an effect. Furthermore, in the present embodiment, a method of defining the discharge potential of the charge / discharge circuit is used as a method of applying a 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 method of applying the bias voltage.
[0037]
【The invention's effect】
According to the present invention, the driving operation is always performed in a distortion state in the same direction without the distortion of the piezoelectric element exceeding 0 and being reversed. Therefore, the fatigue breakage of the piezoelectric element is prevented, and the life is remarkably improved. In addition, since the piezoelectric element is driven at a portion where the electromechanical conversion efficiency is high, a high-speed and high-efficiency driving operation can be performed. In addition, when a plurality of piezoelectric elements are used, the driving operation is performed not in the vicinity of the strain 0 where the electromechanical conversion characteristics vary greatly, but in a region where the characteristics are uniform. Therefore, the characteristic variation between the elements can be substantially reduced with a simple structure. . Therefore, high-speed repetitive driving using a large number of piezoelectric elements becomes substantially possible, and a high-speed ink jet recording apparatus can be realized.
[0038]
Further, according to the present invention, in addition to the effects described in the embodiment, the rigidity of the piezoelectric element is increased by always keeping the piezoelectric element in a distorted state, and unnecessary vibration of the pressure wall of the ink chamber is suppressed. There is also an effect that crosstalk due to inter-propagation vibration can be reduced. In addition, since a voltage in the same direction as that during polarization is always applied to the piezoelectric element, there is an effect that the polarization state is stably maintained even in long-term repeated driving.
[Brief description of the drawings]
FIG. 1 is a cross-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 driving waveforms and displacements 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 the 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 driving waveform and 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 a drive waveform and displacement according to a 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 a structure and a polarization direction of a piezoelectric element.
FIG. 16 is a sectional view of an apparatus showing a conventional example.
FIG. 17 is a sectional view of an apparatus showing a conventional example.
FIG. 18 is a diagram showing a driving waveform and displacement of a conventional example.
FIG. 19 is a sectional view showing an example of destruction 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 applying device
7: Switching device
8: Compression member

Claims (6)

A nozzle for discharging ink, an ink chamber communicating with the nozzle, a pressure wall for changing a volume of the ink chamber provided in a part of the ink chamber, a supply path for supplying ink to the ink chamber, and a voltage A piezoelectric element that changes its shape by the application of the pressure wall and the piezoelectric element are mechanically connected, and the volume of the ink chamber is expanded or reduced by a change in the voltage applied to the piezoelectric element, or In an on-demand type ink jet recording apparatus for discharging ink from the nozzles by reducing and expanding,
A mechanism for applying a bias voltage in the same direction as the polarization voltage of the piezoelectric element as a mechanism for always applying a unidirectional strain to the piezoelectric element during expansion or contraction of the pressure chamber or before or after the expansion,
The mechanism for applying the bias voltage is to apply the bias voltage so that the distortion width of the piezoelectric element due to the applied bias voltage is larger than the maximum value of the amplitude of the residual vibration of the piezoelectric element.
An ink jet recording apparatus, wherein the distortion direction of the piezoelectric element is not reversed by the residual vibration of the piezoelectric element generated when the pressure chamber expands or contracts. The bias voltage has a maximum value ΔD of the amplitude of the residual vibration during the discharging operation of the piezoelectric element, a total thickness H of a voltage applying portion of the piezoelectric element, a distance h between adjacent electrodes of the piezoelectric element, 2. An ink jet recording apparatus according to claim 1, wherein the value is equal to or more than a value ([Delta] D * h) / (d * H) defined by a constant d. 3. The ink jet recording apparatus according to claim 2 , wherein the bias voltage is at least twice the specified value. A nozzle for discharging ink, an ink chamber communicating with the nozzle, a pressure wall for changing a volume of the ink chamber provided in a part of the ink chamber, a supply path for supplying ink to the ink chamber, and a voltage A piezoelectric element that changes its shape by the application of the pressure wall and the piezoelectric element are mechanically connected, and the volume of the ink chamber is expanded or reduced by a change in the voltage applied to the piezoelectric element, or In an on-demand type ink jet recording apparatus for discharging ink from the nozzles by reducing and expanding,
A structure that holds the piezoelectric element in a compressed state as a mechanism for always applying a unidirectional strain to the piezoelectric element at the time of expansion or contraction of the pressure chamber or before or after the expansion or contraction , or a pressure mechanism ,
An ink jet recording apparatus, wherein the distortion direction of the piezoelectric element is not reversed by the residual vibration of the piezoelectric element generated when the pressure chamber expands or contracts. 4. The ink jet recording apparatus according to claim 1 , wherein the ink ejection amount is modulated by changing a maximum voltage applied to the piezoelectric element. 5. 2. The ink jet recording apparatus according to claim 1, wherein an electric field applied to the voltage applying section of the piezoelectric element is driven in the same direction as the polarization, and is always driven at an electric field strength of 50 kV / m or more.

Images