JPH0224218B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving an on-demand ink jet head.

In a conventional driving method for an on-demand inkjet, a driving method known from Japanese Patent Laid-Open No. 52-56928 applies a voltage to the wall of the pressure chamber in the opposite direction to the polarization voltage of the piezoelectric element so as to expand the volume of the pressure chamber. Therefore, the pressure is maintained for a predetermined period of time, and the polarity of the voltage applied to the piezoelectric element is further reversed to shift the volume of the pressure chamber from an expanded state to a reduced state, thereby ejecting droplets. A voltage converter is used to invert the polarity of this voltage, and this secondary inductance forms an oscillating circuit together with the capacitance of the piezoelectric element. The resonant frequency of the vibrating circuit is equal to the resonant frequency of the ink column, and the length of the current shock on the primary side is equal to the length of a half cycle of the resonant frequency.

This type of drive method requires one voltage converter and control circuit for one nozzle, and especially in the case of multi-noise, a combination of voltage converters and control circuits is required for the number of nozzles, which is extremely expensive. increases.

Also, from the point of view of maximum efficiency, i.e., the highest drop velocity at the lowest voltage, the length of the current impulse on the primary side of the voltage converter does not correspond to a half period of the resonant frequency of the ink column. This is because the vibration of the ink column is perceived as a transient response in response to the current shock on the primary side of the voltage converter of the vibration system consisting of the wall of the pressure chamber, the piezoelectric element, and the ink, and the vibration of the ink column is accompanied by a phase lag. This results in a damped vibration. Therefore, the point in time when the volume of the pressure chamber changes from an enlarged state to a reduced state due to polarity change of the control voltage applied to the piezoelectric element corresponds to a certain phase of the damped vibration accompanied by a phase lag of the ink column. When selected, the maximum efficiency described above is obtained. In other words, the length of the current shock on the primary side of the voltage converter is not simply determined by the half period of the natural frequency of the ink column, but is determined by the actual pressure chamber and the ink column in the nozzle. It has been experimentally confirmed that driving at a low voltage can be achieved when the current shock is determined in accordance with the optimum phase of the damped vibration, and that the length of the current shock is longer than a half period of the natural vibration of the ink column.

Furthermore, in order to expand the volume of the pressure chamber, a voltage in the opposite direction to the polarization voltage is applied to the piezoelectric element, which may cause attenuation of the piezoelectric element.

In view of these points, the purpose of the present invention is to actively utilize the damped vibration of a vibration system consisting of the wall of a pressure chamber, an electromechanical conversion means, and ink.
It can be driven by a simple and inexpensive drive circuit, it can apply a voltage in the same direction as the polarization voltage to the piezoelectric element as an electromechanical transducer, and it is also possible to obtain a predetermined droplet ejection speed with a low drive voltage. An object of the present invention is to provide a method for driving a demand type ink jet head.

Hereinafter, a specific example of the present invention will be described based on illustrated embodiments.

In FIGS. 1 and 2, a circular pressure chamber 2, a nozzle 3, and a supply port 4 are formed in a concave shape in a substrate 1, and a wall 5 is bonded to cover the entire concave shape. There is. The ink 6 is led from an ink reservoir 7 to an ink supply pipe 8 and further to a pressure chamber 2 and a nozzle 3 via a supply port 4 forming a narrow portion. At the opening 3a of the nozzle 3, the surface tension of the ink 6 and the negative pressure H are balanced, and the ink 6 is kept from seeping into the opening 3a of the nozzle 3. A piezoelectric element 9 serving as an electromechanical transducer is bonded to an electrode surface 5a provided on the wall 5 by means such as vapor deposition, and the piezoelectric element 9 is placed between the wall 5 and the pressure chamber 2.
It is a position facing the The polarity of the lead wire 10 is selected so that when a voltage is applied, the piezoelectric element 9 contracts in the radial direction, the wall 5 bends in a substantially conical direction toward the pressure chamber 2, and the volume of the pressure chamber 2 decreases. . That is, a voltage is applied in the same direction as the polarization voltage of the piezoelectric element 9.

3 shows a drive circuit that electrically controls the piezoelectric element 9, and FIGS. 4a and 4b show the input signal 16 to this drive circuit and the voltage waveform 19 across the piezoelectric element 9 (third
24) in the figure is shown. Before time _t1 , that is, in the standby state, transistor 11 is turned on and transistor 1 is turned on.
Since the piezoelectric element 2 is also turned on, the voltage remains applied to the piezoelectric element 9. Therefore, the wall 5 remains conically deflected inward, as shown in FIG. At time _t1 , at the rising edge 17 of the input signal 16, the transistor 14 is turned off and the transistor 15 is turned on, and a current flows in the direction of arrow B.
The charges stored in the piezoelectric element 9 are discharged via the resistor 13. At this time, the voltage across the piezoelectric element 9 (24 in FIG. 3) becomes like the voltage waveform 19 in FIG. 4b. Fall 1 of input signal 16 at time t ₂
8, the transistor 14 is turned on and the transistor 15 is turned off, and conversely, the transistor 11 is turned on and the transistor 12 is turned on, and a current instantly flows in the direction of arrow A, charging the piezoelectric element 9. The voltage 24 applied across the piezoelectric element 9 is the power supply voltage 25
approximately equal to. As shown in Figure 4b, in the standby state, a charging pulse is applied to the piezoelectric element and the volume of the pressure chamber is reduced, but between t ₁ and _{t 2} the piezoelectric element gradually discharges. After the volume of the pressure chamber increases, a standby electric pulse is applied to the piezoelectric element at timing _t2 , the piezoelectric element is rapidly charged, and the volume of the pressure chamber decreases.

Mechanical operations corresponding to these electrical operations will be explained with reference to FIG. 1, FIGS. 5a and 5b, and FIG. 6. When the power is turned on, as shown in FIG. 3, transistor 11 is turned on and transistor 12 is also turned on, so that a current flows in the direction of arrow A and charges piezoelectric element 9. When this charging is completed, a voltage approximately equal to the power supply voltage 25 is maintained applied to both ends of the piezoelectric element 9.
The piezoelectric element 9 remains radially contracted, so that the wall 5 also remains deflected toward the pressure chamber 2 in an approximately conical manner (as shown in FIG. 1). At time t ₁ , when the electric charge stored in the piezoelectric element 9 begins to be discharged, the wall 5, which had been bent conically toward the pressure chamber 2, is bent due to the elastic energy stored in the wall 5 and the piezoelectric element 9. Then, return to the outside. At this time, ink 6 is sucked from the ink reservoir through the supply port 4, and at the same time, atmospheric air is sucked through the opening 3a of the nozzle 3, resulting in the state shown in FIGS. 5a and 5b. Before and after this air suction amount 20 reaches its maximum, t ₂
When the time is set and voltage is applied to the piezoelectric element 9 again, the wall 5 and the piezoelectric element 9 move into the pressure chamber 2 as shown in FIG.
The ink 6 sucked into the pressure chamber 2 is discharged into the air as droplets 21.
In this way, ink droplets are ejected, but during driving, the piezoelectric element first displaces the wall of the pressure chamber outward and increases the volume of the pressure chamber, since the electric pulse changes slowly. Air is not drawn into the pressure chamber from the nozzle tip. This is because when ink is ejected, with the method of the present invention, in which the volume of the pressure chamber is increased and ink is first taken into the pressure chamber, if the volume of the pressure chamber increases rapidly, air will enter deep into the nozzle from the tip of the nozzle. This is because air is taken into the pressure chamber, making it impossible to eject ink.

Furthermore, when ink is ejected continuously, after the ink is ejected, the ink in the nozzle becomes empty, and then the ink is filled from the pressure chamber toward the nozzle tip with the help of capillary force, but at the beginning of the ejection cycle, Since the pressure chamber increases slowly, the nozzle emptied by the previous ink injection can be filled with ink with the aid of capillary forces. As a result, when ink is ejected continuously, it is possible to eject ink in a fast cycle without causing ink ejection failure due to suction of air from the nozzle, and the inkjet recording speed can be increased. Further, when ejecting the same amount of ink droplets, the driving voltage of the piezoelectric element can be lower.

Then, in the second half of the ink ejection cycle, a sufficient ejection speed can be recorded by rapidly changing the magnitude of the electrical pulse in the opposite direction to the direction of change of the electrical pulse when increasing the volume of the pressure chamber in the first half of the ink ejection cycle. This makes it possible to drive the inkjet head with excellent performance.

When the pulse interval T between time _t1 and time _t2 is lengthened as shown in FIG. 7a, the wall 5 and piezoelectric element 9 undergo damped vibration as shown in FIG. 7b, and this damped vibration 23 is approximately Expressed as an expression. As is clear from FIG. 7a, also in the embodiment shown in FIG. 7, an electrical pulse in the same direction as the polarization voltage of the piezoelectric element is applied to the piezoelectric element in a standby state, charging the piezoelectric element and creating a pressure. The volume of the chamber is reduced, and when ink is ejected, the piezoelectric element is gradually discharged to increase the volume of the pressure chamber, and then an electrical pulse is applied to the piezoelectric element again to rapidly charge the piezoelectric element and increase the volume of the pressure chamber. Ink is ejected from the nozzle by reducing the volume.

X=-Be ^-nt sin(ωt-θ) Here, X is the displacement in the direction shown in Figure 5a, and X=0 is when the pulse width T is made infinitely long.
That is, the wall 5 when no voltage is applied to the piezoelectric element 9
is the displacement of the piezoelectric element 9, and X=-1 indicates the displacement of the wall 5 bent inward and the piezoelectric element 9 when a voltage is applied to the piezoelectric element 9 and held. t is the time when time t ₁ is set to zero, and the constants B, n, ω, θ
are the elastic coefficient and internal friction of the wall 5 and piezoelectric element 9, the fluid mass and fluid resistance near the nozzle 3 and supply port 4,
It is determined by the surface tension of the ink 6 near the opening 3a of the nozzle 3, etc.

Between time t ₁ and time t ₂ , the wall 5 and the piezoelectric element 9 finally settle to the state X=0, but before that, they perform damped vibrations with X=0 as the axis. Further, this damped vibration 23 is caused by the vibration between the wall 5 and the piezoelectric element 9 when a voltage waveform 19 as shown in FIG.
This is a transient response of the vibration system consisting of the ink 6 and the ink 6. Therefore, a time delay occurs in this damped vibration, and θ in the above equation means this.

The above-described damped vibration 23 of the wall 5 and piezoelectric element 9
As a result, the ink 6 near the nozzle 3 also vibrates in a similar manner. This is observed by the time change of the amount of air sucked in through the opening 3a of the nozzle 3 ((shown in FIGS. 5a and b).
0 causes a damped oscillation 22 as shown in _FIG . They almost match.

By setting the power supply voltage 25 (shown in Figure 3) to the piezoelectric element 9 to a certain value and gradually decreasing the pulse width T in Figure 7a, the velocity of the droplet corresponding to this pulse width T is determined. When plotted, a velocity curve 26 as shown in FIG. 7d is obtained. That is, when the pulse width T is long,
Although the droplet 21 is not ejected from the nozzle, the droplet 21 is ejected when the pulse width T is set near the time _t3 at which the amount of air sucked is maximum. The speed at which the droplets 21 are ejected becomes maximum when the length of the pulse width T is around time _t3 . When the power supply voltage 25 (shown in FIG. 3) to the piezoelectric element 9 is low, if the voltage is applied to the piezoelectric element 9 when the damped vibration 23 of the wall 5 and the piezoelectric element 9 has settled down to X=0, that is, the pulse width When T is made long, the wall 5 and piezoelectric element 9
is from X= to X=- at a speed sufficient to jet the droplet 21.
It does not shift to state 1. However, when voltage is applied again to the piezoelectric element 9 around time _t3 , the state of the wall 5 and the piezoelectric element 9 shifts to The state transition in which the displacement of the wall of the pressure chamber (volume of the pressure chamber) is returned to the standby state again depends on the way the displacement from X = 0 to X = -1 when the pulse width T is increased at time t _3. It becomes as if the subsequent damped vibration 23 is superimposed. Therefore, especially the damped oscillation 2 between time t ₃ and time t ₄
3 shows the wall 5 and piezoelectric element 9 when the pulse width T is long.
Since the wall 5 and the piezoelectric element 9 take part in the state transition to X=-1, the wall 5 and the piezoelectric element 9 shift to the state X=-1 more quickly, and the droplet 21 can be ejected. In this way, the damped vibration 23 that occurs when the ink 6 is sucked into the pressure chamber 2
By setting the pulse width T in synchronization with
In other words, by setting the pulse width T near the maximum value of the damped vibration 23, a predetermined droplet velocity can be obtained with a low voltage applied to the piezoelectric element 9.

Since the damped vibration 23 described above is not occurring when the power is turned on, the droplet 21 is not ejected even if the wall 5 is displaced toward the pressure chamber 2.

After the droplet 21 is ejected, the damped vibration of the vibration system made up of the wall 5, the piezoelectric element 9, and the ink 6 about X=-1 causes the ink 6 inside the pressure chamber 2 to eject the droplet 21 from the nozzle 3. The ink 6 inside the pressure chamber 2 is rapidly terminated due to the simultaneous movement of the fluid escaping through the supply port 4. Therefore, this damped vibration does not have much influence on the ejection of the next droplet 21, and good results can be obtained in terms of frequency response.

Although the drive circuit has been explained using FIG. 3, it can also be driven by a simpler drive circuit as shown in FIG. Since the input signal 16 is the same, it will be explained with reference to FIG. Before time _t1 , transistor 30 is off, so transistor 3
1 is on, and the voltage 32 across the piezoelectric element 9
is approximately equal to the power supply voltage 25. time from time t ₁
During t ₂ , transistor 30 is turned on, transistor 31 is turned off, and the charge stored in piezoelectric element 9 is discharged via resistor 13 . At the falling edge 18 of the input signal 16 at time _t2 , the transistor 30 is turned off and the transistor 31 is turned on again, and a voltage is applied to the piezoelectric element 6. Regarding the voltage application to the piezoelectric element 9, it is completely equivalent to the circuit shown in FIG. If the power consumed by the resistor 13 at times other than the pulse width T is not a problem, the drive circuit shown in FIG. 8 is sufficient.

As described above, a voltage is applied and maintained in the same direction as the polarization voltage to the piezoelectric element 9, and when printing, this voltage application is removed to return the wall 5 to the outside, and the ink 6 at this time is Near the maximum value 27 of the damped vibration 23 of the vibration system consisting of the piezoelectric element 9, the wall 5, and the ink 6 that occurs during inhalation, voltage is applied to the piezoelectric element 9 again, causing the piezoelectric element 9 and the wall 5 to move toward the pressure chamber 2 side. By displacing it inward, it became possible to eject the droplet 21 with a low voltage. Since the damped vibration 23 is a transient phenomenon that corresponds to the removal of voltage to the piezoelectric element 9,
Essentially, there is a time delay and the efficient pulse width T
is desirably set near the maximum value 27 of the damped vibration 23. From this point of view, even if the pulse width T is set to half the period of the resonant frequency of the piezoelectric element 9, the wall 5, and the ink 6, the good efficiency point will be at a point longer than half the period due to the existence of the above-mentioned time delay. exist. In this invention, the inkjet head can be driven at a high efficiency point by using the damped vibration 23, so the voltage to be supplied can be low, and since the voltage is applied in the same direction as the polarization voltage, the piezoelectric element 9 depolarization is also completely avoided. Furthermore, it is not necessary to apply both the positive and negative polarization voltages to the piezoelectric element 9, resulting in the advantage that the circuit configuration can be made extremely simple and inexpensive.

Further, according to the driving method according to the present invention, driving at a high efficiency point can be performed simply by selecting the pulse width T. Therefore, piezoelectric element 9, wall 5
Even if the vibration system consisting of the ink and the ink 6 changes and the position of the maximum value 27 of the damped vibration 23 changes over time, driving at a high efficiency point is possible by changing the pulse width T. On the other hand, when a voltage converter is used, it is necessary to change the winding specifications on the primary side and the secondary side, which is very complicated.

As described above, according to the present invention, an electrical pulse in the same direction as the polarization voltage of the piezoelectric element is applied to the piezoelectric element in a standby state to charge the piezoelectric element and reduce the volume of the pressure chamber. Sometimes, after gradually discharging the piezoelectric element and increasing the volume of the pressure chamber,
Ink is ejected from the nozzle by applying electric pulses to the piezoelectric element again to rapidly charge the piezoelectric element and reduce the volume of the pressure chamber. Therefore, when driving the ink jet head, the piezoelectric element generates a reverse voltage. Its properties are not impaired by the application of Further, even when ink is continuously ejected, high-speed and clear inkjet recording is possible without causing ink ejection failure due to air being taken into the pressure chamber.

[Brief explanation of drawings]

1 to 8 show one embodiment of the present invention, FIG. 1 is a side sectional view showing the structure of an ink jet head, and FIG. 2 is a top view of FIG. 1 including a partially broken surface. , FIG. 3 is a drive circuit diagram of the inkjet head, FIG. 4a shows an input signal to the drive circuit of FIG. FIG. 5a is a side sectional view showing the situation when ink is sucked into the pressure chamber, FIG. 5b is a top view of FIG. 5a including a partially broken surface, and FIG.
7a is a diagram showing the voltage across the piezoelectric element; FIG. 7b is a diagram showing the damped vibration of the wall and the piezoelectric element; and FIG. 7c is a diagram showing the voltage across the nozzle. A diagram showing the time change of air entering through an opening,
FIG. 7d is a chart showing the droplet velocity when the pulse width T is varied, and FIG. 8 is a chart showing another example of the drive circuit. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Pressure chamber, 3... Nozzle, 4... Supply port, 5... Wall, 6... Ink, 9... Piezoelectric element, 10...
Lead wire, 16...Input signal, 17...Voltage waveform.

Claims (1)

[Scope of Claims] 1. A nozzle that ejects ink; a pressure chamber that communicates with the nozzle and generates ink ejection pressure; and a pressure chamber that is provided on a wall of the pressure chamber and that is activated by applying electrical pulses to the pressure chamber. and an electromechanical transducer made of a piezoelectric element that changes volume, supplies ink from an ink reservoir communicating with the pressure chamber, and increases and then decreases the volume of the pressure chamber by the electromechanical transducer. the electrical pulse applied to the piezoelectric element in synchronization with damped vibrations occurring in the pressure chamber, the piezoelectric element, and the ink system supplied to the pressure chamber; In the method for driving an on-demand ink jet head in which the volume of the pressure chamber is returned to a standby state and ink is ejected from the nozzle, in the standby state, the piezoelectric element is previously electrically applied in the same direction as the polarization voltage of the piezoelectric element. A pulse is applied to charge the piezoelectric element to reduce the volume of the rear force chamber, and when ink is ejected, the piezoelectric element is gradually discharged to increase the volume of the pressure chamber, and then the piezoelectric element is charged again. A method for driving an on-demand ink jet head, characterized in that ink is ejected from the nozzle by applying the electrical pulse to rapidly charge the piezoelectric element and reducing the volume of the pressure chamber.