JPH0250867B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving an on-demand type ink jet head, and more specifically, a piezoelectric element is used as an electromechanical transducer, the piezoelectric element is charged in advance, and the pressure chamber wall is The volume of the pressure chamber is maintained in a reduced state by displacing the piezoelectric element inward, and when an ink droplet needs to be ejected, the charge applied to the piezoelectric element is removed to move the wall of the pressure chamber outward. The purpose is to suck ink from the ink reservoir into the pressure chamber by displacing the piezoelectric element, and after a predetermined period of time has elapsed, to rapidly apply an electric charge to the piezoelectric element again, thereby rapidly reducing the volume of the pressure chamber and ejecting droplets from the nozzle. The present invention relates to a method for driving an on-demand ink jet head.

In the conventional driving method of an on-demand type 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 that the volume of the pressure chamber is expanded. The pressure is maintained for a predetermined period of time, and then the polarity of the voltage applied to the piezoelectric element is 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 reverse the polarity of this voltage, and its 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 each nozzle, and especially in the case of multiple nozzles, a combination of voltage converters and control circuits is required for each nozzle, which is extremely expensive. increases.

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 lead to depolarization of the piezoelectric element.

Another example of a conventional ink jet head drive circuit is shown in FIGS. 1 and 2. First, the conventional example shown in FIG. 1 will be described. As mentioned above, when ink is not ejected, the switching element 1 is in the on state so that the voltage _VH is always applied to the piezoelectric element 2. When ink ejection is required, first the switching element 1 is turned off to remove the charge accumulated in the piezoelectric element 2. At this time, the charges accumulated in the piezoelectric element 2 are discharged by the resistive elements 3 and 4. Next, by turning on the switching element 1 again, charges are accumulated in the piezoelectric element 2. At this time, in order to increase the speed at which charges are accumulated, the resistance value of the resistor 4 is selected to be approximately one to several tenths of the resistance value of the resistor 3. However, when ink is not ejected,
Since the switching element 1 is always on, a current always flows through the resistor 3 for discharging the charge accumulated in the piezoelectric element 2 when the switching element 1 is turned off. The piezoelectric element has an equivalent circuit as shown in FIG. 2, and its equivalent parallel resistance R is approximately several tens of MΩ, which is close to that of a pure capacitive element. Therefore, in FIG. 1, current flows through the piezoelectric element 2 only when the state of the switching element 1 changes, and the average current is a very small value of about several tens of microamperes. On the other hand, as mentioned earlier, a current of about several mA flows on average in the resistor, consuming significantly more power than that required for ejecting ink, and the power supply for supplying the voltage V _H is also required. It becomes larger and costs more. Also, in the case of multi-nozzle,
Since the power consumption in the resistor 3 increases by the number of nozzles, the cost will further increase.

In view of these points, it is an object of the present invention to provide an inexpensive driving method with low power consumption, and to provide an on-demand ink cartridge that can apply a voltage in the same direction as a polarization voltage to a piezoelectric element as an electromechanical conversion means. An object of the present invention is to provide a method of driving an ethead.

Hereinafter, a specific example of the present invention will be described based on the embodiments shown in FIGS. 3, 4, and 5.

FIG. 3 shows an embodiment of a drive circuit for an on-demand type ink jet head having one piezoelectric element, and FIG. This is an example of a driving method.

First, the embodiment shown in FIG. 3 will be explained using FIG. 4. In order to eject ink droplets from the nozzle using the ink jet head drive circuit according to the embodiment of FIG. 3, the switching element 7 and the switching element 5 of FIG. This is possible by turning on and off.

FIG. 4a is a diagram showing the on/off state of the switching element 7 in FIG. 3, from time t ₁ to time t _{2 .}
The switching element 7 is held in the on state until the time t 2 and is in the off state at other times. FIG. 4b is a diagram showing the on/off state of the switching element 5 in FIG _. to time t ₃ ,
The switching element 5 is held in the on state, and is in the off state at other times. Third
In the figure, the parallel equivalent resistance value of piezoelectric element 2 is R _P ,
Assuming that the resistance value of resistor 8 is R ₈ , the resistance value of resistor 4 is R ₄ , and the resistance value of resistor 3 is R ₃ , the magnitude relationship of the resistance values is as follows: R _P ≫R ₈ ≫R ₃ ≫R ₄ configured. Furthermore, if the capacitance of the piezoelectric element 2 is C, then R ₄ C≪R ₃ C
Must. This is a condition for driving the ink jet head using the ink jet head drive circuit of FIG.

Before time t1 shown in FIG. _4a , all the switching elements are in the OFF state, and the piezoelectric element 2 is charged to approximately the voltage _VH by the resistors 8 and 4. This state is when the inkjet head drive circuit of the present invention and the inkjet head connected thereto are in a standby state.
Now, as shown in FIG. 4a, the switching element 7 in FIG. 3 is turned on at time _t1 . At this time, the other switching elements remain in the off state. At this time, the charges accumulated in the piezoelectric element 2 are gradually discharged through the resistor 3, and the voltage across the piezoelectric element 2 gradually decreases for a predetermined period of time as shown in FIG. 4c. After T ₁ has elapsed, at time t ₂ the switching element 7 is turned off and the switching element 5 is turned on, and the switching element 6 is turned on in conjunction with this.
The piezoelectric element 2 is rapidly charged through the resistor 4 whose resistance value is sufficiently smaller than that of the resistor 3 as described above. At this time, the volume of the pressure chamber (not shown) rapidly decreases, and ink droplets are ejected from the nozzle (not shown). On the other hand, the switching element 5 is turned off at time _t3 after a time T2 _several times the time constant determined by _R4 ×C has passed, and the switching element 6 is also turned off in conjunction with this. It is clear from T ₂ >R ₄ ×C that the piezoelectric element 2 is charged to approximately V _H again at this time. At a time after time _t3 , all switching elements are turned off again and enter a standby state. In this standby state, if
If the resistor 8 is not added, the piezoelectric element 2
has an equivalent parallel resistance R as shown in Figure 2, so it self-discharges and its terminal voltage decreases.
In addition, the voltage between the terminals of the piezoelectric element 2 changes and decreases due to leakage current generated because the off-states of the switching elements 5, 6, and 7 are not ideal, environmental conditions between the terminals of the piezoelectric element 2, and the like. Here, switching element 7
Since the inkjet head is turned on, the change in voltage V _R required to drive the ink jet head cannot be obtained. Therefore, the effect of inserting the resistor 8 is very large. Furthermore, in the embodiment of FIG. 3, the power consumption in the standby state varies depending on the equivalent parallel resistance R of the piezoelectric element 2 and the environmental conditions of the piezoelectric element 2, but when the voltage applied to the piezoelectric element 2 is 200 V or less, , at most several milliwatts, which is extremely low compared to the conventional inkjet head drive circuit. Also, the driving conditions of the inkjet head, that is, the times T ₁ , T ₂ ,
Even if the voltage V _H , voltage _VR , etc. change, there is no need to change the winding specifications of the voltage converter.

Another embodiment of the invention is shown in FIG. This embodiment is an inkjet head drive circuit most suitable for a so-called multi-nozzle ink-on-demand type inkjet head. As a drive circuit for a multi-nozzle ink-on-demand head that uses a plurality of piezoelectric elements and can eject ink droplets from a plurality of corresponding nozzles, the circuit shown in the previous embodiment of the present invention can be used. Although it is possible to use a diode matrix in the same way, by configuring the drive circuit using a diode matrix, the number of switching elements can be reduced, resulting in a cheaper drive circuit. This ink jet head drive circuit is connected to the fifth ink jet head drive circuit.
This will be explained using figures. Piezoelectric element 2-1...2-n
In the standby state, the resistors 8-1...8-n and the resistors 4-1...
...It is charged to a voltage near V _H via 4-n. When ink ejection is required, switching elements 7-1...7 are used to discharge the charge of the piezoelectric element corresponding to the nozzle that requires ink ejection.
- η is turned on as appropriate. At this time, diodes 9-1...9-n are installed in order to prevent discharge of electric charges in piezoelectric elements corresponding to nozzles that do not require ink ejection.
Use. The polarities of the diodes 9-1...9-n must be selected in an appropriate polarity direction depending on the polarity of the voltage _VH . In the direction of Figure 5, the voltage
This is a valid direction when V _H is positive with respect to GND. Charges of only the necessary piezoelectric elements are discharged, and after a predetermined time (time T ₁ in FIG. 4a), the switching elements that are in the on state are turned off, and at the same time, the switching elements 6 are turned on. Then, only the appropriately selected and discharged piezoelectric elements are charged, and ink droplets are ejected from the linked nozzles. In the embodiment shown in FIG. 5, as in the previous embodiment, the resistors 8-1 .
... Operates to compensate for changes in voltage between terminals 2-n. Furthermore, it is possible to easily and inexpensively realize a multi-nozzle ink-on-demand type ink jet head driving method that consumes very little power during standby.

As described above, according to the present invention, there is provided an on-demand type ink jet head driving method which provides an inexpensive driving circuit with low power consumption, and which can apply a voltage in the same direction as the polarization direction of the piezoelectric element as an electromechanical conversion means. It is possible to provide

[Brief explanation of the drawing]

Figures 1 and 2 are conventional examples;
Figures 4 and 5 show embodiments of the present invention. 2, 2-1...2-n is a piezoelectric element, 8, 8-1
...8-n is a resistance element.

Claims (1)

[Claims] 1. A piezoelectric element is provided corresponding to a pressure chamber that communicates with an ink reservoir and a nozzle, and an appropriate pressure is applied to the pressure chamber by charging and discharging the piezoelectric element, and droplets are ejected from the nozzle. A method for driving an on-demand type inkjet head, comprising a first resistor, a second resistor, a third resistor, a first switching element, and a second switching element between a pair of power supply terminals, The first switching element, the third resistor, the second resistor, and the second switching element are connected in series between the pair of power supply terminals, and the piezoelectric element is connected to the second resistor and the second switching element. the first resistor is connected in parallel to a series circuit of two switching elements, and the first resistor is inserted between the power supply terminals so as to be in parallel with the first switching element and in series with the second switching element, The drive circuit for the inkjet head is configured such that the resistance value of the second resistor is sufficiently larger than that of the third resistor, and the resistance value of the first resistor is sufficiently larger than that of the second resistor. and in a standby state, the first and second switching elements are turned off, charging the piezoelectric element through at least the first resistor and displacing the wall of the pressure chamber inward, At the time of injection, the second switching element is turned on to discharge the electric charge charged in the piezoelectric element through the second resistor, displacing the wall of the pressure chamber outward, and then turning off the switching element, the first
The switching element is turned on to charge the piezoelectric element through the third resistor, displacing the wall of the pressure chamber inward and rapidly reducing the volume of the pressure chamber, thereby causing the liquid to flow from the nozzle. A method for driving an on-demand ink jet head characterized by ejecting droplets.