Circuit and method for driving print head

Abstract

To provide a circuit and a method for driving a print head.SOLUTION: An electronic circuit for driving an individual print element in a print head is disclosed. The print head comprises an array of print elements, each print element having a piezoelectric actuator that is capable of generating an acoustic wave in an ink chamber of the print element, resulting in ejection of an ink drop out of a nozzle that is in fluid connection with the ink chamber. The electronic circuit comprises a number of analog drivers for amplifying an output current at a voltage level between two source voltage levels, where the drivers can be switched into a tristate condition at a voltage level that does not equal one of the source voltage levels.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an electronic circuit for driving individual print elements in a printhead comprising an array of print elements, each print element having a piezoelectric actuator capable of generating acoustic waves in an ink chamber of the print element that result in the ejection of an ink drop from a nozzle in fluid communication with the ink chamber, and further to a method for using the circuit in an ink jet printer. [Background technology]

[0002] Inkjet printers are becoming more sophisticated as experience with the flow of liquid in small ink chambers expands. This is manifested in the use of different types of inks, jetted by much smaller print elements. One of the technologies enabling this development is the employment of specialized electronic circuits to drive actuators in the printhead. The present invention is directed to circuits for driving piezoelectric actuators that convert electrical energy into mechanical energy in the form of acoustic waves in an ink chamber connected to the actuator. The terms "printhead", "print", and derivatives of those terms should be understood to include any device or technique that deposits or creates material on a surface in a controlled manner.

[0003] Many drive circuits are designed to switch a common drive waveform generated outside the printhead to one or more of the actuators in the printhead. The functionality of this circuit, usually embodied as an ASIC, is therefore limited to multiplexing functionality, with the generation of drive currents and associated dissipation kept outside the printhead. This facilitates thermal control of the printhead, keeping it at or near a suitable operating temperature. To adapt the common drive waveform to the individual actuators, which behave electrically as capacitances, additional drive currents may be added at appropriate times in the waveform. This is provided from an additional voltage source that is temporarily switched, for a few microseconds or so, to the actuators of the print elements. A limited number of power sources are then used to shape various drive waveforms with different effectiveness. An example of this type of circuit, sometimes referred to as a "cold switch circuit" or switch circuit, is given in US Patent Application Publication No. 2018 / 056648.

[0004] Another type of circuit, sometimes referred to as a "hot-switch circuit", also known as a driver circuit, comprises several power supplies with fixed voltage levels that are used to drive the required currents to and from the actuators according to individualized drive waveforms, characterized by several parameters indicating the timing of the waveforms and the charging currents. These currents are controlled by several switches that allow exactly one of the power supplies to have electrical access to the actuators, which are then charged up or down by the predefined current to the voltage level of the power supplies.

[0005] The waveform for controlling the ink behavior in the print element by means of a piezoelectric actuator may include several electric pulses, i.e. voltage transitions from one voltage level to another. The mechanical response of the actuator to these voltage transitions causes the ink chamber to expand or compress, which results in a movement of the ink in the chamber leading to a movement of the ink meniscus in the nozzle in fluid communication with the ink chamber. Depending on the amount of movement, ejection of ink from the nozzle, also known as a drop ejection port, may result. However, the movement of ink in the nozzle may also be induced to prevent clogging of the ink in the nozzle, without resulting in the ejection of an ink drop. Other effects may also be pursued.

[0006] Thus, the waveforms required become more complex and there is a need for more extensive circuits. In particular, complex waveforms may require more power supplies and more switches to select one of these sources to provide the required current. Circuits are implemented as ASICs (application specific integrated circuits), which are designed to specifications and become more expensive with these increasing requirements. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] US Patent Application Publication No. 2018 / 056648 Summary of the Invention [Problem to be solved by the invention]

[0008] It is an object of the present invention to provide a circuit and method for driving a printhead with any waveform without increasing the number of power supplies, thereby reducing ASIC costs. [Means for solving the problem]

[0009] To this end, the electronic circuit according to the invention comprises several analog drivers for amplifying an output current at a voltage level intermediate between two power supply voltage levels, which drivers can be switched into a tri-state state at a voltage level not equal to one of the power supply voltage levels. The tri-state state is obtained when the current from the drivers to the piezoelectric actuator is interrupted and the capacitive load, the piezoelectric actuator, is left in a currentless state. Depending on the amount of leakage current, the voltage will only be stable for a limited amount of time, which is usually much longer than the time before a new charging pulse is applied. In this way, complex waveforms can be designed without applying many voltage sources.

[0010] Further details of the invention are given in the dependent claims. In an embodiment, the circuit is configured to control the time the current is switched on with precise sub-microsecond timing. This results in precise control of the resulting voltage levels. A full waveform can take as long as 10 to 30 microseconds, while a pulse, which is a single transition from one voltage to another, can take as long as 1 microsecond. The timing of the circuit is therefore preferably more precise than this.

[0011] In a further embodiment, the circuit includes three analog drivers and four power supply voltage levels, one of which may be the ground level. This amount of voltage levels provides an optimal balance between complexity and cost, on the one hand, and versatility and functionality, on the other hand.

[0012] In an embodiment, the circuit is embodied in an application specific integrated circuit (ASIC) where multiple circuits are grouped together and each circuit is assigned to one of the print elements of the printhead. In this way, every print element can be individually controlled.

[0013] The invention further includes a method for driving a piezoelectric actuator with a pre-programmed waveform of voltage levels using an electronic circuit, the waveform including a steady voltage level that is not available as a power supply voltage level, the method including the step of switching to a tri-state condition at the appropriate time, thereby maintaining the voltage level at the time of switching to the tri-state. Instead of using a separate power supply voltage level, a capacitive load in the circuit is used to hold the current voltage at a fixed level for a period of time that is approximately 10 microseconds or so, depending on the amount of current leakage. This voltage is considered a constant voltage within the time frame of the applied waveform.

[0014] The circuit finds particular useful application in printers having a printhead with an array of print elements, each print element having a piezoelectric actuator capable of generating acoustic waves in an ink chamber of the print element that result in the ejection of an ink droplet from a nozzle in fluid communication with the ink chamber to drive the individual print element.

[0015] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while describing preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

[0016] The present invention will be more fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of example only and are therefore not limiting of the present invention. [Brief description of the drawings]

[0017] [Figure 1]1 is a schematic cross-sectional view of a single inkjet print element driven by electronic circuitry. [Diagram 2] FIG. 2 illustrates waveforms for ejecting ink drops from a print element. [Diagram 3] FIG. 2 is a diagram of an analog driver circuit for controlling the output current. [Figure 4] FIG. 2 is a diagram of an electronic circuit according to the invention. [Diagram 5] FIG. 2 is an example of a measured waveform that includes a voltage level that is not a power supply voltage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention will now be described with reference to the accompanying drawings, in which like or similar elements are identified by the same reference numerals, and in which:

[0019] FIG. 1 is a schematic view of an individual inkjet print element 1. The inkjet print element 1 has an ink supply passage 2 that terminates in a pressure chamber 3 that narrows to a nozzle 4. Ink is provided to the inkjet print element from an ink reservoir. One wall of the pressure chamber 3 is constituted by a flexible membrane 5 to which a piezoelectric actuator 6 is attached. When a voltage is applied to electrodes 7, 8 on either side of the actuator 6, the actuator causes the membrane 5 to bend into the pressure chamber 3, thereby creating an acoustic pressure wave in the ink in the pressure chamber. The pressure wave propagates to the nozzle 4, resulting in the ejection of an ink droplet from the nozzle. The electrodes 7, 8 are connected to a power supply 9, from which an electrical signal is generated that can drive the inkjet print element. It is known that the position and movement of the meniscus of the ink inside the nozzle is an important factor for the characteristics of the ink droplets generated by the print element, such as their size and velocity. Therefore, various waveforms are applied to control the behavior of the ink in the pressure chamber 3.

[0020] 2 shows a waveform 10 formed by a series of voltage levels and voltage transitions, in this example five, applied across electrodes 7 and 8 of a piezoelectric actuator of an inkjet print element. Higher voltages in this waveform result in smaller volumes of pressure chamber 3. Starting from an initial voltage V0, the first pulse begins at time t0. s At time t0 e These timings are related to the propagation of the acoustic wave in the ink and to the geometry of the pressure chamber 3. The output current charging the capacitive load is calibrated in such a way that the voltage across the piezoelectric actuator varies from V0 to V1. Further parameters for the waveform are easily understood. In conventional electronic circuits, all voltage levels are provided by external power sources, one for each voltage level. In the circuit according to the invention, some of the voltage levels are not generated by an external power source but are obtained by stopping the current to the actuator at suitable times and leaving the actuator currentless.

[0021] An electronic circuit for generating a suitable output current is given in Figure 3. The circuit 20 comprises two analog amplifiers 21, 22 which control the gate voltages of output FETs 23, 24. This configuration forms a standard way to generate high output currents at voltage levels between High Voltage 1 and High Voltage 2, especially for integration in integrated circuits.

[0022] Three of the circuits 20 are combined into a circuit 30 in FIG. 4, one of the voltage levels being the ground level, to which is also connected one of the electrodes of the piezoelectric capacitance 31. This driver circuit allows the generation of a waveform as shown in FIG. 2. To be able to obtain voltage levels different from the high voltages 1, 2, 3 and ground, the driver can be switched into a tri-state, which leaves the capacitance 31 floating. The timing for this switching is in the range of a few sub-microseconds, in order to have an adequate resolution of the voltage levels that can be obtained. By including the tri-state specification, the circuit is more generally applicable, especially for situations in which the state of a print element is probed.

[0023] Figure 5 shows a measured waveform 40 from a circuit as in Figure 4. The various voltage supplies have values ​​GND (ground), HV1, HV2, and HV3. The first transition is from HV2 to GND, which is the first level 41 of the waveform. However, by switching the circuit into a tri-state, a stable level 42 is obtained that is not equal to one of the supply voltage values. Thus, various waveforms can be designed without making the ASIC driving the print element more complex.

[0024] Those skilled in the art will recognize that other embodiments are possible within the scope of the claims appended hereto. [Explanation of symbols]

[0025] 1 Inkjet Printing Elements 2 Ink supply passage 3. Pressure Chamber 4 Nozzles 7, 8 electrodes 9 Power supply 10 waveforms 20, 30 circuits 21, 22 Analog amplifier 23, 24 Output FET 40 Measured Waveforms 41 First Level 42 Stable Level

Claims

1. 1. An electronic circuit for driving individual print elements in a printhead comprising an array of print elements, each print element having a piezoelectric actuator capable of generating acoustic waves in an ink chamber of the print element resulting in the ejection of an ink drop from a nozzle in fluid communication with the ink chamber, the electronic circuit comprising at least two power supplies with fixed supply voltage levels and several analog drivers for amplifying an output current at a voltage level intermediate two of the at least two supply voltage levels, the drivers being capable of being switched into a tri-state state at a voltage level not equal to one of the supply voltage levels, thereby keeping the piezoelectric actuator at said voltage level.

2. 10. The electronic circuit of claim 1 configured to control the time when current is switched on with precise sub-microsecond timing.

3. 2. The electronic circuit of claim 1, comprising three analog drivers and four fixed power supply voltage levels.

4. 10. The electronic circuit of claim 1 embodied in an ASIC.

5. 13. A method for driving a piezoelectric actuator with a pre-programmed waveform of voltage levels using the electronic circuit of claim 1, the waveform including steady-state voltage levels that are not available as power supply voltage levels, the method including the step of switching to a tri-state condition at the appropriate time, thereby maintaining the voltage levels at the time of switching to the tri-state.

6. 11. A printer comprising a printhead with an array of print elements, each print element having a piezoelectric actuator capable of generating acoustic waves in an ink chamber of the print element that result in the ejection of an ink droplet from a nozzle in fluid communication with the ink chamber, the printer further comprising electronic circuitry for driving the individual print elements as recited in claim 1.

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