CIRCUIT AND METHOD FOR DRIVING A PRINT HEAD

Abstract

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 piezo-electric actuator that is capable of generating an acoustic wave in an ink chamber of the print element, resulting in the 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 in-between two source voltage levels, wherein the drivers can be switched into a tristate condition at a voltage level that does not equal one of the source voltage levels.

Claims

1. An electronic circuit for driving an individual print element in a print head comprising an array of print elements, each print element having a piezo-electric actuator that is capable of generating an acoustic wave in an ink chamber of the print element, resulting in the ejection of an ink drop out of a nozzle that is in fluid connection with the ink chamber, the electronic circuit comprising at least two power sources with fixed source voltage levels and a number of analog drivers for amplifying an output current at a voltage level in-between two of the at least two source voltage levels, wherein the drivers can be switched into a tristate condition at a voltage level that does not equal one of the source voltage levels, thereby keeping the piezo-electric actuator at said voltage level.

2. The electronic circuit according to claim 1, that is configured with accurate, submicrosecond timing for controlling the time that the current is switched on.

3. The electronic circuit according to claim 1, that comprises three analog drivers and four fixed source voltage levels.

4. The electronic circuit according to claim 1, that is embodied in an ASIC.

5. A method for driving a piezo-electric actuator with a preprogrammed waveform of voltage levels using an electronic circuit according to claim 1, the waveform comprising a steady voltage level that is not available as a source voltage level, the method comprising a step of switching to a tristate condition at an appropriate timing, thereby maintaining the voltage level at the moment of switching to the tristate.

6. A printer comprising a print head with an array of print elements, each print element having a piezo-electric actuator that is capable of generating an acoustic wave in an ink chamber of the print element, resulting in the ejection of an ink drop out of a nozzle that is in fluid connection with the ink chamber and furthermore comprising an electronic circuit for driving the individual print elements according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

[0016] FIG. 1 is a schematic cross-sectional view of a single inkjet print element that is driven by the electronic circuit;

[0017] FIG. 2 shows a waveform for jetting ink drops out of the print element;

[0018] FIG. 3 is an analog driver circuit for controlling an output current;

[0019] FIG. 4 is an electronic circuit according to the invention, and

[0020] FIG. 5 is an example of a measured waveform that comprises a voltage level that is not a source voltage.

DETAILED DESCRIPTION OF EMBODIMENTS

[0021] The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numeral.

[0022] FIG. 1 is a schematic view of an individual inkjet print element 1. It has an ink supply passage 2, ending in a pressure chamber 3 that narrows down to a nozzle 4. Ink is provided to the inkjet print element out of an ink reservoir. One wall of the pressure chamber 3 is constituted by a flexible membrane 5 to which a piezoelectric actuator 6 has been attached. When an electric voltage is applied to electrodes 7, 8 on either side of the actuator 6, the actuator causes the membrane 5 to flex 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, with the result that an ink droplet is expelled from the nozzle. The electrodes 7, 8 are connected to an electric source 9, where an electric signal is generated that is able to 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 element for the properties of the ink drops, such as their size and velocity, generated by the print element. Therefore, various waveforms are applied for controlling the ink behavior in the pressure chamber 3.

[0023] FIG. 2 shows a waveform 10, which is formed by a series of voltage levels and voltage transitions, in this case five, that are applied across the electrodes 7 and 8 of the piezo-electric actuator of the inkjet print element. A higher voltage in this waveform causes a smaller volume of the pressure chamber 3. Starting from the initial voltage V.sub.0, a first pulse starts at a time t.sub.0.sup.s and ends at time t.sub.0.sup.e. These timings are related to the propagation of an acoustic wave in the ink and the geometry of the pressure chamber 3. An output current that charges the capacitive load is calibrated in such a way that the voltage across the piezo-electric actuator changes from V.sub.0 to V.sub.1. The further parameters for the waveform are readily understood. In conventional electronic circuits all the 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 a suitable timing and leaving the actuator currentless.

[0024] An electronic circuit for generating an appropriate output current is given in FIG. 3. Circuit 20 comprises two analog amplifiers 21, 22 that control the gate voltage of the output FET 23, 24. This configuration forms a standard way to generate high output currents at a voltage level between high voltage 1 and high voltage 2, in particular for integration in an integrated circuit.

[0025] Three of the circuits 20 are combined to the circuit 30 in FIG. 4, wherein one of the voltage levels is the ground level, to which also one of the electrodes of the piezo-electric capacitance 31 is connected. This driver circuit enables the generation of waveforms, such as indicated in FIG. 2. In order to be able to obtain a different voltage level than the High voltage 1, 2, 3 and ground, the driver can be switched into tristate, which leaves the capacitance 31 floating. The timing for this switching is in the range of submicroseconds in order to have an appropriate resolution of the voltage levels that can be obtained. By including the specification of a tristate, the circuit is more generally applicable, especially for situations wherein the state of a print element is probed.

[0026] FIG. 5 shows a measured waveform 40 from a circuit as in FIG. 4. The various voltage sources have a value GND (ground), HV.sub.1, HV.sub.2 and HV.sub.3. The first transition is from HV.sub.2 to GND, being the first level 41 of the waveform. However, by switching the circuit into tristate, a stable level 42 is obtained, not equal to one of the source voltage values. Thus, a variety of waveforms may be designed without making the ASIC that drives the print elements, more complex.

[0027] The skilled person will recognise that other embodiments are possible within the scope of the appended claims.