Controlling waveforms to reduce nozzle cross talk

文档序号：1255713 发布日期：2020-08-21 浏览：25次中文

阅读说明：本技术 控制波形以减少喷嘴串扰 (Controlling waveforms to reduce nozzle cross talk ) 是由 C.苏 R.L.法格奎斯特 M.F.鲍默于 2018-11-21 设计创作，主要内容包括：喷墨印刷头包括两组交错的喷嘴。液滴形成波形的第一集合和液滴形成波形的第二集合与喷嘴组相关联,以选择性地使得液体射流的部分脱落成液滴。定时延迟设备相对于与第一组波形相关联的波形将第二组波形时移。具有带有第一和第二电势的部分的充电电极波形被提供到充电电极。第二组波形的波形能量大于对应的第一组波形的波形能量,使得当充电电极处于第一电势时印刷液滴从液体射流脱落,并且当充电电极处于第二电势时非印刷液滴从液体射流脱落。(The inkjet print head includes two sets of staggered nozzles. A first set of drop formation waveforms and a second set of drop formation waveforms are associated with the nozzle group to selectively cause portions of the liquid jet to drop into drops. The timing delay device time shifts the second set of waveforms relative to the waveforms associated with the first set of waveforms. A charge electrode waveform having portions with first and second potentials is provided to the charge electrode. The waveform energy of the second set of waveforms is greater than the waveform energy of the corresponding first set of waveforms, such that printed drops are shed from the liquid jet when the charge electrode is at the first potential and non-printed drops are shed from the liquid jet when the charge electrode is at the second potential.)

1. A method of printing, comprising:

providing a liquid chamber having a plurality of nozzles arranged in a nozzle array direction, the plurality of nozzles including a first set of nozzles and a second set of nozzles, the nozzles of the first set interleaved with the nozzles of the second set;

providing liquid in the liquid chamber under pressure sufficient to eject liquid jets through the plurality of nozzles;

providing a drop forming device associated with each of the plurality of nozzles;

providing a first set of drop forming waveforms and a second set of drop forming waveforms, wherein the first set of drop forming waveforms and the second set of drop forming waveforms each comprise:

a drop formation waveform having one or more print drops with a waveform period that, when supplied to a drop formation device associated with a particular nozzle, modulates the liquid jet ejected from the particular nozzle to selectively cause portions of the liquid jet to break off into drop pairs that travel along a path, the drop pairs including small print drops and small non-print drops; and

a drop formation waveform for one or more non-printing drops that, when supplied to a drop formation device associated with a particular nozzle, modulates the liquid jet ejected from the particular nozzle to selectively cause a portion of the liquid jet to break off into a large non-printing drop traveling along the path, the large non-printing drop being larger than the small printing drop and the small non-printing drop, the drop formation waveform for the non-printing drops having the same waveform period as the drop formation waveform for the printing drops;

wherein each of the drop formation waveforms provides an associated waveform energy when supplied to a corresponding drop formation device, and wherein the waveform energy associated with the drop formation waveforms in the second set of drop formation waveforms is greater than the waveform energy associated with the corresponding drop formation waveforms in the first set of drop formation waveforms;

providing input image data;

controlling the drop formation device associated with each of the plurality of nozzles in response to the provided input image data, wherein the first set of nozzles is controlled with a sequence of drop formation waveforms selected from a first set of the drop formation waveforms and the second set of nozzles is controlled with a sequence of drop formation waveforms selected from a second set of the drop formation waveforms;

providing a timing delay device to time shift the drop formation waveform for controlling the drop formation devices associated with the second set of nozzles by a specified second set of time shifts relative to the drop formation waveform for controlling the drop formation devices associated with the first set of nozzles, wherein the second set of time shifts are a fraction of the waveform period;

providing a charging device, the charging device comprising:

a common charging electrode positioned proximate to the liquid jets ejected through both the first set of nozzles and the second set of nozzles; and

a charge electrode waveform source providing a varying electrical potential between the charge electrode and the liquid jet according to a predefined periodic charge electrode waveform, the charge electrode waveform including a first portion providing a first electrical potential and a second portion providing a second electrical potential, wherein the charge electrode waveform has the same waveform period as the drop formation waveform;

synchronizing the drop formation device, the timing delay device, and the charging device, wherein the waveform energies and the second set of time shifts associated with the drop formation waveforms in the first and second sets of drop formation waveforms are selected such that the small printed drop is shed from the liquid jet during a first portion of the charge electrode waveform to provide a first printed drop charge state and the small non-printed drop and the large non-printed drop are shed from the liquid jet during a second portion of the charge electrode waveform to provide a second non-printed drop charge state;

providing a deflection device that causes the print drops having the first print drop charge state to travel along a different path than the non-print drops having the second non-print drop charge state; and

intercepting the non-printing drops using an ink catcher while allowing the printing drops to travel along a path toward a receiver.

2. The method of claim 1, wherein each of the drop formation waveforms of the first and second sets of drop formation waveforms comprises one or more waveform pulses.

3. The method of claim 2, wherein the amplitude of the waveform pulses in the second set of drop formation waveforms is greater than the amplitude of the waveform pulses in the first set of drop formation waveforms.

4. The method of claim 2, wherein each waveform pulse in the second set of drop formation waveforms corresponds to a waveform pulse in the first set of drop formation waveforms.

5. The method of claim 4, wherein at least one of the waveform pulses in each of the drop formation waveforms in the second set of drop formation waveforms has a pulse width that is greater than the corresponding waveform pulse in the corresponding drop formation waveform in the first set of drop formation waveforms.

6. The method of claim 4, wherein at least one of the waveform pulses in each of the drop formation waveforms in the second set of drop formation waveforms has a pulse width equal to the corresponding waveform pulse in the corresponding drop formation waveform in the first set of drop formation waveforms.

7. The method of claim 2, wherein at least one of the drop formation waveforms in the second set of drop formation waveforms comprises more waveform pulses than the corresponding drop formation waveform in the first set of drop formation waveforms.

8. The method of claim 2, wherein at least one of the drop formation waveforms comprises a reverse waveform pulse that reduces energy provided by the drop formation device.

9. The method of claim 1, wherein each of the drop formation devices includes a heater having a heater resistance, and wherein the heater resistance of the heater in the drop formation device associated with the first set of nozzles is higher than the heater resistance of the heater in the drop formation device associated with the second set of nozzles.

10. The method of claim 1, wherein the second set of time shifts is within a range of 1/4-3/4 of the waveform period.

11. The method of claim 1, further comprising a detector for detecting a time difference between a drop time of a droplet formed by the first set of nozzles and a drop time of a corresponding droplet formed by the second set of nozzles.

12. The method of claim 11, wherein the second set of time shifts is adjusted in response to the detected time difference.

13. The method of claim 1, wherein each droplet forming device comprises a droplet forming transducer, and wherein the droplet forming transducer is a thermal device, a piezoelectric device, a MEMS actuator, an electrohydrodynamic device, an optical device, or an electrostrictive device.

14. The method of claim 1, wherein the plurality of nozzles further comprises a third set of nozzles interleaved with the nozzles of the first set and the nozzles of the second set, and wherein the timing delay device time shifts a third set of drop formation waveforms for controlling the drop formation devices associated with the third set of nozzles by a specified third set of time shifts that are different from the second set of time shifts, and wherein waveform energies associated with the drop formation waveforms in the third set of drop formation waveforms are different from the waveform energies associated with the corresponding drop formation waveforms in the first and second sets of drop formation waveforms.

15. The method of claim 1, wherein the large non-printing droplet is formed by merging two or more droplets.

16. The method of claim 1, wherein the first print drop charge state of the print drop has a lower charge than the second non-print drop charge state of the non-print drop.

17. The method of claim 17, wherein the print drops are uncharged.

18. The method of claim 1, wherein the drop pairs formed from the drop forming waveform of the print drops are preceded or followed by large non-print drops.

19. A method of printing, comprising:

providing liquid in the liquid chamber under pressure sufficient to eject liquid jets through the plurality of nozzles;

providing a drop forming device associated with each of the plurality of nozzles;

providing a first set of drop forming waveforms and a second set of drop forming waveforms, wherein the first set of drop forming waveforms and the second set of drop forming waveforms each comprise:

providing input image data;

providing phase control means for controlling the phase of the drop formation waveform for controlling the drop formation device associated with the second set of nozzles such that the phase is shifted by a second set of phase shifts relative to the drop formation waveform for controlling the drop formation device associated with the first set of nozzles, wherein the second set of phase shifts is a fraction of the waveform period;

providing a charging device, the charging device comprising:

a common charging electrode positioned proximate to the liquid jets ejected through both the first set of nozzles and the second set of nozzles; and

synchronizing the drop formation device, the phase control apparatus, and the charging device, wherein the waveform energies and the second set of phase shifts associated with the drop formation waveforms in the first and second sets of drop formation waveforms are selected such that the small printed drops are shed from the liquid jet during the first portion of the charge electrode waveform to provide a first printed drop charge state and the small non-printed drops and the large non-printed drops are shed from the liquid jet during the second portion of the charge electrode waveform to provide a second non-printed drop charge state;

intercepting the non-printing drops using an ink catcher while allowing the printing drops to travel along a path toward a receiver.

20. The method of claim 19, wherein the phase control device is a timing delay apparatus that time shifts the drop formation waveforms used to control the drop formation devices associated with the second set of nozzles by a specified second set of time shifts relative to the drop formation waveforms used to control the drop formation devices associated with the first set of nozzles.

21. The method of claim 19, wherein the drop formation waveform has a waveform boundary and comprises one or more waveform pulses, and wherein the phase control device modifies the drop formation waveform supplied to the drop formation devices associated with the second set of nozzles by shifting the position of the waveform boundary relative to the position of the waveform pulse.

Technical Field

The present invention relates to the field of inkjet printing, and more particularly to a method of controlling drop formation waveforms to an array of nozzles to reduce print artifacts (artifacts).

Background

Continuous inkjet printing is a printing technique well suited for high speed printing applications, with high throughput and low cost per page. Recent advances in continuous ink jet printing technology have included thermally-induced droplet formation that can selectively alter the drop break-off phase or selectively alter the velocity of a pair of droplets, one of which is charged and the other of which is uncharged, relative to a charging electrode waveform to cause them to coalesce, and electrostatic deflection of the charged droplets to separate charged non-printing droplets from charged printing droplets, as disclosed in U.S. patent 7,938,516 (pitat et al), U.S. patent 8,382,259 (panchawaw et al), U.S. patent 8,465,129 (panchawaw et al), U.S. patent 8,469,496 (panchawaw et al), U.S. patent 8,585,189 (Marcus et al), U.S. patent 8,651,632 (Marcus et al), U.S. patent 8,651,633 (Marcus et al), and U.S. patent 8,657,419 (pancawagh et al), all of which are commonly assigned. These advances enable significant improvements in print resolution while maintaining printer throughput.

It has been found that under certain printing conditions, printing artifacts can be created. There is a need for a more efficient method for preventing the formation of such printing artifacts.

Disclosure of Invention

The present invention represents a printing method comprising:

providing liquid at a pressure in the liquid chamber sufficient to eject liquid jets through the plurality of nozzles;

providing a drop forming device associated with each of the plurality of nozzles;

providing a first set of drop forming waveforms and a second set of drop forming waveforms, wherein the first set of drop forming waveforms and the second set of drop forming waveforms each comprise:

providing input image data;

providing a charging device, the charging device comprising:

a common charging electrode positioned proximate to the liquid jets ejected through both the first set of nozzles and the second set of nozzles; and

synchronizing the drop formation device, the timing delay device, and the charging device, wherein the waveform energies and the second set of time shifts associated with the drop formation waveforms in the first and second sets of drop formation waveforms are selected such that the small printed drop is shed from the liquid jet during the first portion of the charge electrode waveform to provide a first printed drop charge state and the small non-printed drop and the large non-printed drop are shed from the liquid jet during the second portion of the charge electrode waveform to provide a second non-printed drop charge state;

intercepting the non-printing drops using an ink catcher while allowing the printing drops to travel along a path toward a receiver.

The invention has the advantage that offsetting the phase of the droplet formation waveforms applied to interleaved sets of droplet formation devices reduces cross talk artefacts and appropriately modifies the waveform energy for the sets of droplet formation devices to synchronise droplet drop times, thereby enabling electrostatic droplet deflection using a common charging electrode.

Drawings

FIG. 1 is a simplified schematic block diagram of an exemplary continuous ink jet system;

FIG. 2 illustrates a liquid jet ejected from a drop generator and its subsequent shedding of drops at regular intervals;

FIG. 3 illustrates a cross-sectional view of an exemplary inkjet printhead of a continuous liquid ejection system according to the present invention;

FIG. 4 shows an exemplary timing diagram illustrating drop formation pulses and charge electrode waveforms;

FIG. 5 illustrates a liquid jet ejected from a drop generator and its subsequent shedding of drops;

FIG. 6 is a representation of a portion of a print medium including a spatially periodic print pattern and an induced print defect;

FIG. 7 is a simplified schematic block diagram of four adjacent nozzles and associated drop forming devices arranged in two groups according to an exemplary embodiment;

FIG. 8 shows a timing diagram illustrating drop formation pulses applied to two groups of drop formation transducers, where the drop formation pulses applied to the second group are time delayed and have a higher amplitude than the drop formation pulses applied to the first group;

FIG. 9 shows a timing diagram illustrating drop formation pulses applied to two groups of drop formation transducers, where the drop formation pulses applied to the second group are time delayed and have a larger pulse width than the drop formation pulses applied to the first group;

FIG. 10 is a simplified schematic block diagram of four adjacent nozzles and associated drop forming transducers arranged in two groups, wherein the drop forming transducers associated with the second group have a lower resistance than the drop forming transducers associated with the first group to provide higher waveform energy;

FIG. 11 shows a timing diagram illustrating drop formation pulses applied to two sets of drop formation transducers, where the drop formation waveform includes secondary pulses in addition to primary drop formation pulses;

FIG. 12 shows a timing diagram illustrating drop formation pulses applied to two groups of drop formation transducers, where the drop formation waveforms associated with the second group have more drop formation pulses than the drop formation waveforms associated with the first group;

FIG. 13 shows a timing diagram illustrating drop formation pulses applied to two groups of drop formation transducers, where the drop formation waveforms associated with the second group have inverted drop formation pulses;

FIG. 14 shows a timing diagram of a sequence of drop formation waveforms illustrating the flexibility of defining the start and end points of each waveform;

FIG. 15 shows a timing diagram illustrating drop formation pulses applied to two sets of drop formation transducers, where the time delay for the second set is introduced by offsetting the drop formation pulses within the boundaries of the drop formation waveform;

FIG. 16 is a simplified schematic block diagram of four adjacent nozzles and associated drop forming devices arranged in three groups according to another exemplary embodiment;

FIG. 17 shows a timing diagram illustrating drop formation pulses applied to three groups of drop formation transducers, where the drop formation pulses applied to the second group are time delayed and have a higher waveform energy relative to the drop formation pulses applied to the first group, and the drop formation pulses applied to the third group are time delayed and have a higher waveform energy relative to the drop formation pulses applied to the second group; and

fig. 18A-18B are photographs comparing droplets formed using a droplet formation waveform according to the present invention with droplets formed using a prior art method.

It is to be understood that the drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.

Detailed Description

The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. References to "a particular embodiment" or the like refer to features that are present in at least one embodiment of the invention. Separate references to "an embodiment" or "particular embodiments" or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as would be readily apparent to one of ordinary skill in the art. The use of the singular or plural in referring to "a method" or "methods" and the like is not limiting. It should be noted that the word "or" is used in this disclosure in a non-exclusive sense unless otherwise explicitly stated or required by context.

For purposes of clarity, exemplary embodiments of the invention are illustrated schematically and not to scale. Those of ordinary skill in the art will be readily able to determine the specific dimensions and interconnections of the elements of the exemplary embodiments of the present invention.

As described herein, exemplary embodiments of the present invention provide a print head or print head component generally for use in an inkjet printing system. However, many other applications are emerging that use printheads to eject liquids (other than ink) that need to be finely metered and deposited with high spatial accuracy. Thus, as described herein, the terms "liquid" and "ink" refer to any material that may be ejected by a printhead or printhead components described below.

Referring to fig. 1, a continuous printing system 20 includes an image source 22, such as a scanner or computer, which provides raster image data, outline image data in a page description language, or other forms of digital image data. The image data is converted into halftone bitmap image data by an image processing unit (image processor) 24, and the image processing unit 24 also stores the image data in a memory. A plurality of drop forming transducer control circuits 26 read data from the image memory and apply time-varying electrical pulses to drop forming transducers 28 associated with one or more nozzles of a printhead 30. These pulses are applied at the appropriate time and to the appropriate nozzles so that drops formed from the continuous ink jet stream will form a spot on print medium 32 at the appropriate location specified by the data in the image memory.

Print medium 32 is moved relative to printhead 30 by a print medium transport system 34, print medium transport system 34 being electronically controlled by a media transport controller 36 in response to signals from a velocity measurement device 35. The media transport controller 36 is in turn controlled by a microcontroller 38. The print media transport system 34 shown in fig. 1 is merely illustrative, and many different mechanical configurations are possible. For example, transport rollers may be used in print media transport system 34 to facilitate the transport of ink drops to print media 32. Such conveyor roller technology is well known in the art. In the case of a page width print head, it is most convenient to move the print media 32 along the media path past a stationary print head. However, in the case of a scanning printing system, it is often most convenient to move the print head along one axis (the sub-scan direction) and the print medium 32 along the orthogonal axis (the main-scan direction) in a relative raster motion.

The ink is contained under pressure in an ink reservoir 40. In the non-printing state, the stream of continuous inkjet droplets cannot reach print medium 32 due to ink catcher 72 blocking the stream of droplets, and it may allow a portion of the ink to be recovered by ink recovery unit 44. The ink recovery unit 44 reconditions the ink and feeds it back to the ink reservoir 40. Such ink recovery units are well known in the art. The ink pressure suitable for optimum operation will depend on a number of factors, including the geometry and thermal properties of the nozzles and the thermal properties of the ink. Constant ink pressure may be achieved by applying pressure to ink reservoir 40 under the control of ink pressure regulator 46. Alternatively, the ink reservoir 40 may remain unpressurized, or even under reduced pressure (vacuum), and a pump may be employed to transfer ink under pressure from the ink reservoir 40 to the printhead 30. In such an embodiment, the ink pressure regulator 46 may include an ink pump control system. Ink is dispensed to the printhead 30 through an ink channel 47. The ink preferably flows through slots or holes etched through the silicon substrate of the printhead 30 to its front surface where a plurality of nozzles and drop forming transducers (e.g., heaters) are located. When the print head 30 is made of silicon, the drop forming transducer control circuitry 26 may be integrated with the print head 30. The print head 30 also includes a deflection mechanism 70, which is described in more detail below with reference to fig. 2 and 3.

Referring to fig. 2, a schematic diagram of a continuous liquid printhead 30 is shown. The jetting module 48 of the printhead 30 includes an array of nozzles 50 formed in a nozzle plate 49. In fig. 2, nozzle plate 49 is secured to jetting module 48. Alternatively, nozzle plate 49 may be integrally formed with jetting module 48. Liquid (e.g., ink) is supplied to the nozzles 50 via the ink channels 47 at a pressure sufficient to form a continuous stream of liquid 52 (sometimes referred to as a filament) from each nozzle 50. In fig. 2, the array of nozzles 50 extends into and out of the figure.

Jetting module 48 is operable to break off liquid droplets 54 from liquid stream 52 in response to image data. To accomplish this, jetting module 48 includes a drop stimulation or drop formation transducer 28 that, when selectively activated, perturbs liquid stream 52 to cause portions of each filament to break off and coalesce to form drops 54. Examples of drop forming transducers 28 include thermal devices such as heaters for heating ink, MEMS piezoelectrics, electrostrictive or thermal actuators such as those disclosed in commonly assigned et.s. patent 8,087,740 (Piatt et al), electrohydrodynamic devices such as those disclosed in et.s. patent 3,949,410 (basous et al), or optical devices such as those disclosed in U.S. patent 3,878,519 (Eaton). Depending on the type of transducer used, the transducer may be located in or adjacent to a liquid chamber that supplies liquid to the nozzle 50 to act on the liquid in the liquid chamber, may be located in the nozzle 50 or immediately around the nozzle 50 to act on the liquid as it passes through the nozzle, or may be located adjacent to the liquid stream 52 to act on the liquid stream 50 after the liquid stream 50 passes through the nozzle 50.

In FIG. 2, drop forming transducer 28 is a heater 51, such as an asymmetric heater or a ring heater (segmented or unsegmented), in nozzle plate 49 on one or both sides of nozzle 50. This type of droplet formation is known and has been described, for example, in: et.s. patent 6,457,807 (Hawkins et al); et.s. patent 6,491,362 (Jeanmaire); et.s. patent 6,505,921 (Chwalek et al); et.s. patent 6,554,410 (Jeanmaire et al); et.s. patent 6,575,566 (Jeanmaire et al); et.s. patent 6,588,888 (Jeanmaire et al); et.s. patent 6,793,328 (Jeanmaire); et.s. patent 6,827,429 (Jeanmaire et al); and et.s. patent 6,851,796 (Jeanmaire et al), each of which is incorporated herein by reference.

Typically, one drop forming transducer 28 is associated with each nozzle 50 of the nozzle array. However, in some configurations, the drop forming transducers 28 may be associated with groups of nozzles 50 in a nozzle array.

Referring to fig. 2, the printing system has a print head 30 associated therewith, the print head 30 being operable to generate an array of liquid streams 52 (also referred to as liquid jets) from an array of nozzles 50. A drop forming device is associated with each liquid stream 52. The drop forming device includes a drop forming transducer 28 and a drop forming waveform source 55 that supplies a sequence 60 of drop forming waveforms to the drop forming transducer 28. The drop forming waveform source 55 is part of the mechanism control circuit 26. In some embodiments where the nozzle plate is made of silicon, drop forming waveform source 55 is formed at least partially on nozzle plate 49. The drop formation waveform source 55 supplies a drop formation waveform sequence 60 to the drop formation transducer 28, the drop formation waveform sequence 60 generally including a waveform having a fundamental frequency f₀And a basic period T₀=1/f₀The drop forming transducer 28 produces a modulation of the diameter of the liquid stream; the modulation has a wavelength λ along the liquid flow. The jet diameter modulation moves down the liquid stream with the flowing liquid and it grows in amplitude, resulting in a larger liquid streamThe diameter of the diameter portion is further increased and the diameter of the smaller diameter portion of the liquid stream is further decreased. The modulation amplitude increases until at a distance BL from the nozzle plate 49, the small diameter portion of the liquid stream shrinks to zero diameter, causing the end portion of the liquid stream 52 to break off into droplets 54. By the action of the drop forming device, a sequence of drops 54 is produced. The droplets 54 are formed at a fundamental frequency f according to a droplet formation waveform sequence 60₀With T₀=1/f₀The fundamental period of (a) is formed. In fig. 2, the liquid stream 52 drops into droplets at regular periods at drop locations 59, the drop locations 59 being the distance from the nozzle 50 (referred to as the drop length) BL. The distance between a pair of consecutive droplets 54 is substantially equal to the wavelength λ of the perturbation on the liquid stream 52. The stream of droplets 54 formed by the liquid stream 52 follows an initial trajectory 57.

The time from when a drop forming waveform pulse is applied to the drop forming transducer until the jet diameter modulation produced by the waveform pulse causes a portion of the liquid stream to break off into drops is referred to as the break off time BOT. Drop-off time BOT for a drop for a particular printhead can be altered by changing at least one of the amplitude, duty cycle, or number of stimulation pulses to the respective resistive element surrounding the respective resistive nozzle aperture, all of which alter the initial modulation amplitude on the fluid stream. In this way, small variations in pulse duty cycle or amplitude allow for modulation of drop break off time in a predictable manner within one tenth of a drop generation period.

Also shown in fig. 2 is a charging device 61, which includes a charging electrode 62 and a charging electrode waveform source 63. A charging electrode 62 associated with the liquid jet is positioned adjacent the drop off point 59 of the liquid stream 52. If a voltage is applied to the charging electrode 62, an electric field is created between the charging electrode and the electrically grounded liquid jet, and the capacitive coupling between the two creates a net charge on the end of the stream of conducting liquid 52. (liquid stream 52 is grounded by virtue of contact with the liquid chamber of a grounded drop generator.) if an end portion of the liquid jet breaks off to form a drop while there is a net charge on the end of liquid stream 52, the charge of that end portion of liquid stream 52 collects on the newly formed drop 54.

The voltage on charging electrode 62 is controlled by charging electrode waveform source 63, charging electrode waveform source 63 providing charging electrode waveform 64 that operates at charging electrode waveform 64 period 80 (shown in fig. 4). A charge electrode waveform source 63 provides a varying electrical potential between the charge electrode 62 and the liquid stream 52. Charging electrode waveform source 63 generates charging electrode waveform 64, charging electrode waveform 64 including a first voltage state and a second voltage state; the first voltage state is different from the second voltage state. An example of the charging electrode waveform is shown in part B of fig. 4. The two voltages are selected such that drops 54 that fall during the first voltage state acquire a first charge state and drops 54 that fall during the second voltage state acquire a second charge state. The charging electrode waveform 64 supplied to the charging electrode 62 is independent of, or not responsive to, the image data to be printed. The charging device 61 is synchronized with the drop formation device using a conventional synchronization device 27 as part of the control circuit 26, (see fig. 1) such that a fixed phase relationship is maintained between the charging electrode waveform 64 generated by the charging electrode waveform source 63 and the clock of the drop formation waveform source 55. Thus, the phase of drop 54 shedding from the liquid stream 52 by the drop formation waveforms 92-1, 92-2, 92-3, 94-1, 94-2, 94-3, 94-4 (see FIG. 4) is phase locked to the charge electrode waveform 64. As indicated in FIG. 4, there may be a phase shift 109 (or equivalently a time shift) between the charge electrode waveform 64 and the drop formation waveforms 92-1, 92-2, 92-3, 94-1, 94-2, 94-3, 94-4.

Referring now to FIG. 3, the print head 30 includes a drop forming transducer 28 that produces a stream of liquid 52 that is broken up into ink drops 54. The selection of the drop 54 as either a printed drop 66 or a non-printed drop 68 will depend on the drop shedding phase relative to the charge electrode voltage pulse applied to the charge electrode 62 as part of the deflection mechanism 70, as will be described below. The charge electrode 62 is variably biased by a charge electrode waveform source 63. The charge electrode waveform source 63 provides a charge electrode waveform 64 in the form of a sequence of charge pulses. The charge electrode waveform 64 is periodic with a charge electrode waveform period 80 (fig. 4).

An embodiment of the charge electrode waveform 64 is shown in part B of fig. 4. The charge electrode waveform 64 includes a first voltage state 82 and a second voltage state 84. Drops that are shed during the first voltage state 82 are charged to a first charge state and drops that are shed during the second voltage state 84 are charged to a second charge state. The second voltage state 84 is typically at a high level, biased sufficiently to charge the droplet 54 when the droplet 54 drops. The first voltage state 82 is typically at a low level relative to the printhead 30 such that the first charge state is relatively uncharged when compared to the second charge state. An exemplary range of values for the potential difference between the first voltage state 82 and the second voltage state 84 is 50 to 300 volts, and more preferably 90 to 150 volts.

Returning to the discussion of fig. 3, when a relatively high level of voltage or potential is applied to the charge electrode 62 and the droplets 54 break off from the liquid stream 52 in front of the charge electrode 62, the droplets 54 acquire an electrical charge and are deflected by the deflection mechanism 70 toward the ink catcher 72 as non-printing droplets 68. The non-printing drops 68 that strike the catcher face 74 form an ink film 76 on the face of the ink catcher 72. The ink film 76 flows down the catcher face 74 and into a liquid channel 78 (also referred to as an ink channel), through which the ink film 76 flows to the ink recovery unit 44. A liquid passage 78 is generally formed between the body of the ink catcher 72 and the lower plate 79.

The deflection occurs when a droplet 54 is shed from the liquid stream 52, while the potential of the charge electrode 62 is supplied with an appropriate voltage. The droplet 54 will then acquire an induced charge that remains on the surface of the droplet. The charge on a single drop 54 has a polarity opposite to that of the charge electrode 62 and is dependent on the magnitude of the voltage and the coupling capacitance between the charge electrode 62 and the drop 54 at the instant the drop 54 separates from the liquid jet. When the droplet 54 drops, the coupling capacitance depends in part on the separation between the charging electrode 62 and the droplet 54. It may also depend on the vertical position of the drop off point 59 relative to the center of the charging electrode 62. After the charged droplets 54 have escaped the liquid flow 52, they continue to pass through the electric field created by the charge plates. These electric fields provide forces on the charged droplets that deflect them toward the charging electrode 62. Even if the charge electrode 62 cycles between the first voltage state and the second voltage state, the charge electrode 62 therefore acts as a deflection electrode to help deflect the charged droplets away from the initial trajectory 57 and toward the ink catcher 72. After passing through the charge electrode 62, the droplet 54 will travel against the catcher face 74, the catcher face 74 typically being comprised of a conductor or dielectric. The charge on the surface of the non-printed drop 68 will induce a surface charge density charge (for a catcher face 74 comprised of a conductor) or a polarization density charge (for a catcher face 74 comprised of a dielectric). The induced charge on the catcher face 74 creates an attractive force on the charged non-print drop 68. The attractive force on the non-printing drop 68 is the same as the attractive force generated by an imaginary charge (of opposite polarity and equal magnitude) located inside the ink catcher 72 at a distance from the surface equal to the distance between the ink catcher 72 and the non-printing drop 68. The dummy charge is referred to as an image charge. The attractive force exerted by the catcher face 74 on the charged non-print drops 68 causes the charged non-print drops 68 to deflect away from their original trajectory 57 and accelerate toward the catcher face 74 along the non-print trajectory 86 at a rate proportional to the square of the drop charge and inversely proportional to the drop mass. In this embodiment, the ink catcher 72 comprises a portion of the deflection mechanism 70 due to the induced charge distribution. In other embodiments, the deflection mechanism 70 may include one or more additional electrodes to generate an electric field through which the charged droplets pass in order to deflect the charged droplets. For example, an optional single biased deflection electrode 71 in front of the upper grounded portion of the trap may be used. In some embodiments, the charging electrode 62 may include a second portion, represented by a dashed charging electrode 62', on a second side of the fluidic array that is supplied with the same charging electrode waveform 64 as the first portion of the charging electrode 62.

In the alternative, when the drop formation waveform sequence 60 supplied to the drop formation transducer 28 causes the drop 54 to break off from the liquid stream 52 when the potential of the charge electrode 62 is at the first voltage state 82 (fig. 4) (i.e., at a relatively low potential or at zero potential), the drop 54 acquires no charge. Such uncharged droplets are not affected by the electric field that deflects the charged droplets during their flight. Thus, the uncharged drops become print drops 66, and as the recording medium moves past printhead 30 at velocity Vm, print drops 66 travel in a substantially undeflected path along trajectory 57 and affect print medium 32 to form print dots 88 on print medium 32. The charge electrode 62, the deflection electrode 71 and the ink catcher 72 serve as a drop selection system 69 for the print head 30.

Fig. 4 illustrates how selected drops are printed by controlling the drop formation waveform 60 supplied to the drop formation transducer 28. Portion A of FIG. 4 shows a drop formation waveform sequence 60 that includes three large drop formation waveforms 92-1, 92-2, 92-3 and four small drop formation waveforms 94-1, 94-2, 94-3, 94-4. The drop formation waveforms 94-1, 94-2, 94-3, 94-4 for small drops each have a period 96 and include a pulse 98, and each of the drop formation waveforms 92-1, 92-2, 92-3 for large drops has a longer period 100 and includes a longer pulse 102. In this example, the period 96 of the drop formation waveform 94-1, 94-2, 94-3, 94-4 of the small drop is the fundamental period T₀And the period 100 of the drop formation waveform 92-1, 92-2, 92-3 of the large drop is twice the fundamental period (2T)₀). The drop formation waveforms 94-1, 94-2, 94-3, 94-4 of the small drops each cause a single drop to be shed from the liquid stream. The large drop formation waveforms 92-1, 92-2, 92-3 each result in the formation of a larger drop 54 from the liquid stream 52 due to their longer periods. The larger drops 54 formed by the large drop formation waveforms 92-1, 92-2, 92-3 each have a volume approximately equal to twice the volume of the drops 54 formed by the small drop formation waveforms 94-1, 94-2, 94-3, 94-4.

As mentioned previously, the charge induced on the drop 54 depends on the voltage state of the charging electrode at the instant of drop shedding. Section B of fig. 4 shows the charge electrode waveform 64 and the time indicated by the diamond at which the drop 54 is shed from the liquid stream 52. When the charge electrode waveform 64 is in the second voltage state 84, the drop formation waveforms 92-1, 92-2, 92-3 for the large drops cause the large drops 104-1, 104-2, 104-3 to drop from the liquid stream 52. Due to the high voltage applied to the charge electrode 62 in the second voltage state 84, the large drops 104-1, 104-2, 104-3 are charged to a level that causes them to deflect into non-print drops 68, causing them to strike the catcher face 74 of the ink catcher 72 in FIG. 3. The drop formation waveform 94-1, 94-2, 94-3, 94-4 of the small drops results in the formation of small drops 106-1, 106-2, 106-3, 106-4. Arrows 99 represent the links between the waveform and the drops that it causes to form. As previously mentioned, there is a break-off time interval BOT between the application of the waveform to the drop forming transducer and the resulting break-off of the drop 54. Breaks in the arrows 99 and BOT arrows are present to indicate that the break off time BOT is typically many times longer than the drop formation waveform period 100. The small droplets 106-1 and 106-3 break off during the first voltage state 82 and therefore will be relatively uncharged. As a result, they are not deflected into ink catcher 72, but rather pass through ink catcher 72 as print drops 66 and strike print media 32 (see fig. 3). The small drops 106-2 and 106-4 break off during the second voltage state 84 and deflect to strike the catcher face 74 as non-printing drops 68. The drop formation waveform sequence 60 is determined by the print data, while the charge electrode waveform 64 is not controlled by the pixel data to be printed. This type of drop deflection is known and has been described, for example, in: U.S. patent 8,585,189 (Marcus et al); us patent 8,651,632 (Marcus); U.S. patent 8,651,633 (Marcus et al); U.S. patent 8,696,094 (Marcus et al); and U.S. patent 8,888,256 (Marcus et al), each of which is incorporated herein by reference.

As illustrated in part (a) of fig. 5, the large drops 65 produced by the drop formation waveforms 92-1, 92-2, 92-3 (fig. 4) of the large drops may be formed as a single drop that holds a single drop. Under other conditions as illustrated in part (B) of fig. 5, the large droplet 65 may be formed into two droplets 65a and 65B, the two droplets 65a and 65B being shed from the liquid stream 52 at approximately the same time and subsequently merged to form the large droplet 65. Alternatively, as indicated in part (C) of fig. 5, the large droplets may be formed into large droplets 65, which large droplets 65 are shed from the liquid stream that separates into two droplets 65a, 65b, and then coalesce back into a single large droplet 65. The distance below the drop-off point 59 is referred to as the coalescence distance CD at which the drops 65a and 65b coalesce to form a large drop 65. It is generally desirable to keep the coalescence distance CD small. The large drop formation process of part (a) of fig. 5 is represented in fig. 4 by the large diamonds for the large drop 104-1. The large drop formation process of section (B) of fig. 5 is represented in fig. 4 by two closely spaced diamonds for large drop 104-2, and the large drop formation process of section (C) of fig. 5 is represented in fig. 4 by double diamonds for large drop 104-3.

For each nozzle in the nozzle array, the drop formation waveform sequence 60 includes a sequence of large drop formation waveforms 92 (e.g., 92-1, 92-2, 92-3 of FIG. 4) and small drop formation waveforms 94 (e.g., 94-1, 94-2, 94-3, 94-4 of FIG. 4), the drop formation waveform sequence 60 being generated by the drop formation waveform source 55 in response to image data to be printed. When image data for a particular nozzle requires a print drop to be formed, a drop formation waveform 94 for a pair of small drops is added to the waveform sequence 60 for that nozzle, and conversely, when no print drop is to be generated, a drop formation waveform 92 for a large drop (which may also be referred to as a non-print drop formation waveform) is added to the waveform sequence 60 for that nozzle. Since the drop formation waveform 94 of a small drop is always added in pairs to the drop formation waveform sequence 60 whenever a drop needs to be printed, the drop formation waveform 94 (e.g., 94-1, 94-2) of the small drop in the pair is referred to herein as the drop formation waveform 97 (e.g., 97-1) of the print drop. The drop formation waveform 97 of the print drops may also be referred to as the drop formation waveform of a drop pair or more simply as the print drop formation waveform. The drop formation waveform 97 of the print drop has the same period 96 as the non-print drop formation waveform 92. In FIG. 4, the drop formation waveforms 94-1, 94-2 of the small drops together form a drop formation waveform 97-1 of the print drop, and the drop formation waveforms 94-3, 94-4 of the small drops together form a drop formation waveform 97-2 of the print drop.

While the example of FIG. 4 shows that each of the drop formation waveforms 92-1, 92-2, 92-3 of the non-printing large drops are identical to each other and each of the drop formation waveforms 97-1, 97-2 of the printing drops are identical to each other, this is not required. In some embodiments, there may be multiple variations of the drop formation waveform 92 for non-printing large drops and multiple variations of the drop formation waveform 97 for printing drops. In this case, as disclosed in us patent 8,469,495 (Gerstenberger et al), the selection of a particular one of the waveforms may depend not only on the print/non-print state of the corresponding pixel, but also on the print/non-print state for one or both of the preceding and following drops.

Referring to fig. 6, while the above-described printing system has been found to work generally well, certain printing situations have been found to produce printing defects, commonly referred to as printing artifacts. When printing images of certain periodic patterns 110 comprising spaced, wide character strokes 120, diffuse areas 124 of scattering ink dots have been found in the spaces 122 between the character strokes 120. The presence of these unwanted diffuse areas of ink dots 124 depends on the spatial period 125 of the pattern of character strokes 120 and on the printing speed; at high printing speeds, print defects are more pronounced. Without being bound by an understanding of the physics involved, this form of print imperfection appears to be the result of resonance of the spatially periodic application of stimulation by the drop formation waveform required to print the periodic pattern 110.

It has been found that by segmenting the array of nozzles 50 into a first and second set of interleaved nozzles 50, and introducing a phase shift and drop formation waveform energy difference between the drop formation waveforms supplied to the drop formation devices associated with the two sets of nozzles 50, the formation of these diffusive regions 124 of scattering dots of ink can be suppressed. To accomplish this, the plurality of nozzles 50 are arranged or grouped into a first group G1 and a second group G2, wherein the nozzles 50 of the first group G1 and the second group G2 are staggered such that the nozzles 50 of the first group G1 are positioned between adjacent nozzles 50 in the second group G2 and the nozzles 50 of the second group G2 are positioned between adjacent nozzles 50 in the first group G1, as shown in fig. 7.

Each of the nozzles 50 in the first group G1 has an associated drop forming device (which includes a drop forming transducer 28, such as a heater 51), which for brevity will be referred to as a first group of drop forming devices. Each of the nozzles 50 in the second group G2 has an associated drop forming device, which for brevity will be referred to as the second group of drop forming devices.

The timing delay device 134 supplies a first set of trigger pulses 130 to control the start time of the drop formation waveforms 60 supplied to the first set of drop formation devices, and supplies a second set of trigger pulses 132 to control the start time of the drop formation waveforms 60' supplied to the second set of drop formation devices. In a preferred embodiment, the timing delay device 134 shifts the timing of the drop formation waveforms 60, 60 'supplied to one or both of the first and second sets of drop formation devices such that the waveform pulses in the drop formation waveforms 60 supplied to the first set of drop formation devices precede the waveform pulses in the corresponding drop formation waveforms 60' supplied to the second set of drop formation devices by a defined second set of time shifts 108. (the second set of time shifts 108 may be equivalently referred to as a "second set of phase shifts" because they shift the phase of the drop formation waveform 60' relative to the phase of the drop formation waveform 60).

Further, the waveform energy of the drop formation waveform 60' supplied to the second set of drop formation devices is increased relative to the waveform energy of the drop formation waveform 60 supplied to the first set of drop formation devices. In this way, the drop-off times BOT' of the droplets from the second set of nozzles 50 are controlled such that they are less than the drop-off times BOT of the droplets from the first set of nozzles 50.

The waveform energies and timing delays are selected such that the print small drops 106-1, 106-3, 106-1', 106-3' are shed from the liquid jet during the first voltage state 82 of the charge electrode waveform 64 to provide a first print drop charge state, and the non-print small drops 106-2, 106-4, 106-2', 106-4' and the non-print large drops 104-1, 104-2, 104-3, 104-1', 104-2', 104-3' are shed from the liquid jet during the second voltage state 84 of the charge electrode waveform 64 to provide a second non-print drop charge state.

This embodiment is illustrated in fig. 8. The upper part of fig. 8 shows a portion of a drop formation waveform sequence 60 supplied to a first set of drop formation devices. The drop formation waveform sequence 60 is formed in response to image data for the first set of nozzles 50. In this example, the drop formation waveform sequence 60 includes drop formation waveforms 92-1, 92-2, 92-3 for large drops and drop formation waveforms 97-1, 97-2 for print drops. The lower portion of fig. 8 shows a portion of a drop formation waveform sequence 60' supplied to a second set of drop formation devices. A drop formation waveform sequence 60' is formed in response to the image data for the second set of nozzles 50. In this example, the drop formation waveform sequence 60 'includes drop formation waveforms 92-1', 92-2', 92-3' for large drops and drop formation waveforms 97-1', 97-2' for print drops.

For simplicity, the first drop formation waveform sequence 60 may be referred to as a first set of waveforms, and the second drop formation waveform sequence 60' may be referred to as a second set of waveforms. The first and second sets of waveforms each include one or more print drop formation waveforms 97 (e.g., 97-1, 97-2, 97-1', 97-2') that, when supplied to a drop formation device associated with a particular nozzle, modulate a liquid jet ejected from the particular nozzle to selectively cause portions of the liquid jet to break off into a pair of drops that travel along a path. The first and second sets of waveforms also each include a drop formation waveform 92 (e.g., 92-1, 92-2, 92-3, 92-1', 92-2', 92-3 ') of non-printing large drops that, when supplied to a drop formation device associated with a particular nozzle, modulate a liquid jet ejected from the particular nozzle to selectively cause a portion of the liquid jet to break off into large non-printing drops that travel along a path. Each of these printed and non-printed drop forming waveforms has the same waveform period.

The central portion of fig. 8 shows a portion of the charge electrode waveform 64, along with the time at which the drop 54 (fig. 3) is shed from the liquid stream 52 (fig. 3) in response to the illustrated portions of the drop formation waveforms 60 and 60'. The time at which the droplets 54 drop from the liquid stream 52 from the first set of nozzles is represented by filled diamonds, while the time at which the droplets 54 drop from the liquid stream 52 from the second set of nozzles is represented by open diamonds. For clarity, the first drop formation waveform sequence 60 and the second drop formation waveform sequence 60' are shown as printed drops and non-printed drops having the same pattern. However, in practice, the first and second sequences may differ in response to their corresponding image data. It can be seen that the second drop formation waveform sequence 60' has been delayed relative to the first drop formation waveform sequence 60 by a second set of time shifts 108.

The first and second sets of waveforms from which the first and second drop formation waveform sequences 60, 60' are formed are of different amplitudes. The amplitude 140' of the second set of waveforms is greater than the amplitude 140 of the first set of waveforms. Since each of the drop formation waveforms has its associated waveform energy supplied to its corresponding drop formation device, the larger waveform amplitude 140' of the second set of waveforms supplies a greater waveform energy to the second group of drop formation transducers 28 (fig. 3) than the waveform energy supplied to the first group of drop formation transducers 28 by the corresponding drop formation waveform from the first set of waveforms.

More particularly, the energy level of the fourier component of the drop formation waveform 97 (e.g., 97-1', 97-2 ') for forming a print drop of a small print drop and the energy level of the fourier component of the drop formation waveform 92 (e.g., 92-1', 92-2', 92-3 ') for forming a large drop of a large non-print drop are greater for the second set of waveforms than for the corresponding drop formation waveforms in the first set of waveforms. For the sake of brevity, the term waveform energy of the drop formation waveform 97 (e.g., 97-1 ') of the print drop shall refer to the energy level of the Fourier component of the drop formation waveform at a frequency suitable for modulating the liquid flow to form a pair of small drops 106 (e.g., 106-1', 106-2 '), and the waveform energy of the drop formation waveform 92 (e.g., 92-1 ') of the non-print large drops shall refer to the energy level of the Fourier component of the drop formation waveform at a frequency suitable for modulating the liquid flow to form a non-print larger drop 104 (e.g., 104-1 ').

As a result of the larger waveform energy associated with the second set of waveforms, the second group of drop forming devices modulate the diameter of the liquid streams ejected from the second group of nozzles with an initial modulation amplitude that is higher than the initial modulation amplitude generated by the first group of drop forming devices on the liquid streams 52 ejected from the first group of nozzles 50. Since the higher initial modulation amplitude produced on the liquid stream 52 from the second set of nozzles 50 reduces the time required for the modulation amplitude to grow sufficiently to cause drops 54 to break off from the liquid stream 52, the break off time BOT' for drops from the second set of nozzles 50 of G2 will be less than the break off time BOT for drops from the first set of nozzles 50 of G1.

Consider now the time at which the large droplets 104-3, 104-3' fall off the liquid stream 52 from the first set of nozzles 50 and the second set of nozzles 50, respectively. If the same waveform energy is supplied to both sets of drop forming devices, the second set of time shifts 108 between the first and second drop forming waveform sequences 60, 60' will result in the same drop time delays for drops from the second set of nozzles as the first set, as indicated by the position of the large drop 104-3 ″. However, if at this time a large droplet 104-3 ″ from the second set of nozzles is to be dropped, it will be dropped during the first voltage state 82, rather than as it would be during the second voltage state 84 as was the case with the large droplet 104-3 from the first set of nozzles. This will cause the large drops 104-3 "to have a first charge state rather than the desired second charge state, and will cause the large drops 104-3" to be printed rather than deflected to the catcher as intended. The difference in drop times BOT and BOT 'resulting from the waveform energy difference between the first and second sets of waveforms advances the drop time for large drops back to the position of large drop 104-3'. Thus, the large droplet 104-3 'drops off during the second voltage state 84, such that the large droplet 104-3' is charged to the second charge state as expected.

The increased waveform energy associated with the drop formation waveform 92-3' of the second set of large drops at least partially compensates for the second set of time shifts 108 relative to the waveform energy associated with the drop formation waveform 92-3 of the first set of large drops. In a similar manner, the increased waveform energy associated with each of the second set of printed and non-printed drop formation waveforms 97', 92' at least partially compensates for the second set of time shifts 108 between the waveforms relative to the waveform energy associated with the corresponding first set of printed and non-printed drop formation waveforms 97, 92. This enables each of the droplets from the nozzles 50 in the second group G2 to be shed during the expected voltage state of the charge electrode waveform 64, while still having a time shift between the first and second sets of waveforms that suppresses the formation of the diffusion region 124 of scattering ink dots discussed with respect to fig. 5. For acceptable suppression of the diffusing region 124 of scattered ink dots, it has been found that the drop forming waveform sequence 60' supplied to the drop forming devices associated with the second group of G2 nozzles 50 should be delayed by a second group time shift 108 within the range of waveform periods 100 of 1/4 to 3/4 relative to the drop forming waveform sequence 60 used to control the drop forming devices associated with the first group of G1 nozzles 50. In a preferred embodiment, the second set of time shifts 108 should be approximately 1/2 waveform cycles 100.

In the exemplary configuration of fig. 8, the second aggregate drop formation waveform sequence 60' is delayed relative to the first aggregate drop formation waveform sequence 60 by increasing the voltage amplitude of the second aggregate drop formation waveform sequence 60' relative to the voltage amplitude of the first aggregate drop formation waveform sequence 60 by a second set of time shifts 108, and the waveform energy associated with the second aggregate drop formation waveform sequence 60' is increased relative to the waveform energy associated with the first aggregate drop formation waveform sequence 60. Within the purview of the present invention, an alternative means may be used to supply a second aggregate drop formation waveform sequence 60' having an associated waveform energy that is higher than the waveform energy of the first aggregate drop formation waveform sequence 60.

Fig. 9 illustrates an alternative configuration in which the waveform energy is adjusted by changing the pulse width/duty cycle rather than changing the waveform amplitude. In this example, the amplitude 140 'of the second ensemble of drop forming waveform sequences 60' is the same as the amplitude 140 of the first ensemble of drop forming waveform sequences 60, but the duty cycle or pulse width of the waveform pulses of the drop forming waveforms is different. The drop formation waveforms in the first and second collective drop formation waveform sequences 60, 60 'are similar to one another such that each waveform pulse in the drop formation waveform in the second collective drop formation waveform sequence 60' corresponds to a waveform pulse in the corresponding drop formation waveform in the first collective drop formation waveform sequence 60. That is, for each pulse in the first set of drop formation waveforms, there is exactly one pulse in the corresponding second set of drop formation waveforms, and the phases of the pulses that lie within the drop formation waveforms are similar (i.e., to within 45 °) for the first and second sets of drop formation waveforms. The drop forming pulses also have a similar shape. In this case, the drop forming pulses have a square wave shape, although this is not essential. In other configurations, the drop forming pulses can have other shapes, such as triangular pulse shapes or trapezoidal pulse shapes.

In the example of fig. 9, the first and second sets of drop formation waveforms differ in that the drop formation pulse of each of the second set of drop formation waveforms has an increased duty cycle (or pulse width) relative to the corresponding drop formation pulse of the first set of drop formation waveforms. In the upper portion of FIG. 9, drop formation waveforms 97-1, 97-2 for the print drops of first aggregate drop formation waveform sequence 60 each include two drop formation pulses 98 having pulse widths 150, and non-print drop formation waveforms 92-1, 92-2, 92-3 each include a drop formation pulse 102 having a pulse width 152. Similarly, in the lower portion of FIG. 9, drop formation waveforms 97-1', 97-2' for the print drops of the second aggregate drop formation waveform sequence 60 'each include two drop formation pulses 98' having pulse widths 150', while non-print drop formation waveforms 92-1', 92-2', 92-3' each include a drop formation pulse 102 'having a pulse width 152'. In this exemplary configuration, the drop formation waveform pulses 98 'of the second set of print drops have a pulse width 150' that is greater than the pulse width 150 of the corresponding drop formation waveform pulses 98 of the first set of print drops. Similarly, the pulse width 152 'of the second set of non-print drop formation waveform pulses 102' is greater than the pulse width 150 of the corresponding first set of non-print drop formation waveform pulses 102.

In the exemplary configuration of fig. 9, the rising edge of the pulse within the first ensemble of drop formation waveforms occurs at the same phase from the beginning of the waveform as the rising edge of the pulse within the second ensemble of drop formation waveforms. In other embodiments, it may be the falling edge or midpoint of the corresponding drop forming pulses of the first and second sets of waveforms that coincide within 45 ° of each other from the beginning of the waveform.

Another exemplary embodiment is illustrated in fig. 10. In this case, the difference in waveform energy between the first group G1 and the second group G2 nozzles 50 is provided by the structural difference between the first group of drop forming devices and the second group of drop forming devices, such that when both the first group of drop forming devices and the second group of drop forming devices are supplied with the same drop forming waveform, the second group of drop forming devices produces a greater modulation amplitude of the liquid stream than the first group of drop forming devices.

In the exemplary embodiment of fig. 10, the drop formation device is a heater 51 formed in nozzle plate 49 (fig. 3) around each nozzle 50. The geometry of the heaters 51 associated with the two sets of nozzles 50 is different (in this case, the outer diameter 144' of the heaters 51 in the second set G2 is greater than the outer diameter 144 of the heaters 51 in the first set) so that the heaters 51 associated with the second set G2 of nozzles 50 have a lower resistance than the heaters 51 associated with the first set G1 of nozzles 50. Therefore, when both the heater 51 associated with the nozzle 50 of the second group G2 and the heater 51 associated with the nozzle 50 of the second group G2 are supplied with the same droplet formation waveform, the heater 51 associated with the nozzle 50 of the second group G2 generates more heat than the heater 51 associated with the nozzle 50 of the second group G2.

In an alternative embodiment, the physical geometry of the two sets of heaters 51 may be the same, but the heaters 51 associated with the second set of G2 nozzles 50 may have a lower resistance than the heaters 51 associated with the first set of G1 nozzles 50 due to the use of different heater materials having different resistivities. Alternatively, the coupling factor between the heater 51 and the ink may be altered to modify the waveform energy imparted to the liquid stream 52, for example by providing different amounts of thermal insulation between the heater 51 and the nozzle 50.

In a similar manner, differences in the configuration of other types of drop forming transducers 28 (e.g., piezoelectric devices, MEMS actuators, electro-hydrodynamic devices, optical devices, or electrostrictive devices) may enable the drop forming waveforms supplied to the drop forming transducers 28 associated with the second group G2 nozzles 50 to supply more associated waveform energy to the drop forming transducers 28 than the waveform energy supplied to the drop forming transducers 28 associated with the first group G1 nozzles 50 by similar drop forming waveforms, such that the initial modulation amplitude of the liquid stream for the second group G2 nozzles 50 is greater than the initial modulation amplitude of the liquid stream for the first group G1 nozzles 50.

In the previous embodiment, each of the drop formation waveforms includes a single drop formation pulse for each drop to be formed by the drop formation waveform. Thus, the drop formation waveform 97 of a print drop includes two drop formation pulses to produce a print drop and a non-print drop of a drop pair, and the drop formation waveform 92 of a non-print large drop has a single drop formation pulse to produce a single non-print large drop. In the alternative embodiment of fig. 11, the drop formation waveforms include not only primary pulses 154 (i.e., the drop formation pulses primarily responsible for initiating drop formation), but they also include one or more secondary pulses 156. These additional secondary pulses 156 (which may also be referred to as secondary drop forming pulses) typically have a smaller duty cycle than the primary pulses 154.

As discussed in commonly assigned U.S. patent 7,828,420 entitled "Continuous ink jet printer with modified droplet activation waveform" to Fagerquist et al, which is incorporated herein by reference, if the time interval between the secondary pulse 156 and the primary pulse 154 is less than the rayleigh cutoff period, such that the interval between disturbances is less than pi times the diameter of the liquid stream, the secondary pulse 156 will not cause shedding of additional droplets from the liquid stream 52. (Secondary pulses 156 are typically separated in time from primary pulses 154 by more than the thermal response time of the heater, so that they produce a different thermal pulse on the liquid stream than the thermal pulse of primary pulses 154.)

As described in all commonly assigned U.S. patent 7,828,420 (gegerquist et al), U.S. patent 8,714,676 (Grace et al), and U.S. patent 8,684,483 (Grace et al), the inclusion of one or more secondary pulses in the drop formation waveform 92 of the large drops may help stabilize the formation of the non-printing large drops 65 to correspond to the drop formation conditions of section (a) of fig. 5, or help accelerate coalescence of the large drops 65 from the two or more smaller drops 65a and 65B to reduce the coalescence distance CD of sections (B) and (C) of fig. 5. Similarly, including one or more secondary pulses in the drop formation waveform 97 of a print drop may reduce the formation of undesirable satellites or accelerate the coalescence of satellites with the print and non-print drops of a drop pair. The inclusion of secondary pulses may also be used to modify the velocity of drops formed by primary drop forming pulses, as discussed in all commonly assigned U.S. patent application publication 2011/0242169 (Link et al), U.S. patent 8,469,496 (panchawavegh et al), and U.S. patent 8,657,419 (panchawavegh et al).

In the embodiment of fig. 11, the greater waveform energy associated with the second collective drop formation waveform sequence 60 'is provided by the primary pulses 154' in the second collective drop formation waveform sequence 60 'when compared to the first collective drop formation waveform sequence 60', the primary pulses 154 'in the second collective drop formation waveform sequence 60' having a pulse width greater than the corresponding primary pulses 154 in the first collective drop formation waveform sequence 60, and the secondary pulses 156 'in the second collective drop formation waveform sequence 60' having a pulse width equal to the pulse width of the corresponding secondary pulses 156 in the first collective drop formation waveform sequence 60. In some embodiments, the second ensemble of drop formation waveforms may have a different number of secondary pulses 156 than the corresponding drop formation waveforms from the first ensemble of drop formation waveforms.

In certain embodiments, the first and second sets of waveforms may each include drop formation waveforms 97 for multiple print drops to accommodate different print drop/non-print drop sequence options. As discussed in commonly assigned U.S. patent 8,469,495 (Gerstenberger et al), selection of an appropriate drop formation waveform from a collection of predefined drop formation waveforms may depend not only on the print/non-print state of the image data for the current drop formation waveform, but also on the print/non-print state of the image data for the previous drop formation waveform and/or the subsequent drop formation waveform. For example, when the preceding droplet formation waveform is the droplet formation waveform 92 of a non-printing large droplet, the droplet formation waveform 97 of some printing droplets is used, and when the preceding droplet formation waveform is the droplet formation waveform 97 of a printing droplet, the droplet formation waveform 97 of other printing droplets is used. Similarly, when the subsequent drop formation waveform is the drop formation waveform 92 of a non-printing large drop, the drop formation waveform 97 of some printing drops is used, and when the subsequent drop formation waveform is the drop formation waveform 97 of a printing drop, the drop formation waveform 97 of other printing drops is used. The drop formation waveform of the plurality of print drops may vary in the duty cycle and start time of either the primary pulse 154 or the secondary pulse 156. The drop formation waveforms 97 for different print drops may also vary in the number of secondary pulses 156.

Similarly, the first and second sets of drop formation waveforms may each include more than one non-printing large drop formation waveform 92 to accommodate different printing/non-printing sequences. The drop formation waveform 92 of the plurality of non-printing large drops may vary in the duty cycle and start time of the primary pulse 154 or the secondary pulse 156. The drop formation waveforms 92 for different non-printing large drops may also vary in the number of secondary pulses 156.

In some embodiments, the first and second sets of drop formation waveforms each include eight drop formation waveforms (labeled a-H), and the selection of the drop formation waveform for the kth time interval in the sequence of waveforms depends not only on the print/non-print state for time interval k, but also on the print/non-print states for the previous time interval k-1 and the subsequent time interval k +1, respectively, as indicated by the table below.

When consecutive heater pulses are supplied to drop forming heater 51 (which has a time interval between pulses that is less than the thermal response time of drop forming heater 51), these heater pulses act on liquid stream 52 as if a single heater pulse were applied to drop forming heater 51, as described in commonly assigned U.S. patent 8,087,740. Fig. 12 illustrates an embodiment in which the increased waveform energy of the drop formation waveforms in the second sequence of drop formation waveforms 60' is provided by adding additional pulses to the drop formation waveforms, where the additional pulses are separated in time from the primary drop formation pulse by a time less than the thermal response time of the drop formation heater 51. For example, in the drop formation waveform 97-2 'of a print drop, the additional pulse 158 immediately follows the primary drop formation pulse 98' to effectively add more waveform energy to the drop formation pulse. Similarly, in the drop formation waveform 92-3 'of a large drop, an additional pulse 160 immediately follows the primary drop formation pulse 102' to effectively add more waveform energy to the drop formation pulse.

Another embodiment is shown in fig. 13. In this embodiment, the first set of waveforms in the sequence of drop formation waveforms 60 is similar to the waveforms in fig. 9. These first set of waveforms are typically held at a low value (e.g., zero volts) with the pulses rising to some higher potential to generate a heat pulse that causes a drop to form. The second set of waveforms in drop formation waveform sequence 60' differ in that the waveform potential is generally maintained at a non-zero voltage with the pulse dropping down to a lower potential (e.g., to zero volts). This downward pulse produces a temporary reduction in the energy provided to the drop formation device or heater 51. These temporary reductions in energy provided to the drop forming device can be considered "cooling pulses" rather than heating pulses. Such a cooling pulse acts on the liquid stream in a manner similar to the heating pulse to cause the formation of droplets. This reverse drop formation waveform has an associated waveform energy as does the normal drop formation waveform. With an inverted drop formation waveform, the waveform energy of the print drop formation waveform should refer to the energy level of the fourier component of the drop formation waveform at a frequency suitable for modulating the liquid flow to form a pair of small drops, while the waveform energy of the non-print drop formation waveform should refer to the energy level of the fourier component of the drop formation waveform at a frequency suitable for modulating the liquid flow to form a larger non-print drop. In this embodiment, the increased waveform energy of the second set of waveforms is provided by cooling pulses having a pulse width 152' greater than the pulse width 152 of the heating pulses of the first set of waveforms. In the illustrated configuration, the second set of waveforms includes inverted waveform pulses that reduce the energy provided by the drop formation device. In other embodiments, the first set of waveforms may include inverted waveform pulses that reduce the energy provided by the drop formation device. In still other embodiments, both the first and second sets of waveforms comprise inverted waveform pulses.

Since drop shedding phase can vary not only depending on the waveform energy of the drop formation waveform, but also depending on nozzle size, ink pressure, and ink properties, some printhead embodiments also include a drop shedding phase detector (not shown) for determining the phase at which drops shed from the first group G1 of nozzles 50 and from the second group G2 of nozzles 50. A variety of drop-off phase detectors are known in the art, such as those disclosed in U.S. patent 3,761,941, U.S. patent 4,616,234, U.S. patent 7,249,828, and U.S. patent 3,836,912, each of which is incorporated herein by reference. Using such drop-off phase detectors, drop-off phase differences between drops from the first group of G1 nozzles 50 and drops from the second group of G2 nozzles 50 can be determined. As discussed above, the phase difference results from a second set of time shifts 108 (fig. 8) between the first and second sets of waveforms and the waveform energy difference between the first and second sets of waveforms. In order to maximize the latitude for setting the phase of the charge electrode waveform relative to the drop formation waveform, it is desirable that the drop break off time or phase difference between drops from the first group G1 nozzles 50 and drops from the second group G2 nozzles 50 remain small. The drop break off time difference may be adjusted by adjusting the waveform energy of the second set of time shifts 108 or drop formation waveforms applied by the set timing delay device 134 (fig. 7). Since adjusting the second set of time shifts 108 is generally simpler than adjusting the waveform energy, in some embodiments, the time shifts 108 are adjusted in response to the measured drop off time difference to minimize the drop off time difference.

FIG. 14 shows a portion of a drop formation waveform sequence that includes three non-printing large drop formation waveforms 92-1, 92-2, 92-3 and two printing drop formation waveforms 97-1, 97-2. As indicated by the different three boundary sets 162, 164, 166 of the bracket and waveform break-off marks, the boundaries between drop formation waveforms may be offset within a range while still retaining the desired drop formation pulses within the drop formation waveforms 97-1, 97-2 of the print drops for producing print drops and non-print drops, and the desired drop formation pulses for producing large non-print drops in the drop formation waveforms 92-1, 92-2, 92-3 of the large drops.

In the embodiment of fig. 15, the arrangement of the waveform boundaries of the second set of waveforms in the sequence of drop formation waveforms 60' has been offset relative to the drop formation pulses within the waveforms. (when the trailing edge boundaries of the large drop's drop formation waveforms 92-1, 92-2, 92-3 are aligned with the falling edge of the drop formation pulse 102 and the trailing edge boundaries of the print drop's drop formation waveforms 97-1, 97-2 are aligned with the falling edge of one of the drop formation pulses 98 in the first set of waveforms in the drop formation waveform sequence 60, the boundaries have been offset from those locations in the second set of waveforms in the drop formation waveform sequence 60 '. As a result of the offset in the waveform boundaries, even though the waveform boundaries of the first and second sets of waveforms are aligned, there may still be a second set of time shifts 108. thus, the set timing delay device 134 need not delay the second set of trigger pulses relative to the first set of trigger pulses to effectively delay the phase of the second set of waveforms relative to the first set of waveforms, the opposite, "time shifting" is embodied in the collection of drop formation waveforms to provide a phase control means.

In the embodiment of fig. 16, the plurality of nozzles 50 are arranged or grouped into three nozzle groups. The nozzle group includes a third group G3 of nozzles 50 in addition to the first group G1 and the second group G2. The third group G3 of nozzles 50 is interleaved with the first group G1 and the second group G2 of nozzles. Between any two first sets of nozzles there is a second set of nozzles and a third set of nozzles. Similarly, there are a first set of nozzles and a third set of nozzles between any two second sets of nozzles, and a first set of nozzles and a second set of nozzles between any two third sets of nozzles. Each of the nozzles 20 has an associated drop forming device (e.g., heater 51). For the sake of brevity, the drop forming devices associated with the third group G3 of nozzles will be referred to as a third group of drop forming devices. The drop forming waveforms supplied to the third set of drop forming devices are referred to as a third set of waveforms.

The timing delay device 134 supplies a first set of trigger pulses 130 to control the start times of a first set of waveforms in the sequence of drop formation waveforms 60, a second set of trigger pulses 132 to control the start times of a second set of waveforms in the sequence of drop formation waveforms 60', and a third set of trigger pulses 136 to control the start times of a third set of waveforms in the sequence of drop formation waveforms 60 ″. The timing delay device 134 is a specific example of a phase control device that controls the relative phases of the waveforms supplied to the first and second sets of nozzles.

In an exemplary embodiment, the timing delay device 134 shifts the timing of the different sets such that pulses in the first set of waveforms precede corresponding pulses in the second set of waveforms by a time shift 108 and precede corresponding pulses in the third set of waveforms by a time shift 108', the time shift 108' being greater than the time shift 108, as indicated in fig. 17. The second set of waveforms in the sequence of drop formation waveforms 60' thus precedes the third set of waveforms in the sequence of drop formation waveforms 60 ″.

Further, the pulse widths 150 ", 152" for the third set of waveforms are increased relative to the pulse widths 150', 152' of the second set of waveforms such that the waveform energy of the third set of waveforms in the drop formation waveform sequence 60 "is increased relative to the waveform energy of the second set of waveforms 60 '. This results in drop-off times BOT "for droplets from the third group G3 of nozzles 50 being less than drop-off times BOT' for droplets from the second group G2 of nozzles 50, which in turn is less than drop-off times BOT for droplets from the first group G1 of nozzles 50. As with the previous embodiment, the waveform energy of the second set of waveforms is increased relative to the waveform energy of the first set of waveforms such that the drop-off time BOT' for drops from the second set G2 nozzles 50 is less than the drop-off time BOT for drops from the first set G1 nozzles 50.

The printed droplets are relatively uncharged when compared to the charge of small or large non-printed droplets. But even a small charge on the print drops may cause the print drops to experience some drop deflection, altering their position affecting the print medium. To ensure the highest quality printing, it is desirable to ensure that the print drops have a consistent drop charge. Since the charge on a print drop is affected by the charge on a preceding drop, some embodiments require that each pair of drops formed by the drop formation waveform 97 of a print drop be preceded by a large non-print drop. Since the trajectory of the print drops may be affected by drop-to-drop electrostatic and aerodynamic interactions, some embodiments require that each pair of drops formed by the drop formation waveform 97 of the print drops be followed by a large non-print drop.

Although each of the previous embodiments has been directed to a drop formation waveform consisting of a set of one or more waveform pulses, the drop formation waveform is not limited to such a set of waveform pulses. Other waveforms, such as sinusoidal, triangular, chirped waveforms, or portions or combinations thereof, may also be used.

The previous embodiments have described the timing delay device 134 as generating a first set of trigger pulses 130 and a second set of trigger pulses 132 for controlling the timing of a first set of waveforms relative to a second set of waveforms. In alternative embodiments, the timing delay device 134 may use other timing control configurations that do not involve the use of separate trigger pulses for controlling the timing of different sets of drop forming devices. For example, the second set of waveforms may be delayed relative to the first set of waveforms by a predetermined number of clock pulses. Further, in certain embodiments, the different drop formation waveforms in each waveform sequence are linked together with no breaks between the waveforms. In such an embodiment, there is no trigger pulse required to initiate each waveform. In such embodiments, the group timing delay device may refer to a software implementation for delaying the second set of waveforms relative to the first set of waveforms.

Fig. 18A is a photograph of an ink droplet formed according to the present invention. The ink drops formed in this example are non-printing large drops 65. (at this point in time, the drop pairs have not merged into a single large drop 65.) an ink stream 52 is formed by the array of nozzles 50. The odd-numbered nozzles form the first group of nozzles G1, while the even-numbered nozzles 50 form the second group of nozzles G2. The second set of waveforms for controlling the second set of nozzles G2 are time shifted (half of the waveform period) and have higher waveform energy relative to the first set of waveforms for controlling the first set of nozzles G1. It can be seen that at the moment the photograph is captured, the drop drops break off at break off locations 59 for both the first set of nozzles and the second set of nozzles. Thus, the resulting large droplets 65 will all have the same charge state. In contrast, fig. 18B shows the results obtained without the method of the present invention. In this case, the phase of the second set of waveforms has been shifted by 180 degrees, but the same waveform energy is used. It can be seen that the drops drop off at the drop off location 59 for the first set of nozzles G1, while the drops formed by the second set of nozzles G2 do not drop off nearly. Thus, the charge states of the resulting large droplets will not be the same.

List of components

20 printing system

22 image source

24 image processing unit

26 control circuit

27 synchronization device

28 droplet forming transducer

30 print head

32 print medium

34 print media transport system

35 speed measuring device

36 medium conveying controller

38 microcontroller

40 ink reservoir

44 ink recovery unit

46 ink pressure regulator

47 ink channel

48 spray module

49 nozzle plate

50 nozzle

51 heater

52 flow of liquid

54 droplet

55 drop forming waveform source

57 track

59 off position

60 droplet formation waveform sequence

60' droplet formation waveform sequence

60'' drop formation waveform sequence

61 charging device

62 charging electrode

62' charging electrode

63 charging electrode waveform source

64 charging electrode waveform

65 Large droplet

65a droplet

65b liquid droplet

66 print drops

68 non-printing drop

69 droplet selection system

70 deflection mechanism

71 deflection electrode

72 ink catcher

74 trap surface

76 ink film

78 liquid channel

79 lower plate

80 cycle of charging electrode waveform

82 first voltage state

84 second voltage state

86 non-printing tracks

88 printing point

Drop formation waveform for 92 large drops

92-1 large drop formation waveform

Drop formation waveform for 92-1' large drops

Drop formation waveform for 92-2 large drops

Drop formation waveform for 92-2' large drops

92-3 large drop formation waveform

Drop formation waveform for 92-3' large drops

Drop formation waveform for 94 small drops

Drop formation waveform for 94-1 small drops

Drop formation waveform for 94-2 small drops

Drop formation waveform for 94-3 small drops

Drop formation waveform for 94-4 small drops

96 cycles

97 drop formation waveform for print drops

97-1 drop formation waveform for print drops

Drop formation waveform for 97-1' print drops

97-2 drop formation waveform for print drops

Drop formation waveform for 97-2' print drops

98 pulses

98' pulse

99 arrow head

100 cycles

102 pulse

102' pulse

104 large liquid drop

104-1 Large droplets

104-1' Large droplets

104-2 Large droplets

104-2' Large liquid droplet

104-3 Large droplets

104-3' Large liquid droplet

104-3'' large droplets

106-1 droplet

106-1' small liquid droplet

106-2 droplet

106-2' small liquid droplet

106-3 droplet

106-3' small liquid droplet

106-4 droplet

106-4' small liquid droplet

108 time shifting

108' time shifting

109 phase shift

110 periodic pattern

120 strokes

122 space

124 diffusion region

125 space period

130 first set of trigger pulses

132 second set of trigger pulses

134 timing delay device

136 third set of trigger pulses

140 amplitude

140' amplitude

144 outer diameter

144' outer diameter

150 pulse width

150' pulse width

152 pulse width

152' pulse width

154 primary pulse

156 secondary pulse

158 additional pulses

160 additional pulses

162 set of boundaries

164 set of boundaries

166 boundary set

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