EP0932504A1

EP0932504A1 - Inkjet print head for producing variable volume droplets of ink

Info

Publication number: EP0932504A1
Application number: EP97939472A
Authority: EP
Inventors: Mats G. Ottosson; Deane A. Gardner; Andreas Bibl; Herman A. Ferrier
Original assignee: Topaz Technologies Inc
Current assignee: Topaz Technologies Inc
Priority date: 1996-08-27
Filing date: 1997-08-20
Publication date: 1999-08-04
Also published as: AU4155097A; CA2264038A1; JP2000516872A; WO1998008687A1; TW365579B

Abstract

A drop on demand inkjet printer according to the present invention comprises a piezoelectric inkjet print head (20) having a transducer (4) mechanically coupled to an ink channel (16), wherein the electrical actuation of the ink channel transducer results in the expulsion of a drop of ink from an ink channel orifice (12). The volume of the expelled drop of ink can be selectively varied by controlling the number of electrical signal pulses utilized to drive the print head transducer. Changes in the drop speed of the expelled ink drops can be regulated by modifying the amplitude of the electrical signal pulses. The signal pulses are preferably operated near or at the resonant frequency of the ink channel. When expelling larger drops of ink, the amplitude of the initial electrical signal pulses decreases initially, but increases beginning with a predetermined signal pulse. The amplitude of the electrical signal pulses can be increased throughout a burst series as well. The present invention also relates to an apparatus and method to generate cancellation pulses to cancel residual pressure waves in an ink channel. The present invention also can utilize unipolar switches to expel ink droplets in selected channels.

Description

S P E C I F I C A T I O N TITLE OF THE INVENTION

INKJET PRINT HEAD FOR PRODUCING VARIABLE VOLUME DROPLETS OF INK CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending application Serial No. 08/703,974, filed August 27, 1996. This application is hereby incorporated by reference in its entirety. Background of the Invention 1. Field of the Invention

The present invention pertains to the field of inkjet printers, and more specifically, to drop-on-demand piezoelectric inkjet printers. 2. Description of Related Art

Drop-on-demand inkjet printers having piezoelectric components are well known in the art. In general, piezoelectric drop-on-demand inkjet printers are constructed with a piezoelectric transducer component which reacts to the application of an electrical signal with a mechanical movement or distortion, such that a drop of ink is expelled from a print head ink channel or cavity that is in mechanical communication with the transducer component . Prior attempts to expel variable volume drops of ink from known inkjet print head apparatuses have employed the use of an array of print head channels, the outputs of which are selectively combined to generate a larger, variable volume drop of ink. However, this method of printing typically requires a bulky print head apparatus since a plurality of separate ink channels is required to generate a single ink drop. Other attempts to generate variable volume ink drops have focussed on methods to change the amplitude or shape of an electrical drive signal which is applied to the inkjet printer transducers; however, while these methods produce variable volume ink drops, they also generally resulted in inkjet systems where the drop velocities of the expelled ink drops are not consistent from one ink drop to the next. This results in potential printing problems because variations in expelled drop velocities result in drop placement errors on the print medium, degrading output print quality.

Therefore, there is a need for an inkjet print head which can be operated to expel variable volume ink drops from a single ink channel, but which can also be operated such that the variable volume ink drops are expelled at substantially the same drop velocity.

Summary of the Invention A drop on demand inkjet print head apparatus according to the present invention comprises a piezoelectric inkjet print head having a transducer mechanically coupled to an ink channel, wherein the electrical actuation of the ink channel transducer results in the expulsion of a drop of ink from an ink channel orifice. The volume of the expelled drop of ink can be selectively varied by controlling the number of electrical signal pulses utilized to drive the print head transducer. Generally, the more pulses employed to expel a single drop of ink, the greater the volume of the expelled ink drop.

One aspect of the present invention relates to the modification of the amplitude of successive electrical drive signal to compensate for the tendency of the print head to expel ink drops at increasing drop speeds when using multiple pulses to expel a single drop of ink. For this aspect of the present invention, the drive signal comprises a "burst" series of electrical pulses having amplitudes which decrease as a function of an increase in the number of pulses within each burst series. One embodiment comprises decreasing amplitudes for the pulses within a burst series, such that different pulses within a single burst series have different amplitudes. Another embodiment comprises a burst series in which the amplitude of each pulse within the burst series is set at a uniform amplitude, but the overall amplitude of the burst series de- creases in proportion to the number of pulses in that burst series.

Another aspect of the present invention relates to formation of relatively large ink drops. Relatively large ink drops are created by successively reducing the ampli- tude of the initial signal pulses within a burst series and then successively increasing the amplitude of the later occurring signal pulses.

Another aspect of the present invention relates to the operation of the inkjet printer apparatus at approximately the resonant frequency of the ink channel. The preferred frequency time factor (i.e., period) of an applied electrical drive signal is near or at 4L/C, where L equals the length of the ink channel , and C equals the speed of sound of ink contained in the ink- filled ink channel. If a bipolar sinusoidal waveform is employed as the electrical drive signal, then the period of each positive or negative component of the drive signal waveform is preferably set at 2L/C.

Another aspect of the present invention relates to the use of a switch array comprising an array of unipolar switches to control the application of electrical drive signals to an array of ink channel transducers. In this embodiment of the present invention, the electrical drive signals have distinct positive and negative components, and the positive component by itself is not enough to expel a drop of ink from the print head ink channel. However, the combined energy of both the positive and negative components is sufficient to expel an ink drop. Thus, an array of unipolar switches can be used to selectively block only the negative component of the drive signal of selected ink channels to effectively control the firing of certain channels within an array of print head channels. The polarities of the electrical drive signals described in connection with this aspect of the present invention can be reversed to the same effect.

Yet another aspect of the present invention relates to an apparatus and method to generate "cancellation pulses." Typically, firing a given ink channel to expel a drop of ink results in the presence of residual pressure waves which reflect within the ink channel, even after the drop of ink has been fully expelled. These residual pressure waves may interfere, constructively or destructively, with the firing of the next drop of ink from that same ink channel, e.g., by influencing the drop velocity of the next ink drop. in the present invention, cancellation pulses are generated at the appropriate amplitude and phase to create pressures waves to cancel and counter the effects of the residual pressure waves within the ink channels. These and other aspects of the present invention are described more fully in following specification and illustrated in the accompanying drawing figures.

Brief Description of the Drawings Fig. 1 depicts a cross-sectional side view of a single channel of an inkjet print head.

Fig. 2 is a cross-sectional side view of an inkjet print head for a single ink channel according to a preferred embodiment of the present invention.

Fig. 3 is a partial perspective view of the inkjet print head of Fig. 2.

Fig. 4 is a diagram of an embodiment of a sinusoidal multi-pulse drive signal according to the present invention. Figs. 5A-E depict the expulsion of a multi-pulse ink drop corresponding to the sinusoidal waveform of Fig. 4.

Figs. 6-8 depict alternate waveform shapes useful in the present invention. Fig. 9 is a plot illustrating the change in ink drop volume relative to a change in the number of pulses per burst series for an embodiment of the present invention.

Fig. 10 is a plot illustrating the change in ink drop speed relative to a change in the number of pulses per burst series for an embodiment of the present invention.

Fig. 11 is a plot illustrating the change in ink drop speed relative to a change in the amplitude of a fired burst series for an embodiment of the present invention.

Fig. 12 depicts a progression of sinusoidal burst series signals with varying burst series amplitudes according to an embodiment of the present invention.

Fig. 13 depicts a progression of sinusoidal burst series signals having varying pulse amplitudes within the burst series according to an embodiment of the present invention.

Fig. 14 depicts a functional block diagram of a signal generator according to an embodiment of the present invention.

Figs. 15 and 16 depict sinusoidal burst series drive signals according to alternate embodiments of the present invention.

Fig. 17 depicts an alternate embodiment of a waveform generator in which cancellation pulses are generated.

Fig. 18 depicts a sinusoidal burst series signal with varying burst series amplitudes according to an embodiment of the present invention.

Figs. 19 and 20 depict sinusoidal burst series drive signals with varying burst series amplitudes according to alternate embodiments of the present invention. Fig. 21 depicts a sinusoidal burst series signal with varying burst series amplitudes according to an embodiment of the present invention.

Figs. 22 and 23 depict sinusoidal burst series drive signals with varying burst series amplitudes according to alternate embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is diagrammatic representation of the principal components of a single channel of a drop-on-demand piezo- electric inkjet print head structure 20 according to the present invention. In the embodiment of Fig. 1, print head structure 20 comprises an ink channel 16 which is supplied with ink from an ink reservoir 10. A nozzle plate 14 comprising an orifice 12 is disposed along one end of the ink channel 16. A transducer 4 is in mechanical communication with the ink channel 16, and may define part of the inner wall or inner surface area of the ink channel 16. The transducer 4 is typically formed of a piezoelectric material, such as PZT, which responds to the application of an electrical signal with a mechanical distortion of the transducer material. This mechanical distortion causes a change in the positioning and/or dimensions of the transducer material, thereby resulting in a change of the total volume of the ink channel 16. In operation, the applica- tion of a voltage potential across transducer 4 creates volume changes within the ink channel 16 which, in turn, causes the expulsion of ink drops through orifice 12 in nozzle plate 14. A signal generator 6 is employed to generate electrical drive signals to excite the transducer material via two or more electrodes 8. A typical inkjet print head having multiple ink channels may comprise an array of print heads structures 20. The structure of the preferred piezoelectric inkjet print head apparatus useful in conjunction with the present invention is described in more detail in copending U.S. application serial no. (N/A) , Lyon & Lyon Docket No. 220/202, entitled "Inkjet Print Head Apparatus", which is being filed concurrently with the present application, and the details of which are hereby incorporated by reference as if fully set forth herein. The following detailed description with reference to Figs. 2 and 3, sets forth the major features of the inkjet print head apparatus described in that copending application. However, the present invention is capable of operation with a broad range of other piezoelectric inkjet print head structures, and thus, the particular inkjet print head described in connection with Figs. 2 and 3 is presented for illustrative purposes only, and is not intended to be limiting in any way.

Fig. 2 is a cross-sectional side view of a single channel of piezoelectric inkjet print head structure 20 constructed in accordance with one embodiment of the pres- ent invention. Print head structure 20 comprises an ink channel 29 which is supplied with ink from an ink reservoir 10 through an ink passageway 47 in rear cover plate 48. In operation, print head structure 20 expels ink from ink channel 29 though an orifice 38 in nozzle plate 33. Refer- ring to Fig. 3, more detail can be seen of the preferred structure of print head 20. Print head transducer 2 comprises a first wall portion 32, a second wall portion 34, and a base portion 36. The upper surfaces of the first and second wall portions 32 and 34 define a first face 7 of the print head transducer 2, and the lower surface of the base portion 36 defines a second, opposite face 9 of the print head transducer 2. Ink channel 29 is defined on three sides by the inner surface of the base portion 36 and the inner wall surfaces of the wall portions 32 and 34, and is an elongated channel cut into the piezoelectric material of the print head transducer 2 , leaving a lengthwise opening along the first face 7 of the print head transducer 2. A first metallization layer substantially coats the inner surfaces of ink channel 29 and is also deposited along the first face 7 of the print head transducer 2. This first metallization layer forms a common electrode 24 for the print head structure 20, and is preferably connected to ground. An ink channel cover 31 is bonded over the first face 7 of the print head transducer 2, closing off the lengthwise opening in the ink channel 29. A second metallization layer coats the outer surfaces of the base portion 36, and also extends approximately halfway up each of the outer surfaces of the first and second wall portions 32 and 34. This second metallization layer forms the addressable electrode 22. The poling direction (i.e., the overall polarization direction) of the piezoelectric material forming print head transducer 2 preferably lies substantially in the direction shown by arrow 30 in Fig. 3. As disclosed in more detail in copending application serial no. (N/A) , Lyon & Lyon Docket No. 220/202, this poling direction provides for the inkjet print head structure 20 to be actuated in both the normal mode and shear mode upon the generation of a voltage difference between the first and second metallization layers (e.g., when an electrical drive signal is applied to the addressable electrode 22) .

The piezoelectric inkjet print head 20 operates by the application of an electrical drive signal from a signal generator 6 to the piezoelectric material of print head transducer 2. The application of this electrical drive signal produces a dimensional and/or positional distortion of the piezoelectric material of the print head transducer 2, resulting in a change in the interior volume of the ink channel 29. This change of volume within the ink channel 29 generates an acoustic pressure wave within the ink channel 29, and the movement of the pressure wave within the ink channel 29 provides energy to expel ink from the ink channel 29 onto a print medium via orifice 38. Of particular importance to the operation of the inkjet print head 20, and to the creation of these acoustic pressure waves within the ink channel 29, are the particular parameters of the electrical drive signal applied to the piezoelectric material of the print head 20. For example, manipulating the parameters of an applied electrical drive signal (e.g., the amplitude, frequency, and/or shape of the applied electrical waveform) significantly affects the characteristics of the acoustic pressure wave(s) acting within the ink channel 29, which in turn affects the size, volume, shape, speed, and/or quality of the ink drop expelled from the print head 20.

The present specification describes and teaches the use of an electrical drive signal having a numerically selectable series of individual pulses to generate a vari- able volume ink drop from an inkjet print head. This can be best described by way of example, as shown in Figs. 4 and 5A-E. In Fig. 4, there is shown an example of a electrical drive signal having one or more individual signal pulses which may be applied to the electrodes of an inkjet print head transducer according to the present invention, and this type of electrical signal is hereafter referred to as a "burst series". The particular burst series shown in Fig. 4 is a sinusoidal waveform comprising a series of four individual sine-wave electrical signal pulses 72, 74, 76, and 78 ("bursts"). Each of the individual signal pulses or bursts 72, 74, 76, and 78 comprises both the negative and positive components of a sinusoidal signal. To better explain the operation of the burst series of Fig. 4, the effects of the individual signal pulses 72, 74, 76, and 78 upon an inkjet print head will now be explained in detail.

In the first individual signal pulse 72 shown in Fig. 4, the negative portion 72a of the sinusoidal waveform begins the process of expelling a droplet of ink from an inkjet print head. The negative portion 72a of the first signal pulse 72, when applied to the addressable electrode 22 of the preferred print head 20 (Fig 3) , moves the base portion 36 and the wall portions 32 and 34 of the print head transducer 2 outwardly to expand the volume of the ink channel 29, creating an underpressure in the ink channel 29, which generates an acoustic pressure wave that reverberates and reflects within the ink channel 29. The following positive portion 72b of the electrical waveform of signal pulse 72 deflects the base portion 36 and wall portions 32 and 34 of the print head transducer 2 in the opposite direction (inwardly into the ink channel 29) , reducing the volume of the ink channel 29, which generates another pressure wave within the ink channel 29. The characteristics of the waveform of signal pulse 72 are preferably selected and timed such that the energy and movement of the pressure wave(s) from the negative portion 72a of the signal pulse 72 are substantially synchronized with the energy and movement of the pressure wave(s) created by the positive portion 72b of signal pulse 72, such that the substantially combined energy of both the negative and positive portions of signal pulse 72 unite to expel a micro-droplet 60 of ink from the orifice 38 of print head 20.

Fig. 5A corresponds to a view of orifice 38 at time period TI of Fig. 4, and shows the expulsion of a single ink micro-droplet 60 from orifice 38 upon the application of a first electrical signal pulse 72. During time period T2 (Fig. 4) , a second signal pulse 74 is applied to the addressable electrode 22 of print head 20, and as shown in Fig. 5B, a second ink micro-droplet 62 is expelled following the application of signal pulse 74. As illustrated in Fig. 5B, because signal pulse 74 is applied immediately following the application of signal pulse 72, the subse- quently expelled ink micro-droplet 62 is typically still attached to the preceding micro-droplet 60 by a thin segment of ink. Fig. 5C depicts the orifice 38 at time period T3 , after the application of a third electrical signal pulse 76. As is also shown in Fig. 5C, a third ink micro- droplet 64 is expelled as a result of the third signal pulse 76, and this third micro-droplet 64 may also be connected to the preceding micro-droplet 62 by a thin segment of ink. By this time period, the surface tension of the ink may have already begun the process of drawing the first two micro-droplets 60 and 62 together into a single macro-droplet of ink. At time period T4 , Fig. 5D shows the result of the application of a fourth electrical signal pulse 78, which causes the expulsion of a fourth micro-droplet 66 from the orifice 38. As before, micro- droplet 68 may still be connected to the preceding micro- droplet by a thin segment of ink. By this time period, the first three micro-droplets may have already begun coalescing into a single macro-droplet of ink. Fig. 5E depicts the expelled ink macro-droplet 70 at a later period in time, when the surface tension of the ink has pulled all the separate ink micro-droplets 60, 62, 64, and 66 for this particular burst series together, so that the micro-droplets have merged in-flight into a generally spherical ink macro-droplet 70 prior to impact upon a print medium. The micro-droplets may also merge at time of impact upon the print medium.

In general, by repeatedly applying these electrical signal pulses in a "burst series" of such signal pulses, an ink drop can be expelled having a cross-sectional diameter larger than the diameter of the print head orifice 38 from which the ink drop is expelled. The size of the ink macro- droplets is dependent upon the number of electrical signal pulses in a burst series, with the size of the macro-drop- let expelled generally increasing as the number of pulses in a burst series increases. Although the example of Figs. 4 and 5A-E show a burst series consisting of only four individual signal pulses, the number of pulses within a burst series can be significantly higher. The principal limitation as to the number of signal pulses which can be within a single burst series is that, at some point in time, the series of pulses should be discontinued to allow the next burst series to begin firing, in order to maintain the proper spacing between the ink drops to be printed upon the print medium. Fig. 9 graphically displays the results of experiments conducted with a preferred inkjet print head 20, showing the increase in volume of an expelled ink drop as the number of electrical signal pulses per burst series increases. The test data shown in Fig. 9 are for the purposes of illustration only, and the actual volume increases may differ if an inkjet printer having other configurations and/or dimensions is employed, or if different parameters for the electrical drive signal are applied to the inkjet print head. The present invention is preferably practiced in a resonant mode of operation, wherein the frequency of the pulses of an electrical drive signal is near or at the resonant frequency of the print head ink channel 29. Employing a resonant frequency time component (i.e., a period corresponding to the resonant frequency) for the electrical drive signal allows the inherent resonance of the ink channel 29 to assist in the expulsion of ink from the print head, since residual energy from pressure waves generated in the ink channel 29 by earlier signal pulses will combine with the energy of pressure waves from one or more later signal pulse to expel ink drops. In the preferred embodiment of the present invention, the expelled ink drops are produced by generating an energizing electri- cal drive signal having major Fourier components near the ink channel's resonant frequency, where the ink channel resonant frequency is preferably calculated to include the effect of having ink contained within the ink channel. If a sinusoidal electrical drive signal is employed, such as the waveform depicted in Fig. 4, then the period of each signal pulse is preferably approximately 4L/C; thus, the width for each of the positive or negative components for each signal pulse (i.e., one-half period of each signal pulse) is approximately 2L/C, where L equals the length of the ink channel and C equals the velocity of sound of ink contained in the ink channel .

Although the preferred embodiment employs a period of 4L/C, other frequency time components for the signal pulses in each burst series are expressly contemplated within the scope of the present invention, and the actual frequency to be employed is dependent upon the particular application for which the invention is utilized, and upon the specific system and drive signal parameters to be used. For example, drive signals may be utilized having frequencies near the resonant frequency of the ink channel, e.g., having a period within 0-10% of 4L/C. Alternative embodiments within the scope of the present invention may employ drive signals having a period which is also near the resonant frequency of the ink channel, but which varies by more than 10% from the resonant frequency. In addition, the frequency of each burst series may be selectively varied based upon the number of pulses within the burst series. Thus, an alternate embodiment comprises the use of a frequency time component which is less than or is at 4L/C for a burst series having a greater number bursts, but which has a period that increases up to or is greater than 4L/C for a burst series having a lesser number of bursts, with a single-pulse burst series having the largest frequency time component .

One effect of applying multiple pulses to eject a drop of ink is that the velocity of the expelled ink drops tends to increase as the number of pulses within a burst series is increased. Fig. 10 graphically depicts the experimental results with a presently preferred inkjet print head structure 20, where it has been confirmed that in a piezoelectric resonating inkjet printer, the drop speed of an ink drop formed with fewer pulses is more likely to be slower than the drop speed of an ink drop formed with a greater number of pulses. In part, this results from a residual build-up of energy in the ink channel 29 from the multiple number of signal pulses in a burst series. As discussed above, each individual signal pulse generates pressure waves in the ink channel which act to expel a micro-droplet of ink from the ink channel. However, residual energy from each induced pressure wave may remain in the ink channel to add to the pressure wave induced by the next applied burst pulse, which then produces a succeeding ink micro-droplet that is somewhat faster and larger than the preceding ink micro-droplet. Essentially, the sizes of the following sub-droplets are increased as a result of the echo effect of the resonance left over in the channel from the previous signal pulses. In addition, the first micro-droplet that is ejected typically has a slower overall velocity than the ink micro-droplets that follow. This is partially because the energy in the first sub-droplet is partially dissipated in breaking the surface tension of the ink meniscus at the orifice of the ink channel 29. Thus, the net velocity of a combined macro-droplet will be greater for an applied drive signal having more bursts than an applied signal having less bursts .

Visible printing imperfections may occur if the print head expels drops of ink at inconsistent drop speeds, since the inkjet print head typically traverses across a print medium at a substantially constant speed. This contributes to a degradation in the quality of the resulting print, since the ink drops may not uniformly line up or be uniformly spaced on the print medium if the print head is moving at a constant speed while the ink drops are being expelled at different speeds. Therefore, although the use of multiple signal pulses within a burst series allows the expulsion of a variable volume ink drop from a print head, the use of multiple pulses may also create variations in the drop speed, which affects the final quality of an image printed onto a print medium.

The present invention overcomes this problem, by providing a method and apparatus for varying the volume of an expelled ink drop, while simultaneously maintaining a controlled, constant drop speed for the expelled ink drop. This is accomplished by using the principle that decreasing the amplitude of the electrical signal applied to a transducer of a print head to expel a drop of ink results in a decrease of the drop velocity of the expelled ink drop. By varying the amplitude of the electrical drive signal, in conjunction with the use of an electrical drive signal having multiple-pulses per burst series, the volume of the expelled ink drop can be increased while maintaining a substantially constant drop speed. For the preferred embodiment of the present invention, when utilizing the inkjet print head 20 of Figs. 2 and 3, substantially constant drop speeds for expelled variable-size ink drop is produced by varying the amplitude of signal pulses in accordance with the parameters shown in Fig. 11. For example, as indicated in Fig. 11, the amplitude of a four- pulse burst series used to expel an ink drop should be set at approximately 80% of the amplitude of a single-pulse ink drop in order to maintain a constant drop speed. The plot depicted in Fig. 11 is for the purpose of illustration only as it was derived particularly to be used in conjunction with the above-described preferred inkjet print head 20, and is not intended to be limiting in any way, since amplitude compensation levels will necessarily vary depending upon the particular application in which the present invention is utilized, and may also vary depending upon many other conditions, some of which may include, for example, the actual dimensions and structure of the inkjet print head employed, the material used to construct the print head, the shape of the print head, the type, frequency, and amplitudes of the electrical input waveform employed, and the characteristics of the ink employed.

Referring to Fig. 12, shown is an illustrative embodiment of electrical signal waveforms which can be employed to generate variable volume ink drops having substantially constant drop speed. This figure shows varying amplitude burst series signals which may be used to maintain the constant drop speed of an expelled ink drop, wherein the amplitude of each individual pulse within a particular burst series is the same, but the overall amplitude of the burst series varies depending upon the number of pulses within each burst series. As shown in Fig. 12, the amplitude of a single-pulse burst series waveform 100 is at a height of Al, which produces an ink drop at drop velocity V. To maintain a substantially constant drop speed, the amplitude for each pulse in a two-pulse burst series 102 is set at amplitude A2 , which is preferably smaller than amplitude Al, so that the two-pulse burst series 102 expels an ink drop at substantially the same velocity V as the single-pulse burst series 100. The amplitude A3 for three- pulse burst series 104 is smaller than the amplitude A2 for the two-pulse burst series 102, and the amplitude A4 for a four-pulse burst series 106 is even smaller than the ampli- tude A3 for the three-pulse burst series 104. The amplitudes shown in Fig. 12 are for the purposes of illustration only, to allow a pictorial representation of the types of amplitude changes which may be utilized in the present invention. To maintain substantially constant drop speed, the preferred embodiment employs the amplitude changes shown in Fig. 11, as applied to the preferred inkjet print head 20 described further in connection with Figs. 2 and 3. Thus, as indicated in Fig. 11, in the preferred embodiment, the amplitude of a four-pulse burst series is set at ap- proximately 80% the amplitude of a single-pulse burst series to maintain the substantially constant drop speed of the expelled ink drops.

Referring to Fig. 13, an alternate embodiment of the present invention is shown which can be utilized to gener- ate variable volume ink drops having a substantially constant drop velocity. In contrast to the embodiment of Fig. 12, the embodiment shown in Fig. 13 makes use of signal pulses having varying amplitudes within a single burst series to maintain the substantially constant drop velocity of expelled ink drops. As shown in Fig. 13, the amplitude of a single-pulse burst series 110 is at a height of Bl, which produces an ink drop at drop velocity V. In this embodiment, to cause a two-pulse burst series to expel a drop of ink at substantially the same drop velocity V as a single-pulse burst series, a two-pulse burst series 112 preferably has a first pulse 117 which is also set at an amplitude of Bl, but the second pulse 118 of burst series 112 is preferably set at an amplitude of B2, with amplitude B2 being smaller than amplitude Bl. It has been experimen- tally confirmed that in the present invention, the proper selection of decreasing amplitudes for the successive pulses of a multi-pulse burst series will compensate for the tendency of an expelled ink drop to increase its drop velocity when the number of pulses is increased. Thus, for a three-pulse burst series 114 in this embodiment, the first pulse 119 will again be set at an amplitude Bl, and the successive two pulses 120 and 121 will be set at amplitudes of B2 and B3 , respectively, with the amplitude of the successive pulses decreasing as necessary to maintain an expelled ink drop at the consistent drop velocity V. For a four-pulse burst series, or even more pulses per burst series, the same decreasing amplitude pattern is repeated to generate a multi-pulse ink drop with substantially the same drop velocity V. The amplitudes shown in Fig. 13 are for the purposes of illustration, to allow a pictorial representation of the types of amplitude changes which are preferably made to utilize this aspect of the present invention. Another embodiment comprises burst series waveforms in which one or more of the individual signal pulses have the same amplitude, but other signal pulses within that same burst series have different amplitudes to maintain constant drop velocities of the expelled ink drop. This method employs burst series waveforms which are a variation of those shown in Figs. 12 and 13, and can also be utilized to establish constant ink drop velocities.

While the above described burst series waveforms can generate variable volume ink drops having substantially constant drop speed, when producing larger ink drops, for example, ink drops created by more than four pulses in a burst series, these burst series waveforms can provide unsatisfactory print quality. The reason for this is that after approximately the third pulse in a burst series, the resonance energy built in the print head ink channel 29 peaks at its maximum level. Once the resonance energy in print head ink channel 29 peaks, the velocity of the expelled ink micro-droplet will also peak. This can cause the later-expelled ink micro-droplets to fail to combine with the larger ink macro-droplet created by the earlier pulses of the burst series. Furthermore, the earlier expelled ink micro-droplets have traveled a substantial distance to the print medium before the later expelled ink micro-droplets have even been expelled. When this happens, the trailing ink micro-droplets might not strike the print medium at the same location as the larger ink macro-droplet (which is comprised of the earlier expelled ink micro- droplets) . Instead, the later expelled ink micro-droplets could strike the print medium at a location adjacent to where the larger ink macro-droplet struck the medium. This is highly undesirable, as it can lead to poor print quality.

It is therefore desirable to vary the amplitude of the pulses in the burst series so as to maintain the same time of flight to the print medium of the aggregate drop, i.e., the macro-droplet, for all ink drops, regardless of how many micro-droplets comprise the macro-droplet . Thus, in a preferred embodiment, the amplitude of the later pulses in a burst series is increased in order to increase the resonance energy in the print head ink channel 29. The increased resonance energy in the ink channel 29 will increase the velocity of the later expelled ink micro-droplets, thereby allowing those later expelled ink drops to either remain attached to the larger ink macro-droplet formed by the previously expelled micro-droplets or catch up to the earlier expelled ink drops. If the later expelled ink micro-droplets cannot catch up to the ink macro- droplet created by the earlier expelled ink micro-droplets, the increased velocity should allow them to impact the print medium at substantially the same location as the ink macro-droplet created by the earlier expelled micro-droplets. In preferred embodiments of the present invention, the amplitude of the signal pulses are progressively reduced until reaching a predetermined signal pulse in a burst series . Beginning with the predetermined signal pulse, the amplitude of the signal pulses of the burst series is then progressively increased until the burst series is completed.

In the presently preferred embodiment, the predetermined number of signal pulses is four. Thus, in the presently preferred embodiment, the amplitude of the first three signal pulses is progressively reduced. Then, begin- ning with the fourth signal pulse, the amplitude of the signal pulses is progressively increased. An exemplary seven-pulse burst series 180 of the presently preferred embodiment is shown in Fig. 18. In the embodiment of Fig. 18, the amplitude of the earlier occurring signal pulses in the burst series 180 is progressively reduced to maintain the substantially constant drop velocity of the initially expelled ink drops. The amplitude of the first pulse 182 of the seven-pulse burst series 180 has a height of El, which produces an ink droplet at drop velocity V. The second pulse 184 of burst series 180 is preferably set at an amplitude of E2 , with amplitude E2 being smaller than amplitude El. The third pulse 186 of burst series 180 is preferably set at an amplitude of E3, with amplitude E3 being smaller than amplitude E2. Beginning at the fourth pulse 188 of burst series 180, the amplitude is gradually increased to increase the velocity of the later expelled ink micro-droplets. Thus, the amplitude of the fourth pulse 188 of burst series 180 is preferably set at an amplitude of E4 , with amplitude E4 being larger than amplitude E3. The amplitude of the fifth pulse 190 of burst series 180 is preferably set to amplitude E5, with amplitude E5 being larger than amplitude E4. The sixth pulse 192 of burst series 180 is preferably set at an amplitude of E6 , with amplitude E6 being larger than amplitude E5. Finally, the seventh pulse 194 of burst series 180 is preferably set at an amplitude of E7 , with amplitude E7 being larger than amplitude E6.

The amplitudes shown in Fig. 18 are for the purposes of illustration only to allow a pictorial representation of the types of amplitude changes which are preferably made to utilize this aspect of the present invention. Moreover, the number of signal pulses that are decreased in amplitude prior to increasing is dependent upon several factors, including the type and viscosity of ink used, environmental factors such as temperature and humidity, and the type of print head used. As discussed the presently preferred predetermined number of signal pulses is four. However, using the teachings of this invention, one could vary the number signal pulses having progressively decreasing amplitudes to ensure high quality printing.

Another preferred method of varying the amplitude of the pulses in the burst series to maintain the same time of flight to the print medium of larger aggregate drops, i.e., the macro-droplets, for all ink drops is shown in Fig. 21. In this preferred embodiment, the amplitude of all the pulses in a burst series is progressively increased throughout the burst series. This is done to increase the resonance energy in the print head ink channel 29. Just as in the embodiment of Fig. 18, this method allows the later expelled ink drops to either remain attached to the larger ink macro-droplet formed by the previously expelled micro- droplets or catch up to the earlier expelled ink drops. If the later expelled ink micro-droplets cannot catch up to the ink macro-droplet created by the earlier expelled ink micro-droplets, the increased velocity should allow them to impact the print medium at substantially the same location as the ink macro-droplet created by the earlier expelled micro-droplets.

In the embodiment of Fig. 21, the amplitude of the first pulse 202 of the seven-pulse burst series 180 has a height of HI, which produces an ink droplet at drop velocity V. The second pulse 204 of burst series 200 is prefera- bly set at an amplitude of H2 , with amplitude H2 being larger than amplitude Hi. This produces an ink droplet at a drop velocity slightly higher than V. The third pulse 206 of burst series 200 is preferably set at an amplitude of H3 , with amplitude H3 being larger than amplitude H2. This produces an ink droplet at a drop velocity slightly higher than that produced by second pulse 204. The fourth pulse 208 of burst series 200 is preferably set at an amplitude of H4, with amplitude H4 being larger than amplitude H3. This produces an ink droplet at a drop velocity slightly higher than that produced by third pulse 206.

Finally, the fifth pulse 210 of burst series 200 is preferably set at an amplitude of H5, with amplitude H5 being larger than amplitude H4. This produces an ink droplet at a drop velocity slightly higher than that produced by fourth pulse 208.

The amplitudes shown in Fig. 21 are for the purposes of illustration only to allow a pictorial representation of the types of amplitude changes which are preferably made to utilize this aspect of the present invention. Moreover, the number of signal pulses that are decreased in amplitude prior to increasing is dependent upon several factors, including the type and viscosity of ink used, environmental factors such as temperature and humidity, and the type of print head used. As discussed the presently preferred predetermined number of signal pulses is four. However, using the teachings of this invention, one could vary the number signal pulses having progressively decreasing amplitudes to ensure high quality printing. Further, depending upon these factors, it is possible that no pulses with decreased amplitude will be necessary.

The specific amplitude levels described and depicted in connection with Figs. 11, 12, 13, 18 and 21 are not intended to be limiting in any way as to the scope of the present invention, since it is contemplated that this invention may be employed with other types of piezoelectric inkjet print heads or other types of electrical drive signals, and such other applications may employ the same inventive principles taught herein to utilize amplitude levels that are differ- ent from that described and shown in connection with Figs. 11-13, 18 and 21. For example, some of the factors which may affect the use and/or selection of amplitude changes of the signal pulses or burst series in accordance with the present invention include the viscosity and properties of the particular ink type employed, the physical dimensions of the ink channel, the specific transducer material employed, the configuration of the transducer material, the shape and frequency of the electrical drive signal, and the size of the ink channel orifice. A decrease in the amplitude of an applied electrical signal, while resulting in decreased drop speed, may also decrease the drop volume of an expelled ink drop. However, the reduction in volume of an ink drop resulting from a decrease in signal pulse amplitude is typically less than the volume increase achieved by increasing the number of pulses in a multiple-pulse burst series waveform to create an increase in the volume of the expelled ink drop.

Although the above embodiments depict the utilization of a sinusoidal electrical input waveform, other waveform shapes may also be employed within the scope of the present invention. For example, a trapezoidal waveform, as shown in Fig. 6, can also be employed in the present invention. Other waveforms, such as half-sinusoidal pattern, or a square wave (Fig. 7) , or a triangular wave pattern (Fig. 8) may also be employed in the present invention. Of course, the choice of waveform shapes may change depending upon the particular configuration of the inkjet printer apparatus employed with the present invention or the application for which it is used. For these other waveform shapes, the period of the signal pulses may also be near or at the resonant mode for operation of the inkjet print head. For example, for the square wave shown in Fig. 7, the period of each signal pulse is preferably 4L/C. Fig. 14 is a functional block diagram showing the principal components of a preferred signal generator 6 useful in the present invention. Signal generator 6 generates and transmits the electrical drive signal which drives the transducer material 141 in the inkjet print head. The operational sequence of signal generator 6 begins with the application of a waveform control signal 130 to a burst series waveform generator 134 from an outside signal source 132, such as a print head controller 132 Waveform control signals 130 may also be sent from an external encoder or microprocessor, which outputs control signals linked to the motion of the print head, so that the expelled ink drops are ejected with optimal timing to impact the print medium at the correct position. The waveform generator 134 produces burst series waveform 136, comprising one or more pulses per burst series, which is applied to an amplifier 138, which increases the amplitude of the burst series waveform 136 to an appropriate voltage level to drive the transducer 141 in the print head. The amplified burst waveform 139 from the amplifier 138 is connected to a switch array 140, a series of digitally controlled switches, which selectively controls which individual channels of the array of print head channels will be permitted to receive the actuating amplified burst waveform 139. The amplified burst waveform is then applied to selected channels of the print head transducer 141.

The preferred burst series waveform generator 134 comprises a lookup table controller 150 which directs the operation of a lookup table 152. Lookup table controller 150 receives waveform control signals 130 from an outside signal source 132 which provides control signals pertaining to the timing and waveform parameters of the burst series waveform to be generated by the signal generator 6. Some of the parameters which may be specified by the waveform control signal 130 include the frequency of the waveform, the number of pulses within a burst series, and the shape of the waveform.

Lookup table 152 is programmed with a table of waveform-data points which define electrical burst series waveforms that can be utilized to drive the print head transducers 141. In the preferred embodiment, the burst series waveform is defined by a series of (x, y) coordinate points, where the x-coordinate represents a point in time on a burst series waveform plot, and the y-coordinate represents the voltage or amplitude of the waveform at that particular x-coordinate. Each individual waveform-data point in the lookup table 152 corresponds to an (x, y) coordinate location on a desired waveform. For each burst series, the lookup table controller 150 outputs a stream of time-factor coordinates (x-coordinates) to the lookup table 152, and the lookup table 152 in turn outputs a series of amplitude values (y-coordinates) corresponding to each x- coordinate. The lookup table controller 150 contains a counter which increments through the lookup table's input range at a rate determined by the specific contents of the waveform control signal 130. The waveform-data points from the lookup table 152 are applied in sequence to a digital/analog converter ("DAC") 154, which outputs an analog burst series waveform 136 that corresponds to the series of (x, y) coordinate points, and which is a low power version of the signal that will be applied to transducer 141 of the print head. Other DACs may also receive control inputs signals which partially determine several parameters of the burst series waveform 136. For example, a separate DAC may be employed to control the overall amplitude of the output burst series waveform.

The function of the switch array 140 is to selectively allow the amplified burst series waveform to fire only certain ink channels of the print head 141. In the preferred embodiment, switch array 140 generally comprises an array of switches, e.g., field effect transistors, which are controlled to selectively allow the amplified burst wavefor (s) to pass only to certain ink channels of the print head 141. The preferred embodiment employs an opto- isolator 144 to apply switch control signals 146 from the print head controller 132 to the switch array 142. The print head controller 132 provides switch control signals 146 that control whether a given channel of the print head is or is not printing at a given point in time as the print head 141 is moved across the print medium.

In one embodiment of the present invention, switch array 140 comprises an array of bi-directional switches, with each individual ink channel within the print head having a corresponding switch within the switch array 140. This type of switch array allows the inkjet print head to operate with the waveforms characterized by Figs. 12, 13, 18 and 21, wherein the transmission of both the positive and negative half portions of each signal pulse to the array of inkjet channel transducers can be selectively controlled by the switch array 140, thereby controlling which individual ink channel is allowed to fire a drop of ink. However, bi-directional switches typically cost more than equivalent unipolar switches, and the greater manufacturing cost resulting from using bi-directional switches increases proportionally when a print head contains a greater number of ink channels that needs to be controlled. Thus, an alternate embodiment of the present invention comprises a switch array 140 which employs an array of unipolar power switches, where only the positive component or only the negative component of each pulse of a burst series waveform need to be controlled to operate an inkjet print head. In this alternate embodiment when applying a bipolar pulse waveform, the application of a first half pulse waveform to the transducer, which can be either the positive or negative component, generates a pressure wave inside the ink channel, which reflects and reverberates within the inner walls of the ink channel. The application of the second half pulse waveform, which is opposite in polarity to the first half pulse waveform one-half period later, creates another pressure wave which is timed to coincide with the reflected first pressure wave such that the combined energy of both pressure waves unite to expel a droplet of ink from the ink channel orifice. Because the energy of the two half pulse waveforms are combined to produce the desired expulsion of ink, the second half pulse waveform need not necessarily be as large as the first waveform to generate the desired droplet size. In fact, each half pulse waveform may be small enough in amplitude such that it will not, by itself, expel a drop of ink, but the combined energy of both half-pulse waveforms will be enough to expel a drop of ink. Thus, in this embodiment, the firing of a particular ink channel can be effectively controlled by selectively blocking the application of one- half of each pulse waveform to the transducer, while the other half pulse waveform of opposite polarity is allowed to be transmitted to each desired ink channel. Therefore, this embodiment allows the utilization of an array of unipolar switches in switch array 140, instead of a more costly array of bipolar switches.

The array of unipolar switches can be employed with the alternate electrical waveforms shown in Figs. 15, 19 and 22. Figs. 15, 19 and 22 correspond to a modification of the signal waveforms shown in Figs. 13, 18 and 22, wherein the amplitude of the positive portion of each pulse in the burst series is set at approximately one-half of the amplitude of the immediately preceding negative portion. In this embodiment, the positive portion of each pulse waveform of the burst series will always be applied to each ink channel of the print head; however, the positive portion is not enough, by itself, to generate the necessary energy in the ink channels to expel a drop of ink. Thus, the selective transmission of the negative portions of the pulse waveforms of the burst series will allow the firing of the particular ink channels to which both the positive and negative portions of the waveforms are applied. The amplitude of the positive portion of each pulse waveform is preferably set high enough such that a meaningful pressure wave contribution can be made within the ink channels toward the expulsion of ink drops from the ink channels, but the amplitude is set low enough that spurious or unintended expulsion of ink is prevented from occurring when only the positive portions of the pulse waveforms are transmitted to the ink channels.

The preferred signal generator described in more detail above in conjunction with Fig. 14 can be easily modified to implement this aspect of the present invention. The modi fied switch array 140 consists of an array of unipolar switches which can selectively switch the application of the negative portions of the amplified pulse waveforms 139 to the print head transducers 141, but would universally allow the transmission of all of the positive portions of the pulse waveform. The lookup table 152 is programmed with waveform-data coordinate points corresponding to a varying waveform, such as illustrated in Fig. 15, wherein the amplitude of the positive portion of each output pulse is set at approximately one-half the amplitude of the preceding negative portion, and this output is converted into the desired analog signal by D/A converter 154. Alternatively, the lookup table 152 outputs a digital sequence corresponding to a typical sinusoidal-like waveform wherein the positive and negative portions are at the same amplitude, but the D/A converter 154 is controlled to output the positive portions at one-half the amplitude of the negative portions.

This embodiment of the present invention can be alter- natively implemented with the described polarities reversed, wherein the negative portion of each pulse waveform is universally allowed to pass to the print head transducer, but the positive portions are selectively switched to fire only selected ink channels. In addition, the ampli- tude levels shown in Figs. 15, 19 and 22 are illustrative only, and numerous other variations upon this theme are expressly within the scope of the present invention. For example, Figs. 16, 20 and 23 depict alternative signal waveforms, where the positive portions of the pulse waveforms for each burst series are at a constant amplitude D5 (Fig. 16), G8 (Fig. 20), or J6 (Fig. 23) and are universally applied to the ink channel transducers, but the negative portions are selectively switched to fire only specific ink channels. In this embodiment, only the change in amplitude of the negative portions of the pulse waveforms is employed to maintain constant drop speed of the expelled ink drop.

Referring now to Fig. 17, another aspect of the present invention comprises the generation of "cancellation pulses" within the print head ink channels. One effect of operating a piezoelectrically actuated print head is that pressure waves induced in the ink channel may create residual pressure waves which reverberate within the ink channel, even after an ink drop is fired from the channel. Residual pressure waves within the ink channel may be even more pronounced when the print head functions in a resonance mode. Because the residual pressure waves may interfere, either constructively or destructively, with the precise operation of the print head for the immediately succeeding firing of the ink channel, it is typically necessary to wait a period of time after each droplet is ejected from an ink channel before the next firing of the ink channel to allow the residual energy from the previous pressure waves to dissipate. Thus, it is desirable to be able to generate cancellation pulses which can be used to substantially cancel the effects of the residual pressure wave energy that is present in the ink channels in order to permit more immediate ejection of successive ink droplets. The present invention comprises an apparatus and method for generating such cancellation pulses. Utilizing cancellation pulses results in improved operation of an inkjet printer, since the firing of a given ink channel will not generate residual pressure waves which interfere with a succeeding firing of that same ink channel. In addition, because a given ink channel does not have to be significantly rested between firings to permit residual pressure waves to dissipate, the number of pulses within a particular burst series can be extended as long as possible before the firing of the next burst series.

A cancellation pulse generated in accordance with the present invention, when applied to a print head transducer, generates counter-pressure waves which substantially cancel the effects of residual pressure waves present in the ink channels. To be effective, a cancellation pulse must produce a counter-pressure wave which is substantially at the same absolute energy level as the existing residual pressure wave in the ink channel, but which is out of phase when compared to the phase of the existing residual pressure wave. This counter-pressure wave is preferably generated by a calculated pulse burst of the same general type and shape as the signals pulses which are used to fire the ink channels, but the cancellation pulse is of a lesser amplitude and out of phase when compared to the prior pulses which created the residual pressure waves.

Fig. 17 shows an embodiment of a waveform generator 160 which can be utilized to practice the cancellation pulse aspect of the present invention. The waveform generator 160 contains a lookup table controller 162 which transmits control signals to both a burst series lookup table 164 and a cancellation pulse lookup table 166. Burst series lookup table 164, like the lookup table 152 of Fig. 14, is preferably programmed with a table of waveform coordinate points that define one or more burst series signal waveforms which can be utilized to drive the print head transducers 141 to controllably eject droplets of ink from the print head. Cancellation pulse lookup table 166 is preferably programmed with a table of waveform coordinate points which define a set of one or more cancellation pulse waveforms optimized for canceling residual pressure waves created from the waveforms programmed into the burst series lookup table 164. In the preferred embodiment, lookup table controller 162 comprises a counter which increments and outputs a set of time factor coordinates (x-coordinates) to the burst series lookup table 164. In response to the signal applied from the lookup table controller 162, the burst series lookup table 164 transmits a series of voltage amplitude coordinates (y-coordinates) , corresponding to each time factor x-coordinate, which together define a desired burst series waveform. This output is applied to a digital- analog converter ("DAC") 168 to generate an analog burst series waveform. The firing of a given burst series waveform sets and starts a timer 167, which is employed to track the total elapsed time since the firing of the burst series waveform. After a designated period of time, the timer 167 triggers the lookup table controller 162, which increments and outputs a set of cancellation pulse time factor coordinates (x-coordinates) to the cancellation pulse lookup table 166. The cancellation pulse lookup table 166 produces a series of voltage amplitudes (y-coor- dinates) , which correspond to each of the x-coordinates, which together define a waveform which is appropriate to generate counter-pressure waves to cancel the residual pressure waves in the ink channel created by the most recent burst series waveform. The output of the cancella- tion pulse lookup table 166 is then applied to the DAC 168 to produce an analog cancellation pulse signal waveform.

An alternate embodiment comprises the triggering of the cancellation pulse waveform immediately after the firing of the burst series waveform. In this embodiment, a timer 167 is not required, as the lookup table controller 162 is programmed to immediately begin outputting a series of control signals to the cancellation pulse lookup table 166 immediately following the transmission of control signals to the burst series lookup table 164. An alternate embodi- ment triggers the cancellation pulse after a fixed time delay following the end of the previous burst series waveform. Yet another embodiment employs different cancellation pulse waveform shapes depending upon the number of individual pulses in the main burst series waveform.

The cancellation pulse coordinate values within the cancellation pulse lookup table 166 can be generated in a plurality of ways. For example, the lookup table values can be derived empirically, by repeatedly firing a progres- sion of burst series pulses and corresponding "test" cancellation pulses through the transducer of a given ink channel. By incrementally adjusting the amplitude and phase parameters of the sample cancellation pulses which are fired, a set of appropriate cancellation pulse parame- ters can be plotted which correspond to the preprogrammed set of burst series pulses which are used to drive the inkjet print head. The preferred values of the cancellation pulses generally correspond to experimental firings wherein the burst series pulses can be fired at a higher effective repetition rate of firing without interference from prior firings. In general, the higher the effective firing repetition rate when using a particular cancellation pulse, then the more effective that cancellation pulse will be to cancel residual pressure waves in the ink channels. Alternatively, the lookup table values can be obtained by simulating the pressure wave effects resulting from the firing of a given burst series in an ink channel, and mathematically calculating the parameters of a cancellation pulse signal to generate appropriate counter-pressure waves. The simulation determines the amplitude and timing of a pulse which minimizes or cancels the residual pressure waves at a certain time period after the firing of a previous burst series. The character of the cancellation pulse required is related both to the amplitude and phase of the pressure waves present in the ink channel and to the particular geometry and characteristics of the ink channel. The cancellation pulse for the preferred embodiment is preferably either a half pulse, or one full pulse, of a sinusoidal wave displaced in time from the type of pulse in a burst series used to fire an ink drop. Various simulations may be performed to determine the optimal characteristics and parameters of the desired cancellation pulses. The effectiveness of these cancellation pulses can be experimentally confirmed, preferably in conjunction with the repetition rate method described above.

This aspect of the present invention is particularly advantageous when operated in conjunction with the multi- pulse aspect of the present invention, since by appropriate cancellation pulses, the pulses within an individual burst series can thus be fired until very shortly before the firing of the next burst series, without leaving residual pressure waves to interfere with the next firing. However, this aspect of the present invention can also be employed for numerous other applications also, including single pulse applications for example, when there is a need to fire many pulses near each other in time, without having to worry about interfering with the next firing of the ink channel . While embodiments, applications and advantages of the invention have been shown and described with sufficient clarity to enable one skilled in the art to make and use the invention, it will be equally apparent to those skilled in the art that many more embodiments, applications and advantages are possible without deviating from the inventive concepts disclosed and described herein. The invention therefore should only be restricted in accordance with the spirit of the claims appended hereto and to their equivalents, and is not to be restricted by the description of the preferred embodiments, the specification or the drawings.

Claims

What is Claimed is:

1. A piezoelectric printer apparatus for expelling variable volume ink drops : an ink channel; a transducer in mechanical communication with said ink channel ; and a signal generator selectively communicating an electrical drive signal to said transducer, said electrical drive signal comprising a series of one or more signal pulses, said signal pulses having a variable amplitude in relation to the number of said signal pulses in said series of one or more signal pulses.

2. The printer apparatus of claim 1 wherein each of said signal pulses has the same amplitude within said series of one or more signal pulses.

3. The printer apparatus of claim 1 wherein said signal pulses have a different amplitude within said series of one or more signal pulses.

4. The printer apparatus of claim 3 wherein said series of one or more signal pulses comprise more than one of said signal pulses of a first amplitude and one or more of said signal pulses of a second amplitude, said first amplitude different from said second amplitude.

5. The printer apparatus of claim 3 wherein said signal pulses of said series of one or more signal pulses progressively decreases in amplitude.

6. The printer apparatus of claim 3 wherein said signal pulses of said series of one or more signal pulses progressively increases in amplitude.

7. The printer apparatus of claim 1 wherein said signal pulses have a decreasing amplitude as a function of an increase in the number of said signal pulses in said series of one or more signal pulses.

8. The printer apparatus of claim 1 wherein said signal generator comprises a burst series waveform generator having a lookup table .

9. The printer apparatus of claim 8 wherein said lookup table comprises waveform coordinates defining selected burst series waveforms .

10. The printer apparatus of claim 1 wherein said electrical drive signal comprises a generally sinusoidal waveform.

11. The printer apparatus of claim 1 wherein said electri- cal drive signal comprises a positive polarity component and a negative polarity component .

12. The printer apparatus of claim 11 wherein said signal generator comprises a switch array, said switch array selectively switching only said negative polarity component of said electrical drive signal.

13. The printer apparatus of claim 11 wherein said signal generator comprises a switch array, said switch array selectively switching only said positive polarity component of said electrical drive signal.

14. The printer apparatus of claim 1 wherein said electrical drive signal is generated near or at a resonant frequency of said ink channel .

15. The printer apparatus of claim 14, wherein said signal pulses have a period within 0-10% of 4L/C, where L equals the length of said ink channel and C equals the speed of sound of ink contained in said ink channel .

16. The printer apparatus of claim 14 wherein said signal pulses have a period of 4L/C, where L equals the length of said ink channel and C equals the speed of sound of ink contained in said ink channel.

17. The printer apparatus of claim 16 wherein said signal pulses comprise a negative polarity portion and a positive polarity portion, each of said negative and said positive polarity porions having a period of 2L/C.

18. The printer apparatus of claim 1 wherein said signal generator further comprises a waveform generator which generates cancellation pressure waves.

19. The printer apparatus of claim 1 wherein said electri- cal drive signal comprises a trapezoidal waveform.

20. The printer apparatus of claim 1 wherein said electrical drive signal comprises a triangular waveform.

21. An inkjet printer apparatus comprising: a print head having an ink channel and a transducer in mechanical communication with said ink channel; said ink channel having an orifice through which an ink drop is expelled upon actuation of said transducer; a signal generator selectively applying an electrical drive signal to said transducer, said electrical drive signal having a series of one or more pulses; and means for selectively varying the amplitude of said electrical drive signal to expel a variable volume ink drop through said orifice.

22. The apparatus of claim 21 wherein said series of one or more pulses comprises signal pulses having the same amplitude.

23. The apparatus of claim 21 wherein said pulses have a decreasing amplitude as a function of an increase in the number of said pulses in said series of one or more pulses.

24. The apparatus of claim 21 wherein said pulses have a increasing amplitude as a function of an increase in the number of said pulses in said series of one or more pulses.

25. The apparatus of claim 21 wherein said means for selectively varying the amplitude of said electrical drive signal comprises a waveform generator having a lookup table.

26. The apparatus of claim 25 wherein said lookup table comprises a table of coordinate points corresponding to selected burst series waveforms.

27. The apparatus of claim 25 wherein said waveform generator further comprises a cancellation pulse lookup table, said cancellation pulse lookup table comprising coordinate data points corresponding to a cancellation pulse waveform for canceling residual pressure waves in said ink channel.

28. The apparatus of claim 21 wherein said electrical drive signal further comprises a substantially sinusoidal signal waveform.

29. A piezoelectric print head comprising: an ink channel, a transducer mechanically coupled to said inkjet channel, and means for receiving an electrical drive signal from a signal generator to expel variable volume ink drops; said transducer electrically coupled to said electrical drive signal; and said electrical drive signal comprising a burst series of one or more pulses, said pulses having amplitudes which are variable in relation to the number of said pulses in said series of one or more pulses.

30. The print head of claim 29 wherein each of said pulses in said burst series of one or more pulses has the same amplitude.

31. The print head of claim 29 wherein the frequency of said electrical drive signal is near or at a resonant frequency of said ink channel.

32. The print head of claim 29 wherein said pulses in said burst series of one or more pulses have different amplitudes .

33. The print head of claim 29 wherein said pulses have a variably decreasing amplitude as a function of an increase in the number of said pulses in said burst series of one or more pulses.

34. The print head of claim 33 wherein the amplitude of said pulses in said burst series of one or more pulses levels off asymtopically.

35. The print head of claim 29 wherein said pulses have a variably increasing amplitude as a function of an increase in the number of said pulses in said burst series of one or more pulses.

36. A method to operate an inkjet print head to expel variable volume ink drops comprising the steps of : generating a series of one or more electrical signal pulses for each ink drop to be expelled from an orifice of an inkjet print head channel, the volume of said ink drop varying as a function of the number of electrical signal pulses within said series of one or more electrical signal pulses; varying the amplitude of said electrical signal pulses in correlation to said number of electrical signal pulses within said series of one or more electrical signal pulses; and coupling said series of one or more electrical signal pulses to a print head transducer.

37. The method of claim 36 wherein said varying step comprises the step of decreasing the amplitude of said electrical signal pulses in correlation to an increase in said number of electrical signal pulses within said series of one or more electrical signal pulses.

38. The method of claim 37 wherein said decreasing step comprises the step of successively decreasing the amplitude of each of said electrical signal pulses within said series of one or more electrical signal pulses.

39. The method of claim 37 wherein said decreasing step comprises the step of uniformly decreasing the amplitude of all the electrical signal pulses within said series of one or more electrical signal pulses.

40. The method of claim 36 wherein said series of one or more electrical signal pulses is generated near or at a resonant frequency of said inkjet print head channel.

41. The method of claim 40 wherein each of said electrical signal pulses has a period of 4L/C, where L equals the length of said inkjet print head channel, and C equals the speed of sound of ink contained in said inkjet print head channel .

42. The method of claim 40 wherein each of said electrical signal pulses has a period within 0-10% of 4 L/C, where L equals the length of said inkjet print head channel, and C equals the speed of sound of ink contained in said inkjet print head channel .

43. The method of claim 36 wherein said electrical signal pulses are sinusoidal waveforms.

44. The method of claim 36 wherein each of said electrical signal pulses has a positive polarity component and a negative polarity component, said coupling step further comprising the steps of: uniformly applying said positive polarity component to said transducer; and selectively switching the application of said negative polarity component to said transducer.

45. The method of claim 36 wherein each of said electrical signal pulses has a positive polarity component and a negative polarity component, said coupling step further comprising the steps of: uniformly applying said negative polarity component to said transducer; and selectively switching the application of said positive polarity component to said transducer.

46. The method of claim 36 further comprising the steps of: generating a cancellation pulse; applying said cancellation pulse to said print head transducer; and actuating said print head transducer with said cancellation pulse to cancel residual pressure waves in said inkjet printer channel.

47. An inkjet printer apparatus comprising: an ink channel having an ink channel orifice, said ink channel supplied with ink from an ink channel reservoir; a transducer, said transducer mechanically coupled to said ink channel; a signal generator, said signal generator comprising a burst series waveform generator, said burst series waveform generator having a burst series output signal coupled to an amplifier; and a switch array, said amplifier having an amplified burst series signal coupled to said switch array, said switch array selectively coupling said amplified burst series output signal to said transducer.

48. The apparatus of claim 47 wherein said burst series waveform generator comprises: a lookup table controller having control signal outputs communicating with a burst series lookup table; said burst series lookup table having a digital waveform output coupled to a digital/analog converter; said digital/analog converter generating said burst series output signal which is coupled to said amplifier.

49. The apparatus of claim 48 further comprising: a cancellation pulse timer in electrical communication with said lookup table controller; a cancellation pulse lookup table, generated by controller cancellation pulse output signals said lookup table said cancellation pulse lookup table coupled to cancellation pulse output signals; and said cancellation pulse lookup table outputting a digital cancellation pulse waveform to said digital/analog converter.

50. The apparatus of claim 47 wherein said switch array comprises an array of bipolar switches.

51. the apparatus of claim 47 wherein said switch array comprises an array of unipolar switches.

52. A piezoelectric printer apparatus for expelling variable volume ink drops comprising: an ink channel; a transducer in mechanical communication with said ink channel; and a signal generator selectively communicating an electrical drive signal to said transducer, said electrical drive signal comprising a series of one or more signal pulses, said series of one or more signal pulses progressively decreasing in amplitude until reaching a predeter- mined signal pulse, said series of one or more signal pulses progressively increasing in amplitude beginning with said predetermined signal pulse.

53. The printer apparatus of claim 52 wherein said signal generator comprises a burst series waveform generator having a lookup table.

54. The printer apparatus of claim 53 wherein said lookup table comprises waveform coordinates defining selected burst series waveforms.

55. The printer apparatus of claim 52 wherein said electri- cal drive signal comprises a generally sinusoidal waveform.

56. The printer apparatus of claim 52 wherein said electrical drive signal comprises a positive polarity component and a negative polarity component .

57. The printer apparatus of claim 56 wherein said signal generator comprises a switch array, said switch array selectively switching only said negative polarity component of said electrical drive signal.

58. The printer apparatus of claim 56 wherein said signal generator comprises a switch array, said switch array selectively switching only said positive polarity component of said electrical drive signal.

59. The printer apparatus of claim 52 wherein said electrical drive signal is generated near or at a resonant fre- quency of said ink channel.

60. The printer apparatus of claim 59, wherein said signal pulses have a period within 0-10% of 4L/C, where L equals the length of said ink channel and C equals the speed of sound of ink contained in said ink channel.

61. The printer apparatus of claim 59 wherein said signal pulses have a period of 4L/C, where L equals the length of said ink channel and C equals the speed of sound of ink contained in said ink channel.

62. The printer apparatus of claim 61 wherein said signal pulses comprise a negative polarity portion and a positive polarity portion, each of said negative and said positive polarity porions having a period of 2L/C.

63. The printer apparatus of claim 52 wherein said signal generator further comprises a waveform generator which generates cancellation pressure waves.

64. The printer apparatus of claim 52 wherein said electrical drive signal comprises a trapezoidal waveform.

65. The printer apparatus of claim 52 wherein said electrical drive signal comprises a triangular waveform.

66. An inkjet printer apparatus comprising: a print head having an ink channel and a transducer in mechanical communication with said ink channel; said ink channel having an orifice through which an ink drop is expelled upon actuation of said transducer; a signal generator selectively applying an electrical drive signal to said transducer, said electrical drive signal having a series of one or more pulses; and means for selectively varying the amplitude of said electrical drive signal to expel a variable volume ink drop through said orifice, said means for selectively varying the amplitude of said electrical drive signal progressively decreasing the amplitude of said one or more pulses until reaching a predetermined signal pulse, said means for selectively varying the amplitude of said electrical drive signal then progressively increasing the amplitude of said one or more signal pulses beginning with said predetermined signal pulse.

67. The apparatus of claim 66 wherein said means for selec- tively varying the amplitude of said electrical drive signal comprises a waveform generator having a lookup table.

68. The apparatus of claim 67 wherein said lookup table comprises a table of coordinate points corresponding to selected burst series waveforms.

69. The apparatus of claim 67 wherein said waveform generator further comprises a cancellation pulse lookup table, said cancellation pulse lookup table comprising coordinate data points corresponding to a cancellation pulse waveform for canceling residual pressure waves in said ink channel .

70. The apparatus of claim 66 wherein said electrical drive signal further comprises a substantially sinusoidal signal waveform.

71. A piezoelectric print head comprising: an ink channel, a transducer mechanically coupled to said inkjet channel, and means for receiving an electrical drive signal from a signal generator to expel variable volume ink drops from said ink channel; said transducer electrically coupled to said electrical drive signal; and said electrical drive signal comprising a burst series of one or more pulses, each of said pulses of said burst series progressively decreasing in amplitude until reaching a predetermined pulse, said burst series progressively increasing in amplitude beginning with said predetermined pulse .

72. The print head of claim 71 wherein the frequency of said electrical drive signal is near or at a resonant frequency of said ink channel .

73. A method to operate an inkjet print head to expel variable volume ink comprising the steps of : generating a series of one or more electrical signal pulses for each ink drop to be expelled from an orifice of an inkjet print head channel, the volume of said ink drop varying as a function of the number of electrical signal pulses within said series of one or more electrical signal pulses; successively decreasing the amplitude of said electri- cal signal pulses in correlation to an increase in said number of electrical signal pulses within said series of one or more electrical signal pulses until reaching a predetermined signal pulse; successively increasing the amplitude of said electri- cal signal pulses within said series of one or more electrical signal pulses beginning with said predetermined signal pulse; coupling said series of one or more electrical signal pulses to a print head transducer.

74. The method of claim 73 wherein said series of one or more electrical signal pulses is generated near or at a resonant frequency of said inkjet print head channel.

75. The method of claim 74 wherein each of said electrical signal pulses has a period of 4L/C, where L equals the length of said inkjet print head channel, and C equals the speed of sound of ink contained in said inkjet print head channel .

76. The method of claim 74 wherein each of said electrical signal pulses has a period within 0-10% of 4 L/C, where L equals the length of said inkjet print head channel, and C equals the speed of sound of ink contained in said inkjet print head channel .

77. The method of claim 73 wherein said electrical signal pulses are sinusoidal waveforms.

78. The method of claim 73 wherein each of said electrical signal pulses has a positive polarity component and a negative polarity component, said coupling step further comprising the steps of: uniformly applying said positive polarity component to said transducer; and selectively switching the application of said negative polarity component to said transducer.

79. The method of claim 73 wherein each of said electrical signal pulses has a positive polarity component and a negative polarity component, said coupling step further comprising the steps of: uniformly applying said negative polarity component to said transducer; and selectively switching the application of said positive polarity component to said transducer.

80. The method of claim 73 further comprising the steps of: generating a cancellation pulse; applying said cancellation pulse to said print head transducer; and actuating said print head transducer with said cancellation pulse to cancel residual pressure waves in said inkjet printer channel.