JPH02219656A - Acoustic ink printer - Google Patents

Abstract

PURPOSE: To improve the tolerance of an accoustic ink printer to change in its free ink surface level by reducing the effect of half wave resonance on the accoustic power density of the accoustic beam being incident on the free ink surface and reducing focusing sensitivity of the accoustic ink printer. CONSTITUTION: The print head 22 of an accoustic ink printer 21 has a focusing lens 31 and for lighting accoustically a lens 31, a piezoelectric transducer 36 is excited by a pulse modulated high frequency voltage applied between both faces during actuation to link the acoustic wave to a base 32. In addition, the lens 31 rearranges the wave face of the incident acoustic radiation beam and forwards it as a converging acoustic beam 30 approximately focusing on the free ink surface into an ink 26. As the acoustic power density changes as a function of the ink surface level caused by convergence characteristics of the acoustic beam 30, by using an acoustically lossy ink, the half wave resonance is dampened to such an extent that has almost no effect.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to acoustic ink printers, and more particularly to methods and means for reducing the focusing sensitivity of acoustic ink printers.

BACKGROUND OF THE INVENTION Acoustic ink printers of the type to which the present invention pertains are characterized in that they emit each converging acoustic beam into a pool of ink liquid, and -M a primary bundle of each beam, essentially a free surface of the ink. one or more drop ejectors at substantially normal angles of incidence with respect to each other, and the angular convergence of each beam is selected such that the beam converges substantially on the free ink surface.

Printing is typically accomplished by modulating the radiation pressure (acoustic pressure) that each beam applies to the free surface of the ink. This modulation causes the effective pressure of each beam to be at a high enough pressure level to cause a short, restrained excursion motion to cause individual ink droplets to deposit imagewise from the free ink surface onto a nearby recording medium on command. This allows the bullet to fire at sufficient velocity to overcome the restraining force of surface tension.

Previous research has demonstrated that acoustic ink printers with droplet ejectors constructed from acoustically illuminated spherical focusing lenses have been shown to be capable of producing high-quality prints of relatively complex images at sufficient resolution for high-quality printing.
It has been demonstrated that accurately positioned pixels can be printed. See, for example, U.S. Patent Application No. 9 to Elrod et al.
No. 44,490, No. 944.698 and No. 944.7
See issue 01. It has also been shown that by providing such printers with means for dynamically changing the size of the printed pixels, it is possible to easily print variable gray level images, for example. 1986
Blr filed on December 19, 2013 and assigned to the applicant
See also another US patent application entitled "Variable Spot Size Acoustic Print" by od et al.

Although acoustic lenses are currently the preferred focusing mechanism for acoustic ink printer drop ejectors, alternatives include (1) U.S. Pat. No. 4.308 to Lovelady et al., issued December 29, 1981 .547, entitled "Droplet Ejector" (2) 1987 as a continuation of U.S. Patent Application No. 776.291, filed September 16, 1985. Pending U.S. Pat.
Others are known, including planar piezoelectric transducers with concentric interlocking electrodes (IDTs) as described in US Pat. Additionally, existing droplet ejector technology includes (1) a single ejector aspect for raster scan printing, (2) a matrix configuration ejector aspect for matrix printing, and (3) (i) parallel/serial. Single line, sparse for compound formats in print
) from the array, (ii) multiple rows with an individual jetter for each pixel position or address within a page-width image area (i.e., a single jetter/pixel/line) for regular line printing; Zigzag (stagger)
It is also clear that it is sufficient to design a variety of printhead configurations, including several different types of pagewidth jetter arrays ranging from . Furthermore, because practical considerations may influence or even govern the selection of drop ejectors for some printhead configurations, each of the above-mentioned patent applications does not provide a general discussion here. It will also be understood that they are incorporated herein by reference as complementary.

The size of the ink droplets ejected by an acoustic ink printer and the velocity at which they are ejected are substantially affected by minute changes in the free ink surface of the printer, such as those caused by attrition and/or evaporation of the ink over time. It is preferable not to be affected. Although relatively simple measures can be taken to compensate for easily detected level changes in the free ink surface, it is difficult to detect small surface level changes with the precision necessary to effectively compensate for them. It is technically difficult and costly. Therefore, the tolerance of an acoustic ink printer to small level changes in the ink free surface is an important factor to consider.

Unfortunately, conventional acoustic ink printers are too sensitive to changes in the level of the free surface of the ink. For example, spherical acoustic focusing lenses with practical depths of focus on the order of one wavelength of acoustic radiation in the ink have been developed for such printers. However, in printers implementing these lenses, only 1/1/2 of the free ink surface
It has been found that even level changes of four wavelengths or less tend to significantly affect the size of the ejected droplets and the velocity at which those droplets are ejected. Research has shown that half-wave resonances caused by acoustic reflections within one or more resonant cavities of these printers are primarily responsible for the above problems.

Obviously, since the ink/air interface is inherently misaligned, most of the incident acoustic radiation is generally reflected from the ink free surface of an acoustic ink printer. Also, since the ink is necessarily contained within a finite acoustic cavity, a significant portion of the reflected radiation will depend on whether the legal ejector is acoustically aligned with the ink or not, and the droplet ejector bag' 11/The ink attempts to return to the free surface after being reflected from the ink interface or an acoustically mismatched interface behind the droplet ejector. In general, the round trip propagation time for reflected radiation back to the free ink surface is:
The very narrow band (i.e., single frequency) radio frequency (RF) wave that has been proposed to drive the droplet ejection device of conventional acoustic ink printers is shorter than the duration of the ink burst, so that the ink is incident on the free surface. The reflected radiation and unreacted radiation coherently interfere. Although this interference can be constructively amplified, destructively annihilated, or partially amplified and partially annihilated, the levels of the ink free surface at which the resonant amplification interference and the antiresonant annihilation interference occur are mutually dependent on the acoustics within the ink. They differ by only 1/4 wavelength of radiation. Therefore, unless appropriate measures are taken to prevent, stop or suppress such resonances, only a fraction of the ink free surface
A level change of /4 wavelength or less can significantly change the effective radiation pressure (sound pressure) of one or more focused beams.

SUMMARY OF THE INVENTION According to the present invention, the influence of half-wave resonance on the acoustic power density of one or more acoustic beams incident on the free ink surface of an acoustic ink printer is significantly reduced, thereby A means is provided for reducing the focusing sensitivity of an acoustic ink printer. Some of the methods taken to achieve this measure rely on acoustic losses to dampen half-wave resonances and anti-resonances, while others rely on acoustic losses to dampen half-wave resonances and anti-resonances, while others are used to drive one or more droplet ejectors. Using multi-frequency high-frequency voltage pulses, the fluctuations in acoustic power caused by resonance and anti-resonance of half-waves of different frequencies (parkbage gin) are mutually neutralized. In practice, the use of acoustic loss inks to damp half-wave resonances and anti-resonances is compatible with selecting the frequency components of the acoustic radiation such that they neutralize them.
If desired, a combination of the two methods described above can also be used to implement the invention.

Further features and advantages of the invention will become apparent as the following detailed description is read in conjunction with the accompanying drawings.

Embodiments Referring now to the drawings, and in particular first to FIG. 1, an acoustic ink printer 21 (only relevant parts shown) shows a print head 22
, the printhead 22 having, upon command, a supply of ink fluid, or a droplet 24, sufficient to rapidly deposit individual droplets 24 from the free surface 25 of the ink 26 onto a nearby recording medium 27 in the form of an image. one or more droplet ejectors 23 (only one shown) for ejecting at a suitable ejection velocity. As shown, droplet ejectors 23 are immersed in ink 26, but their droplets are transferred to ink 26 by one or more liquid or solid intermediate acoustic coupling media (not shown). It will be appreciated that the ejectors may be coupled acoustically, and in the illustrated embodiment, the recording medium 27 is in operation line printed by a page-width array of drop ejectors 23, as indicated by arrow 29. etc., at a predetermined speed (by means not shown) relative to printhead 22 in a transverse or process direction.However, to accommodate different printhead configurations and different print patterns, Of course, the relative movement between print head 22 and recording medium 27 can be changed as desired.

In operation, each droplet ejector 23 produces a converging acoustic beam 30
into the ink 26 such that the primary bundle of the beam 30, or the upper beam, is at an angle of incidence approximately normal to the ink free surface 25. In accordance with conventional teachings, the angular convergence of each beam 30 is selected such that the beam is approximately focused on the ink free surface 25. Additionally, the radiation pressure (acoustic pressure) that each beam 30 exerts on the free ink surface 25 is modulated according to the image data applied to the corresponding droplet ejector 23, so that when a "black" pixel is to be printed, The radiation pressure is temporarily raised to a level above the threshold pressure for initiation of droplet ejection, while the radiation pressure is maintained at a level below that threshold pressure when a "white" pixel is to be printed.

As shown, each droplet ejector 223 suitably includes a spherical acoustic focusing lens 31 defined by a small spherical depression or recess in the top or front surface of the substrate 32 . Although only one lens 31 is shown, for example -
Of course, if a page-width printhead with a second or two-dimensional array of droplet ejectors 23 is desired, many lenses can be distributed across the top surface of the substrate 32, each spaced apart from center to center. However, regardless of the particular shape of the printhead, the substrate 32 may be made of a material that has an acoustic velocity significantly higher than that of the ink 26, such as silicon, silicon nitride, silicon, silicon carbide, alumina, sapphire, fused quartz, and certain glasses. Consisting of Since a printhead 22 with a single droplet ejector 23 is sufficient to demonstrate the problem addressed by the present invention and the solution provided, for the sake of brevity the printhead 22 with only one focusing lens will be described below. 31.

To acoustically illuminate the lens 31, a piezoelectric transducer 36 deposited on or closely bonded to the lower or back surface of the substrate 32 is excited with a pulse modulated high frequency voltage applied between the surfaces during operation. is vibrated to couple acoustic waves to the substrate 32.

Transducer 36 suitably comprises a piezoelectric film, such as a zinc oxide (ZnO) film sandwiched between a pair of electrodes 38 and 39, although it will be appreciated that other piezoelectric materials and transducer configurations may be used. Will. and lens 31
reshapes the wavefront of the incident acoustic radiation and directs the acoustic radiation into the ink 26 as a converging acoustic beam 30 that is approximately focused on the ink free surface 25 .

As shown in FIG. 2, the acoustic power density at the ink free surface 25 varies as a function of the ink surface level due to the focusing nature of the acoustic beam 30. In the absence of other factors here, the level of the ink free surface is within the range (
For example, if the lens 31 is F#~1, the wavelength of the acoustic radiation within the ink 26, λ, may vary.

Unfortunately, however, as shown in FIG. 3, half-wave resonance has generally been the main factor, although hitherto not recognized, as influencing the focusing sensitivity of conventional acoustic ink printers. As previously discussed, such resonances generally arise due to coherent interference between previously unreflected and reflected components of the acoustic radiation incident on the ink free surface 25. Also, the boundary conditions at the ink free surface level that cause the resonance enhancement interference and the antiresonance annihilation interference differ from each other by only 1/4 wavelength. Therefore, changes in the ink free surface level of the printer 21 (FIG. 1) by just a quarter wavelength or less can affect the efficiency with which acoustic power is transferred from the droplet ejector 23 to the ink free surface 25 (i.e., the acoustic coupling efficiency)
is likely to vary significantly enough to significantly affect the size of the ejected droplets and/or the speed at which those droplets are ejected.

One possible solution to this problem is to use acoustically lossy inks for acoustic ink printing,
This attenuates counter-wave resonance to such an extent that it has little, if any, effect. As a result of the acoustic loss (d87m) caused by the ink 26 tending to be greater for more viscous inks, the significant reduction in the amplitude of the existing half-wave resonance is more pronounced for inks with an absolute viscosity significantly higher than that of water. , and with even higher absolute viscosities, half-wave resonances do not appear to have a substantial effect on the focusing sensitivity of acoustic ink printers. The particular viscosity at which significant attenuation of the half-wave resonance occurs depends on the length of the acoustic path within the ink 26 and the high frequency frequency used, but generally an ink with an absolute viscosity of at least 5 to 10 centipoise can be used. An easily perceptible reduction in the acoustic power fluctuations at the free surface 25 is observed. However, of course, the use of acoustic loss inks provides either a partial or complete solution to the problem of half-wave resonance. This is because the acoustic loss ink significantly attenuates the reflected radiation during its return trip to the ink free surface 25, thereby reducing the magnitude of the fluctuations that the ink free surface undergoes.

Another method for reducing the sensitivity of acoustic ink printers to half-wave resonance, which can be used alone or in combination with acoustic loss ink, is to drive the droplet ejector 23 of the printer 21 with multi-frequency high-frequency tone bursts, which can reduce the sensitivity of one frequency component to The power fluctuation caused by the resonance of the frequency component substantially cancels out or neutralizes the power fluctuation caused by the anti-resonance of another frequency component. That is, referring to the dual tone case shown in FIG.
2 and transducer 36 (i.e., printhead 22), the ink free surface level at which one frequency, f, is resonant and the other frequency, f2, is anti-resonant, is determined by the central portion of the lens surface (i.e., lens 31
Displacement of the ink free surface 25 from the "acoustic center" of 1.
It will be understood that it is required as a function of . Due to the characteristics of the acoustic impedance of the lens substrate 32, the ink 72
Since the acoustic impedance is higher than 6, the acoustic velocity field is phase shifted by 180° upon reflection at the lens/ink interface. Therefore, since antiresonance occurs when the ink free surface 25 is displaced from the acoustic center of the lens 3I by an integer n times a half wavelength, the condition for antiresonance exists for the frequency f, if: f, = nV, /211 However, Vi = sound speed of mini ink On the other hand, resonance occurs when the ink free surface 25 changes from the acoustic center of the lens 31 by an odd multiple of 1/4 wavelength, so the condition for resonance exists in the following case for the frequency ft Do: fz =nVi /21= +Vt /4ft
(2) As a result, two high frequency frequencies f1
and ft are their frequency spacing Δ in the ink 26
If f is selected so as to satisfy the following equation; Δ r i ='l i /4j? i
(3) The power fluctuations caused by resonance and anti-resonance at the above frequencies cancel each other out, reducing the sensitivity of the printer 21 to small changes in the ink free surface level (see FIG. 5).

To further reduce the influence of half-wave resonance on the power density at the ink free surface 25, the high frequency drive pulse can also have more frequency content. For example, as shown in FIG.
By mixing a high frequency carrier wave, such as a transmit wave, with a periodic pseudo-random pit sequence signal having a frequency of up to about 20 MHz, the drive pulses applied to the transducer 36 by the switch gate or modulator 53 are approximately 13 MHz.
It may be comprised of a number of high frequency frequencies ranging from 0 MH2 to approximately 170 MHz. The pseudorandom pit sequence signal varies periodically with the data rate of printer 21 (i.e., the rate at which data bits are applied to modulator 53) such that the RF power of the drive pulses applied to transducer 36 is substantially variable. It is appropriate to do so. Alternatively, a linear chirp (ch 1rp) signal can be used to modulate the high frequency carrier frequency, but this method has the disadvantage that the carrier must be frequency modulated at a high rate. Yet another possibility is to drive the transducer 36 with modulated data, ie, substantially "white" high frequency noise, although the high frequency power level of such noise may vary widely from pulse to pulse. Therefore, this method is not preferred.

Examining the acoustic coupling characteristics of the illustrated acoustic ink printer in more detail, we can see that the print head 22 is
It will be appreciated that if the printhead 22 is not acoustically matched with the ink 26, the printhead 22 is a resonator that is only weakly coupled to the ink 26. Also, even if such an acoustic matching layer is applied at the printhead/ink interface, the acoustic coupling efficiency will tend to vary as a function of frequency. In the dual-tone embodiment of FIG. 4, the amplitudes of the two frequency components, and f2, are scaled as necessary so that their resonances and anti-resonances produce substantially equal and It can be made to swing in the opposite direction. However, when using a broad spectrum radio frequency source such as in FIG. 6, it is easier to design the radio frequency source to have a relatively flat amplitude across its frequency spectrum. Therefore, in such an embodiment, it is advantageous to use a printhead 22 with a resonant cavity length 11 that is much greater than the thickness of the ink liquid layer 26, ie, the resonant cavity length J. In this case, the frequency spacings of half-wave resonances Δf, and Δf, in printhead 22 and ink 26 are: Δf,=V,/21° and Δf!, respectively. = Vi /212 Therefore, with due consideration to the difference between the speeds of sound in the printhead 22 and the ink 26, the respective resonances should be spaced so that the printhead resonances have a much narrower frequency spacing than the ink resonances. Cavity length 1. and fi. Therefore, many frequency components of the radio frequency source can be coupled from lens 31 to ink 26 within the passband of each resonance of ink 26, and by exciting ink 26 at a sufficient spectral frequency, each frequency half Acoustic power fluctuations caused by wave resonance and anti-resonance can be made to substantially neutralize each other.

EFFECTS OF THE INVENTION From the foregoing, it will be appreciated that the present invention reduces the effect of half-wave resonance on the focusing sensitivity of an acoustic ink printer, thereby increasing the tolerance of the printer to changes in the level of the free surface of the ink. . Additionally, the damping of half-wave resonances may be increased to reduce undesirable acoustic power fluctuations caused by such half-wave resonances, or the acoustic power fluctuations caused by half-wave resonances may be neutralized, or the like. It will also be understood that the invention may be practiced using a combination of both methods or either method.

[Brief explanation of the drawing]

Figure 1 is a simplified partial cross-sectional view of an acoustic ink printer.
Figure 2 schematically shows how the acoustic power density at the center of the focal spot on the ink free surface of the printer shown in Figure 1 changes as a function of level changes at the ink free surface in the absence of half-wave resonance. Figure 3 is a graph showing the effect of a single frequency half-wave resonance on the tolerance of the printer shown in Figure 1 to changes in the level of the free surface of the ink; , a simplified partial cross-sectional view of an acoustic ink printer driven by dual-frequency high-frequency pulses to suppress half-wave resonance;
Figure 6 is a graph showing the improved tolerance of the printer shown in Figure 4 to changes in the level of the ink free surface; Figure 6 is an acoustic ink driven by multi-frequency radio pulses to further suppress half-wave resonance; A simplified partial cross-sectional view of the printer; FIG. 7 is a graph illustrating the near-optimal tolerance of the printer shown in FIG. 6 to changes in the level of the free ink surface. Description of symbols 21-acoustic ink printer, 22--print head, 23-droplet ejector, 24--droplet, 25--ink free surface, 26-ink (ink liquid supply reservoir) 30--. - Acoustic radiation (beam). FIG, 3 FIG, 4 FIG, 7 FIG, 2 1 - One Season Eight One One One

Claims (1)

Claims: at least one radio-frequency wave for emitting pulse-modulated, focused acoustic radiation into a supply of ink liquid such that the acoustic radiation is substantially focused on the free surface of said ink;
rf) having a printhead including an excited droplet ejector;
individual ink droplets of controlled size are ejected from the ink free surface at a controlled ejection velocity in response to a command;
The ink is contained within an acoustic cavity of finite length, and acoustic radiation emitted into the ink reflects off the free surface of the ink and thereafter returns to the free surface of the ink, resulting in an acoustic radiation emitted from the droplet ejector. In an acoustic ink printer that produces acoustic power fluctuations at the ink free surface by coherently interfering with reflected radiation: the acoustic power fluctuations are sufficiently suppressed so that the ink free surface Even if a change in level occurs,
An acoustic ink printer comprising means for preventing fluctuations in acoustic power from significantly affecting the droplet size or ejection velocity.