US 3832579 A
A reservoir supplies liquid through a conduit to a nozzle. The liquid is under small or zero static pressure. Surface tension at the nozzle prevents liquid flow when the system is not actuated. A section of the conduit terminating at the nozzle is designed to be capable of conducting pressure waves in the liquid from end to end of the section without the occurence of significant reflections within the section. An electroacoustic transducer is coupled to the liquid in the reflection-free section. When an electric pulse is applied to the transducer it applies a pressure pulse to the liquid sending a pressure wave to the nozzle where it causes ejection of a droplet. The pressure pulse also sends a pressure wave in the opposite direction. The system has energy absorbing means coupled to the liquid and adapted to absorb substantially all of the energy of the latter wave, thus preventing reflections which could return to the nozzle and interfere with ejection of a subsequent droplet. Two classes of energy absorbing means are described: (a) conduit walls of viscoelastic material which deform under the influence of the pressure wave and absorb energy therefrom, and (b) several forms of acoustic resistance elements within the conduit at the inlet end of the reflection-free section.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Arndt [451 Aug. 27, 1974 PULSED DROPLET EJECTING SYSTEM  Inventor: John P. Arndt, Cleveland, Ohio  Assignee: Gould Inc., Chicago, Ill.
 Filed: Feb. 7, 1973  Appl. N0.: 330,360
Primary ExaminerMark O. Budd Attorney, Agent, or Firm-Eber .l. Hyde  ABSTRACT A reservoir supplies liquid through a conduit to a nozzle. The liquid is under small or zero static pressure. Surface tension at the nozzle prevents liquid flow when the system is not actuated. A section of the conduit terminating at the nozzle is designed to be capable of conducting pressure waves in the liquid from end to end of the section without the occurence of significant reflections within the section. An electroacoustic transducer is coupled to the liquid in the reflection-free section. When an electric pulse is applied to the transducer it applies a pressure pulse to the liquid sending a pressure wave to the nozzle where it causes ejection of a droplet. The pressure pulse also sends a pressure wave in the opposite direction. The system has energy absorbing means coupled to the liquid and adapted to absorb substantially all of the energy of the latter wave, thus preventing reflections which could return to the nozzle and interfere with ejection of a subsequent droplet. Two classes of energy absorbing means are described: (a) conduit walls of viscoelastic material which deform under the influence of the pressure wave and absorb energy therefrom, and (b) several forms of acoustic resistance elements within theconduit at the inlet end of the reflection-free section.
4 Claims, 13 Drawing Figures PATENI AUBZ 71914 SREHMIF 5 IVIIIII 1 Pmnmwszmu 7 3.832.579 sum sur 5 PULSED DROPLET EJECTING SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to a system for ejecting droplets of liquid on command suitable for use in apparatus such as ink jet printers and facsimile recorders.
2. Description of the Prior Art This invention is an improvement on the system described in U.S. Pat. No. 3,683,212, issued to Steven I. Zoltan on Aug. 8, 1972, assigned to the same assignee as the present invention.
A system constructed as described in the Zoltan patent having the dimensions cited by way of example works very well when the pulse rate is less than about one kiloHertz. If the pulsing is continuous and the pulse rate is gradually increased above about one kiloHertz, alternate increases and decreases in droplet velocity may be observed.
When a burst of pulses equally spaced in time is applied to the system, and the time interval between pulses exceeds about one millisecond, the resulting droplets are ejected with uniform spacing. However, when the time between pulses is decreased to a fraction of a millisecond, the first several droplets which are ejected generally have irregular spacing.
The above described irregularities are undesirable in many applications. An experimental and theoretical investigation has shown that they are caused by acoustic resonances, reflections, and interference phenomena in the liquid in the system.
OBJECT AND SUMMARY OF THE INVENTION The object of this invention is to provide a droplet on command system generally similar to the system described in U.S. Pat. No. 3,683,212 but which is substantially free of the irregular performance at high pulse rates observed in systems constructed as described in that patent.
According to the invention a reservoir supplies liquid through a conduit to a nozzle. A section of the conduit terminating at the nozzle is adapted to conduct pressure waves in the enclosed liquid without the occurence of significant reflection within the section. An electroacoustic transducer is coupled to the liquid in the reflection-free section of the conduit and is adapted to apply a pressure pulse to the liquid whereby a first pressure wave travels in the liquid to the nozzle and causes ejection of a droplet therefrom, and a second pressure wave travels in the liquid toward the inlet end of the reflection-free section. An energy absorbing means is coupled to the liquid in the conduit and is adapted to absorb substantially all of the energy of the second pressure wave.
BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.
In the drawings:
FIG. 1 shows a system according to the invention partly in section and partly schematic;
FIG. 2 shows a test set up for selecting certain system parameters;
FIG. 3 shows graphs obtained with the set up of FIG. 2;
FIGS. 4 to 11 inclusive show modifications of the system of FIG. 1;
FIG. 12 is an exploded view of a system according to the invention which differs substantially in mechanical detail from the system shown in FIG. 1; and
FIG. 13 is a conventional sectional view along lines 13--l3 of FIG. 12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a reservoir shown schematically at 1 contains ink or other liquid 2. A conduit indicated generally by reference character 4 communicates with liquid 2 in the reservoir and is filled with the liquid. Conduit 4 terminates in a nozzle 10 which also is filled with liquid 2. Droplets 13 of the liquid may be ejected on command through orifice ll of the nozzle as will be explained in later paragraphs.
Conduit 4 comprises a section 5 having an inlet end 7. Section 5 is formed of a material such as glass which provides a smooth internal surface and relatively stiff walls. The internal cross-sectional area is substantially constant along the length of section 5. At dashed line 8 the cros-sectional area begins a gradual reduction, forming nozzle 10 having exit orifice 11. Therefore section 5 may be regarded as having an outlet end at 8, and this end is terminated by nozzle 10.
Conduit 4 also comprises a liquid supply section 14 formed of viscoelastic material such as a plasticized polyvinyl chloride.
The internal diameter of supply section 14 is smaller than the internal diameter of section 5. Section 14 is expanded at one end and forced over the outside of section 5 at inlet end 7 thereof. Supply section 14 may continue to reservoir 1 where it may terminate below the surface of liquid 2, or it may be coupled to an additional section 16 leading from reservoir 1.
A tubular electroacoustic transducer 17 surrounds conduit section 5 and is secured thereto in stress transmitting engagement by epoxy cement 19. Preferably transducer 17 comprises a piezoelectric lead zirconatelead titanate ceramic tube 20, having electrodes 22, 23 on the cylindrical surfaces, and is radially polarized. A metal foil strip 25 is inserted to provide electrical contact with electrode 22 prior to introduction of cement l9.
Terminal wire 26 is wrapped around conduit section 5 in contact with foil strip 25 and is secured in electrical contact therewith by conductive epoxy 28. Terminal wire 29 is wrapped around electrode 23 and secured in electrical contact therewith by conductive epoxy 31.
Due to the well known piezoelectric effect, the inside diameter of transducer 17 decreases almost instantaneously when a voltage of suitable polarity is connected between terminal wires 26 and 29. This diameter decrease forces decrease in diameter of the portionof conduit member 5 which is surrounded by transducer 17. Liquid 2 within that portion of section 5 must therefore either be compressed, or experience some displacement. As the voltage between terminals 26 and 29 is reduced to zero, transducer 17 and conduit member 5 return to their original dimensions, again causing pressure change in liquid 2, or displacement thereof.
Thus transducer 17 is coupled to the liquid in conduit section 5.
Reservoir l is maintained at an elevation which applies little or no pressure to the liquid 2 in nozzle 10. A slight negative pressure, on the order of two to three centimeters of head seems to be advantageous. Under quiescent conditions, the surface tension of the liquid in orifice ll prevents flow of liquid 2 in either direction.
When it is desired to have a droplet ejected from nozzle 10, a voltage pulse of the polarity which causes contraction of the transducer is applied between terminals 26 and 29. The transducer contracts in response to the pulse causing slight decrease in the internal volume of conduit member 5. This momentarily compresses that portion of liquid 2 which is within transducer 17 and causes pressure waves to travel in the liquid toward outlet 8 and nozzle and also toward inlet 7 and reservoir 1.
Conduit section 5, surrounded over part of its length by transducer 17, may be regarded as an acoustic transmission line. By virtue of the relatively stiff walls, and the uniform cross-sectional area of the enclosed liquid along the length of the section, it conducts pressure waves in liquid 2 substantially without the occurrence of reflections within the section.
The pressure wave which travels in the liquid toward outlet end 8 of section 5 causes ejection of a droplet from nozzle 10.
When supply section 14 of conduit 4 is formed of appropriate material and is suitably porportioned as hereinafter described, the characteristic acoustic impedance looking into the liquid in section 14 from inlet end 7 of section 5 is approximately matched to the characteristic acoustic impedance of section 5. Thus the pressure wave which travels in the liquid from transducer 17 toward inlet end 7 of conduit section 5, passes into the liquid within section 14 without deleterious reflection. The wave therefore continues in the liquid in section 14 toward reservoir 1. As the wave progresses it causes elastic deformation of the viscoelastic material of supply section 14, progressively along the length thereof. Since the material is viscoelastic, part of the energy transferred from the liquid to the material to cause deformation is converted to heat and is not returned to the liquid as potentialor kinetic energy as the wave passes. Thus, as the wave progresses toward reservoir 1, the energy of the wave is progressively absorbed by conduit section 14.
When the attenuated wave reaches the reservoir end of conduit section 14 it encounters an impedance discontinuity with consequent reflection back toward inlet end 7 of conduit section 5. As the reflected wave travels through section 14 toward inlet 7 of section 5 it is attenuated by absorption in the viscoelastic material in the manner above described. Conduit supply section 14 is made long enough so that the energy of the reflected wave, when it reaches nozzle 10, is too low to have substantial influence on the ejection of a new droplet when a new voltage pulse is applied to terminals 26 and 29. Thus the viscoelastic material of supply section 14 may be regarded as energy absorbing means coupled to the liquid in conduit 4 which absorbs substantially all of the energy of the wave which travels from transducer 17 toward inlet end 7 of conduit section 5.
As the electric drive pulse decays to zero, transducer 17 and conduit section 5 return to their original dimensions. After a droplet has been ejected, the liquid in nozzle 10 withdraws from the end thereof leaving an empty space which is then refilled by liquid from the conduit under the urging of capillary forces in the nozzle. Following refill of the nozzle, quiescent conditions prevail until another electric drive pulse is applied to transducer 17. When a new pulse is applied, the above described process repeats. Thus droplets may be ejected on command, each command being given by applying an electric pulse to transducer 17.
The pulse shape requirement is not critical. It has been found advantageous to have rise time less than two microseconds dwell time of five to flfty microseconds, and fall time greater than two microseconds. Good results also have been obtained using a cosine squared pulse shape with period of ten to one hundred microseconds.
Many electric circuit arrangements can be devised for generating and applying suitable electric drive pulses. For examples of such circuits, reference may be made to US. Pat. No. 3,683,212 to Zoltan.
In order for the system to operate as described it is necessary to have a suitable inter-relationship between the properties of the material forming supply conduit section 14, the dimensions of section 14, the inside diameter of conduit section 5, and properties of liquid 2. If a proper relationship is not established, a pressure wave traveling in the liquid from transducer 17 will be at least partially reflected when it reaches inlet end 7 of section 5. When that reflected wave reaches nozzle 10 it may cause ejection of an additional, undesired droplet, or it may interfere with the desired ejection of a new droplet which happens to be timed to occur as the reflection reaches the nozzle. When the reflected wave reaches the nozzle it will be at least partially reflected back toward inlet 7, and upon arrival at inlet 7 this newly reflected wave will be reflected just as the original wave from transducer 17 was reflected. In severe cases of incorrect matching of supply section 14 to section 5 a large number of reflections may thus take place before the energy decreases enough so as not to interfere with ejection of another droplet initiated by a new command pulse. Thus, the stronger the reflections, the longer the time interval before a new droplet can be ejected without disturbance from the reflecting waves.
Selection of suitable viscoelastic material and dimensions for conduit section 14 to prevent deleterious reflections at inlet end 7 of conduit member 5 may be accomplished by testing, as hereinafter described, a series of sample sections constructed of various materials and with a range of dimensions for each material. For use in such testing, the assembly of FIG. 1 is provided with an additional transducer 32 secured to conduit section 5 close to nozzle 10. Transducer 32 may be identical to transducer 17 except that preferably it is made much shorter. Foil strip 34 is inserted and terminal wires 35, 37 are secured by conducting epoxy 38,40 just as in the case of strip 25, terminal wires 26,29 and epoxy 28,31 associated with transducer 17.
The tests may be performed using the dual transducer assembly of FIG. 1 in a test set-up as shown in FIG. 2. In FIG. 2 the liquid supply section 14 under test is expanded at one end and forced over the inlet end 7 of conduit section 5 as in FIG. 1. A hypodermic syringe 41 fitted with a blunt needle 43 selected for a snug fit in section 14' serves as a reservoir. Syringe 41 is loaded with liquid 2 of the kind that will be used with the droplet ejecting system, and the liquid is forced through the conduit to eject a stream from nozzle until all air is swept out of the system. Thereafter, during the test, no pressure is required but care must be exercised to prevent drawing liquid back out of nozzle 10 into conduit section 5.
A variable frequency sine wave oscillator 44 is connected by coaxial cable 46 to terminals 26,29. Oscillator 44 preferably has a continuous frequency range from below 1,000Hz to 50KHz. A substantially constant output voltage is desirable. A level of about two volts is satisfactory.
A motor drive mechanism 47 is mechanically coupled to the frequency control dial 49 of oscillator 44 to sweep the oscillator slowly over the entire frequency range. A potentiometer 50, supplied with current from dc source 52 also is coupled to motor drive 47. The output of potentiometer 50 goes to the x-axis terminals 52 of an XY plotter 53. Thus, as oscillator 44 is swept over its frequency range, the pen 55 of XY plotter 53 is driven across chart 56.
The swept frequency voltage from oscillator 44 causes transducer 17 and the portion of conduit section 5 surrounded by transducer 17 to alternately increase and decrease in diameter in synchronism with the oscillator voltage. These dimensional variations cause corresponding pressure variations in the liquid 2. The amplitude of the preseure variations is too small to cause ejection of droplets from nozzle 10. However, the pressure variations within transducer 32 adjacent to nozzle 10 stress the transducer sufficiently toproduce measurable AC voltage between terminals 35,37, corresponding to the pressure variations. This voltage may be in the range of one to ten millivolts when conduit section 14' is properly matched to section 5, and much higher when there is a serious mismatch.
The pressure pickup signal which is developed between terminals 35 and 37 is applied to an electronic AC voltmeter 58 via coaxial cable 59. Meter 58 has output terminals 62 between which appears a DC signal proportional to meter deflection. Terminals 62 connect to Y-axis terminals 64 of XY plotter 53. Thus, as oscillator 44 is swept over its frequency range, the X Y plotter draws a graph of pressure behind nozzle 10 .vs frequency. A pressure calibration of the system is not required but it is desirable to have a rough calibration of the X or frequency axis.
Preferably meter 58 is a tuned voltmeter with tuning dial 65 coupled to oscillator dial 49 in a manner which insures'accurate tracking. The use of a tuned meter reduces difficulties that may otherwise be encountered with pickup of noise and stray signals. Instruments are commercially available which combine in one unit the functions of oscillator 44, tracking tuned voltmeter 58, sweep drive 47, potentiometer 50, and DC supply 52. One such instrument is the model 302A Wave Analyzer equipped with Model 297A Sweep Drive, manufactured by the Hewlett Packard Company. One of these instruments was used in the tests hereinafter described.
In continuing the description of the test procedure reference will be made to a particular series of tests that resulted in the selection of material'and dimensions for conduit section 14 that later produced good results in actual pulsed droplet ejection. Referring to FIG. 1, approximate specifications and dimensions were as follows:
Conduit section 5 lime glass length 2.5 cm inside diameter 0.05] cm wall thickness 0.0l cm Transducer 17 Lead Zirconate-Lead Titanate Ceramic length L25 cm inside diameter 0.076 cm wall thickness 0.025 cm Transducer 32 Lead Zirconate-Lead Titanate Ceramic length 0.16 cm inside diameter 0.076 cm wall thickness 0.025 cm Nozzle l0 orifice diameter 0.007 cm Liquid 2 distilled water FIG. 3 is a copy of XY plots for several sample conduit members 14'. Curve 67 was obtained with a conduit section 14 made of soft vinyl material. The inside diameter was 0.063 cm and the outside diameter was 0.16 cm. The pressure peak at about 15' kHz occurred because conduit section 14' was too large in inside diameter, and was too soft, providing very low acoustic impedance, thus allowing almost complete reflection at inlet end 7 of conduit section 5. Nearly complete refiection also took place at nozzle 10 because of the very high acoustic impedance presented by the nozzle, representing nearly a blocked condition. Thus the 15 kHz peak may be regarded as a quarter wave resonance in conduit section 5. The peak at about 40 kHz may be regarded as a three quarter wave resonance. The lack of exact 3 to 1 correspondence probably is due to the imf pedance of conduit member 14' not being zero and the impedance of the nozzle not being infinite.
Curve 68 in FIG. 3 was obtained with a conduit section 14' of much stiffer material. The inside diameter was 0.063 cm and the outside diameter was 0.18 cm. The reduced height of the peaks at 15 kHz and 40 kHz indicates that a significant part of the energy of the wave traveling in the liquid from transducer 17 toward conduit section 14' continued into the liquid in section 14 as desired.
Curve 70 in FIG. 3 was obtained with a conduit section 14' made of the same material and having the same outside diameter involved in curve 68 but the inside diameter was 0.041 cm. This curve indicates substantially reflection-free transmission from conduit section 5 to section 14 and represents a satisfactory selection of material and dimensions. The slight hump in the curve at about 23 kHz suggests that possibly a slightly larger inside diameter would be preferable,-but pulsed operating tests of a system employing this conduit section produced substantially uniform drop ejection at rates up to 10 kHz, which is an order of magnitude improvement over the results obtained'with a droplet on command system constructed as desribed in U.S. Pat. No. 3,683,212. A
Curve 71 in FIG. 3 was obtained with a conduit section 14' made of the same material and having the same outside diameter involved in curves 68 and 70 but having inside diameter of 0.025 cm. The low response at- ISkI-Iz and the pronounced hump at about 25kHz indicate that the inside diameter was too small.
An additional series of tests provides a useful guide to determining the minimum length for conduit section 14'. After selecting suitable material and diameters for section 14 as illustrated by the example referring to FIG. 3, the selected sample is cut to one-half length and a new curve is run and compared with the curve for the original length. The section just tested is again cut to half length and a new curve is run and again compared with the original curve. This process is repeated until a new curve is obtained which is significantly different from the original curve. At this point it may be assumed that the length is too short and the preceding length should be considered to be approximately the minimum length.
The conduit section 14' which resulted in curves 68, 70,71 in FIG. 3 were made of a plastisol prepared as follows:
Ingredients and Source Parts by Weight Resin vinyl chloride homopolymer identified as Geon 121 powder, supplied by B. F. Goodrich Chemical Co., Cleveland, Ohio 70 Plasticizer diactyl phthalate,
identified as Good-Rite GP261 Plasticizer, supplied by B. F.
Goodrich Chemical Co., Cleveland,
Stabilizer identified as 6-V-6-A Stabilizer, supplied by Ferro Chemical Division, Ferro Corp., Bedford, Ohio I The plasticizer and stabilizer were added to the resin powder in a beaker and hand stirred for over thirty minutes. This formed a very stiff mixture which then was placed in a bell jar and evacuated and held for 24 hours.
The conduit section was moulded by forcing the thick mixture to fill the space around a smooth wire tensioned and centered within a glass tube, and then curing at a temperature of 160C for about one to three minutes. The wire then was stretched beyond its elastic limit to reduce the diameter, and withdrawn. The viscoelastic tube then was withdrawn from the surrounding glass tube which formed part of the mold.
Satisfactory pulsed droplet ejections over a wide pulse frequency range has been obtained with a system having the parameters tabulated above and provided with conduit section 14' formulated as described and having inside diameter of 0.041 cm, and length of 10 cm. This was the conduit member resulting in curve 70 of FIG. 3.
Similar results have been obtained employing an extruded section of plasticized polyvinyl chloride tubing supplied by the Norton Company of Akron, Ohio, under the Trade Mark TYGON. The outside diameter was 0.178 cm and the inside diameter was 0.041 cm, and the length was 10 cm. The particular TYGON composition was identified as formulation S54-HL.
Preferably conduit section is formed of glass, but metal, and plastic sections have been used successfully. Preferably nozzle if formed integrally with conduit section 5 in a manner to provide a smooth contour as illustrated in FIG. 1. However, an abrupt transition from the relatively large diameter of conduit section 5 to the small diameter of orifice 11 does not mitigate against satisfactory operation of this invention.
The use of viscoelastic material to form part of conduit 4 is not the only way to provide energy absorbing means coupled to the liquid in the conduit. Another way that has produced satisfactory results is to install a suitable acoustic resistance element at the inlet end 7 of conduit section 5 through which the liquid 2 flows, as shown in FIGS. 4 to 11.
Referring to FIG. 4, the construction may be generally similar to the construction shown in FIG. 1. Conduit section 5, preferably of glass, is terminated at the outlet end at 8 by nozzle 10. Transducer 17 is secured to section 5 by epoxy cement l9, and terminal wires 26,29 are attached as in FIG. 1. A pressure measuring transducer is not shown in FIG. 4 but if desired one may be provided by using a longer conduit section 5 and attaching a transducer 32 as in FIG. 1.
One form of acoustic resistance unit that has given satisfactory results has been provided by pressing a short bundle of glass fibers 73 into the end extension 74 of conduit section 5 as shown in FIGS. 4 and 5. The resistance element terminates at dashed line 7' which marks the effective inlet end of conduit section 5.
Liquid 2 is supplied from a reservoir, now shown, through supply conduit section 76 which may be made of any convenient material. Preferably it is formed of soft plastic and has inside diameter equal to or larger than the inside diameter of conduit section 5. If desired, conduit extension 74 may be attached directly to the reservoir, eliminating section 76.
Another form of acoustic resistance unit that has given good results has been provided by filling conduit extension 74 with minute glass beads 77 and then fusing them together and to the inner wall of extension 74 as shown in FIGS. 6,7.
Still another successful resistance unit has been provided by pressing a cylinder 79 of porous plastic into conduit extension 74 as shown in FIGS. 8,9. The particular material that was employed was cut from a POROSYN tip for a TIP-WIK pen sold by the Eversharp Pen Company of Janesville, Wisconsin.
The flow resistance R of the acoustic resistance elements of FIGS. 4 to 9 should be approximately equal to the characteristic acoustic impedance Zo of the liquid filled conduit section 5. The characteristic impedance Z0 is given approximately by:
Z0 l/S) V B P where S 1r a cross sectional area of the liquid column enclosed by conduit section 5 a inside radius of conduit section 5 B bulk modulus of liquid 2 p density of liquid 2 Suitable dimensions and degree of packing of the glass fibers 73 of FIGS. 4,5; suitable dimensions and degree of fusing of the beads 77 of FIGS. 6,7; or suitable material and length for the porous cylinder 79 of FIGS. 8,9 to obtain the desired value of R may be determined by experiment. The experimental resistance element to be tested should be vacuum impregnated with the liquid to be used, and then such liquid should be forced through the unit under measured pressure P taking care to avoid introducing any air. The quantity Q of liquid which flows in a measured time t should be measured. The resistance then is calculated from:
As an alternative, experimental resistance units may be installed in a double transducer assembly similar to that shown in FIG. 1, but supplied with liquid as in FIG.
4, and then tested in the set-up of FIG. 2. Resistance which is too low will result in curves resembling curves 67 or 68 in FIG. 3. When the resistance is too high the curve should resemble curve 71. The correct resistance should produce a curve resembling curve 70 of FIG. 3.
FIGS. 10 and 11 show how an acoustic resistance may be provided by an annular slit at the inlet end 7 of conduit section 5. In this case, conduit section 5 and the nozzle, not shown, preferably are made of metal or plastic. Small indentations 82 are formed in conduit extension 74. A solid cylinder 80 of metal or plastic is pressed into extension 74 and held in place by indentations 82. The acoustic resistance R is given approximately by R 6p.L/ t 1ra where [.L viscosity of liquid 2 L axial length of cylinder 80 t= clearance between cylinder 80 and conduit extension 74 a inside radius of conduit extension 74 The dimensions t and L should be selected to make R approximately equal to the characteristic impedance Z of conduit section 5.
FIGS. 4 to 11 show various ways in which energy absorbing means in the form of acoustic resistance elements may be coupled to the liquid in the conduit to absorb substantially all of the energy of a pressure wave traveling in the liquid from transducer 17 toward the inlet end 7 of conduit section 5. In each case the inlet end 7 may be located inside of transducer 17 as illustrated in FIGS. 4 and 8 or may be external of the trans ducer as illustrated in FIGS. 6 and 10.
In any of the constructions of FIG. 1 and FIGS. 4 to 11 it is advantageous to enclose the transducer 17 in a jacket of yieldable material such as rubber or plasticized vinyl. The assembly may thus be secured in the apparatus in which it is used, by clamping to the jacket. With such an arrangement there is little danger of applying breaking stresses to the assembly, and the clamp does not interfere significantly with the pulsing changes in diameter of the assembly as droplets are ejected. The jacket may be in the form of a section of tubing dimensioned to fit tightly over the transducer as shown at 78 in FIG. 4, or it may be cast around the assembly. In the latter case it may extend over the end of the transducer embedding also the wrapped-around portions of terminal wires 26, 29 and the exposed portion of conduit section or'74. Another advantage of such a jacket is that it provides damping of mechanical resonances of the assembly which might otherwise cause deleterious effects.
It is not necessary to employ a cylindrical piezoelectric transducer surrounding the conduit as shown in FIGS. 1 and 4 to 11. Other transduction principles may be used, for example, electrostriction and magnetostriction. Further, other geometric configurations and methods of coupling the transducer to the liquid may be employed. For example, in FIGS. 12 and 13 the transducer is a piezoelectric disc which is coupled to the liquid by direct contact.
In FIGS. 12, 13, piezoelectric disc 83, preferably of lead zirconate-lead titanate ceramic, has electrodes 85,86 to which terminal wires 88, 89 are attached by solder or conductive epoxy 91,92.
Piezoelectric disc 83 is clamped between metal or plastic cover plates 94,95 by O-rings 97,98 which fit into grooves 100,101 in the cover plates. Terminal wires 88,89 extend through openings 103,104 in the cover plates.
Also clamped between covers 94,95 is a sheet 106 of I cut-out 109 at one end thereof. Opening 110 through cover 94 communicates with cut-out 109 at the other end. Thus there is formed a conduit comprising tubular member 107, cut-out 109 enclosed by covers 94, 95, opening 110, and an annular space formed by cut-out 1 12, the rim of piezoelectric disc 83, O-rings 97,98 and cover plates 94,95. The conduit is terminated at one end by sapphire watch jewel 113 which serves as a droplet ejecting nozzle. The other end of the conduit, i.e., the open end of tubular member 107, may be immersed in liquid in a reservoir, not shown, or may be coupled to liquid in a reservoir by an additional conduit member such as a flexible tube. The entire conduit and the opening 115 in nozzle 113 are filled with the liquid.
To facilitate further description, the section of the above described conduit extending'from dashed line 118 to the face of watch jewel 113 at dashed line 116 will be identified as conduit section 118-116. Line 118 marks the inlet end and line 116 marks the outlet end. The location selected for line 118 is not critical but preferably it is considered to be near or at conduit member 107. The internal cross sectional areas of the various components of conduit section 118-116 are selected so that pressure waves in the liquid may travel from end-to-end of the section without the occurrence of significant reflection within the section.
The polarization of piezoelectric disc 83 is in the thickness direction. Thus, when a voltage of suitable polarity is connected between terminals 88 and 89, the diameter of the disc increases. When the voltage is reduced to zero, the disc returns to its original diameter.
The rim of piezoelectric disc 83 forms part of conduit section 118-116 and is in direct contact with the liquid. O-rings 97,98 which also form part of the conduit prevent the liquid from contacting electrodes 83,85. Thus, when a voltage pulse with polarity that causes increase of diameter is applied to transducer 83 the liquid surrounding the transducer is momentarily compressed. This causes a pressure wave to travel through the liquid in conduit section 118-116 to the outlet end 116 thereof and eject a droplet from nozzle 113. It also causes a pressure wave to travel through the liquid toward inlet end 118. As the latter wave progresses from the rim of transducer disc 83 it causes elastic deformation of the viscoelastic material of sheet 106 progressively along the length of conduit section 118-116, with consequent absorption of wave energy as described in connection with FIG. 1. After the wave passes inlet end 118 it at some point encounters an impedance discontinuity and therefore it is at least partially reflected. As the reflected wave progresses toward nozzle 113 it experiences further attentuation due to energy absorption in the viscoelastic walls of the conduit. The conduit section 118-116 is made sufficiently long so that the reflected wave energy reaching nozzle 1 13 is too weak to interfere with ejection of subsequently initiated droplets.
While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.
What is claimed is:
l. A pulsed droplet ejecting system comprising:
liquid contained in said reservoir;
a conduit communicating with said liquid in said reservoir and filled with said liquid, said conduit comprising a first section having an inlet end and an outlet end;
a nozzle terminating said outlet end of said section and filled with said liquid; and
an electroacoustic transducer coupled to said liquid in said section;
said section, with said transducer coupled to the liquid therein, being dimensioned to conduct pressure waves in said liquid between said ends substantially free of internal reflections and being dimensioned relative to the properties of said liquid to have a given characteristic acoustic impedance;
said transducer being adapted to apply a pressure pulse to said liquid whereby a first pressure wave travels in said liquid in 'said first section to said nozzle and causes ejection of a droplet therefrom, and whereby a second pressure wave travels in said liquid in said first section toward said inlet end of said section;
said conduit comprising a second section which is attached to the inlet end of said first section and is comprised of viscoelastic material and is dimensioned relative to the properties of said liquid and to the properties of said viscoelastic material to have characteristic acoustic impedance substantially matching said characteristic acoustic impedance of said first section so that the said second pressure wave travels in said liquid from said first section into said second section without deleterious reflection at said inlet end of said first section and which is dimensioned relative to properties of said viscoelastic material so that said second wave is substantially fully absorbed by said viscoelastic material.
2. A pulsed droplet ejecting system as described in claim 1 in which said transducer is a piezoelectric transducer.
3. A pulsed droplet ejecting system as described in claim 2 in which said first conduit section is cylindrical and in which said piezoelectric transducer surrounds said section and is in stress transmitting engagement therewith.
4. A pulsed droplet ejecting system as described in claim 3 in which said second section is a viscoelastic tube having inside diameter smaller than the inside diameter, of the inlet end of said first section, except that said second section is enlarged where it is attached to said inlet end with the inner surface of said enlarged portion engaging the outer surface of said inlet end.