GB2152877A - Droplet ejector with control of fluid inlet to a reservoir - Google Patents

Abstract

A droplet ejector which is particularly useful as the driving head in a "dot-on-demand" ink jet printer selectively connects an ink supply (30) to an auxiliary reservoir (26) from which ink is expelled a droplet at a time in response to driving pulses. The connection is regulated to isolate the auxiliary reservoir from the main supply during drop ejection, and to thereafter open the auxiliary reservoir to the main supply following droplet ejection in order to replenish the contents of the reservoir. The control is achieved by the use of two synchronised transducers (20,22), the first transducer (20) operating to eject a droplet and the second transducer (22) operating to connect and disconnect the reservoir (26) from the supply (30) (Figs. 2-4 show stages of operation). The ejector provides close velocity control and is readily accommodated to variable-density printing. <IMAGE>

Description

SPECIFICATION Droplet ejector Background of the invention A. Field of the Invention The invention relates to droplet ejectors and, more particularly, comprises a droplet ejector that is particularly useful as the driving head in a "dot-on-demand" ink jet printer.

B. Prior Art Ink jet printers combine reasonable printing quality with fairly high printing speeds. Common types of printer driving heads currently available include continuous jet devices and drop-on-demand devices. In continuous jet devices, a continuous jet of droplets is ejected from a printhead and selected droplets are diverted to a return trough; the remaining droplets impinge on a blank sheet to form the desired image. In drop-on-demand devices, in contrast, droplets are formed only as needed, and each droplet contributes to forming the image.

Drop-on-demand devices have a number of inherent advantages over continuous feed devices, including simpler liquid feeding requirements and the lack of need for electric charging of the droplets for flightpath control purposes. However, they are significantly slower than continuous feed devices, and this has limited their usefulness.

Brief Summary of the Invention A. Objects of the Invention Accordingly, it is an object of the invention to provide an improved droplet ejector.

Further, it is an object of the invention to provide an improved droplet ejector which is particularly suited for dot-on-demand ink jet printing.

Still a further object of the invention is to provide an improved droplet ejector which operates at increased rates of speed.

Brief Description of the Invention In accordance with the present invention, a droplet ejector has a reservoir of limited capacity for storing a liquid therein, the reservoir having an outlet port for ejecting ink therefrom and an inlet port through which the supply of liquid is replenished from a main liquid supply. A driver transducer expels droplets from the reservoir in response to signals applied thereto.

Advantageously, the reservoir comprises a cylindrical tube and the transducer comprises a piezoelectric crystal surrounding the tube and alternately constricting and expanding the bore of the tube to expel liquid therefrom. A useful system of this type is shown in U.S. Patent No. 3,683,212 issued August 8, 1972 to S.l. Zoltan. In accordance with the present invention, a second transducer regulates communications between the outlet port and a main liquid supply. The second transducer, which serves as a gating transducer, blocks the reservoir inlet port during the time that a liquid droplet is being expelled from the outlet port, and thereafter opens the inlet port to allow the main supply to replenish the supply of liauid in the reservoir.

This configuration provides significant advantages in dot-on--demand printers. In particular, blocking the inlet port during droplet ejection prevents dissipation of the droplet driving force which is otherwise caused by backflow of the liquid from the reservoir back to the main supply. Accordingly, a smaller transducer, and less driving energy, is required to expel the droplet from the reservoir. Conversely, for a given driving energy input, a greater proportion of the energy is applied to expelling the droplet, which is therefore accelerated to higher speeds and thus contributes to faster printing rates. Thus, in the case of presently-available doton-demand print heads, only approximately ten percent of the applied driving energy is transmitted to the expelled droplet; the remainder is dissipated primarily in the liquid remaining behind in the print head.In the present invention, in contrast, nearly all the applied energy is transmitted to the ejected droplet, thereby significantly increasing its momentum in response to a given energy input and thus increasing printing speed and facilitating placement control.

In addition to increasing efficiency and printing speed, providing a positive barrier or gate between the reservoir and the main liquid supply effectively decouples the main supply from the reservoir during droplet ejection, and thus allows the dynamics of the reservoir and ejection head to be isolated from the dynamics of the main supply. Accordingly, the characteristics of each can be established largely independently of restrictions otherwise imposed by the other. Additionally, the gate effectively damps oscillations in the reservoir and thereby enables the liquid to return to its quiescent state in a shorter period of time than is the case without such a gate. Accordingly, the interval between droplet ejections can be significantly reduced, and the printing rate thus increased.

By controlling the time during which the gate is opened, the reservoir can be replenished from the main supply in such a manner as to ensure that replenishment is accomplished with minimum system disturbance. In particular, in accordance with the present invention, replenishment of the reservoir does not occur until after ejection of a droplet.

Further, the replenishment time and gate displacement can be closely controlled so that the amount of liquid supplied to the reservoir just equals that removed from it by the ejected droplet. Accordingly, successive droplets have constant mass and a constant print density is thereby maintained. Further, the print density can readily be controllably varied with the present invention when desired, e.g., in graphics reproduction, among other applications.

Detailed Description of the Invention The foregoing and other and further objects and features of the invention will be more readily understood from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which; Figure 1 is a vertical sectional view of a droplet ejector in its quiescent state in accordance with the present invention.

Figure 2, 3 and 4 are vertical sectional views of the droplet ejector of Figure 1 in successive stages of operation; and Figure 5 is a sketch of idealized driving signals applied to the transducers of the ejector, and consequent displacement.

In Figure 1, a droplet ejector 10 is formed from a base 12 having a cylindrical collar 14 from which extends a cylindrical tube 16. A bore 18 at the upper end of the tube serves as an outlet port through which liquid droplets are ejected. A cylindrical transducer 20 surrounds the tube 16 and provides the requisite force for ejecting droplets from it. A second transducer, 22, is located at the lower end of the collar 14 and abuts the bottom face 14a of this collar. The transducer 22, illustratively shown as in the form of a flat rectangular plate, spans across the throat 24 of the collar 14 and blocks it as shown in Figure 1. The throat 24 serves as the inlet port for the tube 16 which thus forms a reservoir 26 for liquid. Liquid is supplied to the base 12, and thus to the tube 16, from a main supply source (not shown) via a tube 30 and chamber 13.

The transducers 20 and 22 preferably are piezoe lectric transducers which are electrically driven from a pulse generator 32 having controls 34a, 34b, 36a, 36b, 38a and 38b for controlling such pa rameters as the amplitude, duration, and phase of electrical pulses applied to the transducers 20 and 22, respectively.

Transducer 20 serves as a driving transducer.

When energized with a driving signal from generator 32, it contracts and thereby compresses the tube 16 so as to diminish the volume of reservoir 26 and cause ejection of a droplet therefrom. The transducer 22, in contrast, serves as a gating trans ducer. It regulates liquid flow into the tube 16 and thus allows replenishment of the reservoir 26. Further, it limits transmission of forces between the reservoir 16 and the main supply via the conduit 30 during ejection of a droplet, and thereby not only enhances efficiency of the ejection operation but also prevents communication of oscillations between the main supply and the reservoir 26 which would otherwise delay return of the reser voir to its quiescent state.

The operation of the droplet ejector may now be understood in detail.

In the quiescent state of the ejector, as shown in Figure 1, the transducers 20 and 22 are deener gized, that is,mo driving voltage is applied to them.

In this state, transducer 20 closes off throat 24 from communication with conduit 30. A slight neg ative meniscus is formed at outlet bore 18.

To start the ejecting process, a driving signal such as a voltage pulse 40 is applied to transducer 20 during a first time interval, t,. The transducer thereupon constricts in diameter and in turn presses inwardly on reservoir 26 to reduce its di ameter. The fluid in the reservoir thereupon under goes a displacement 42 which forces the meniscus from a negative to a positive displacement or state as shown at 44 in Figure 5. At its outermost extension, the liquid droplet 46 (Figure 3) forming the meniscus breaks from the remainder of the fluid in reservoir 26 and begins its travel toward the substrate (e.g., sheet of paper) on which a dot is to be formed. The liquid in reservoir 26, and thus the liquid now forming the new meniscus, remains in displaced condition for the remainder of the applied driving signal, as shown in Figure 5.When this signal terminates, reservoir 26 returns toward its quiescent state, together with the meniscus. As shown in Figure 5, the latter undergoes damped oscillation for a period of time after the driving pulse terminates.

Subsequent to the termination of the driving pulse to transducer 20, a driving pulse 50 is applied to transducer 22. Illustratively, the pulse 50 follows the termination of the pulse supplied to transducer 20 within a quarter of a cycle. Transducer 22 then expands outwardly from throat 24 as illustrated at 52 in Figure 5 (in the drawings, the expansion is in the downward direction as shown in Figures 2-4) and places reservoir 26 in communication with the main reservoir via conduit 30. A small amount of fluid 54 then enters reservoir 26 from conduit 30 via chamber 13 to replace the ejected droplet. As shown in Figure 5, this occurs during the oscillations of the meniscus of reservoir 26 following termination of the driving voltage to transducer 20. The droplet ejector is thereupon ready for repetition of the cycle for the next drop.

As noted previously, the generator 32 is provided with means for establishing the amplitude, duration and phase of each of the signals applied to the transducers 20, 22, respectively. By suitably adjusting these quantities, the user may adjust the driving levels, and the relation between the opening and closing of inlet port 24 with respect to the time of ejection of a droplet from bore 18 in such a manner as to insure the ejection of droplets of uniform mass at a defined rate regardless of variations in the density and other significant physical characteristics of the liquid being utilized. Achievement of uniform drop density is an important characteristic of viable ink jet printers. Of course in some applications it may be desired to vary the mass of the droplet, and thus the dot diameter as applied to the ink-receiving medium, in a controlled manner. For example, varying resolution is often desirable in many graphics applications. This is readily achieved in the present invention by varying such quantities as the rise time and fall time of the driving pulses, as well as their amplitude, duration and phase.

Conclusion From the foregoing, it will be seen that we have provided an improved droplet ejector. The ejector is simple to construct and lends itself to ready calibration for achieving uniform droplet ejection and thus uniform print density when used in an ink jet printer. The ejector isolates the liquid being ejected from communication with the main supply during the ejection phase; this increases the efficiency of the ejection process and also aids in damping os ciiiations which would otherwise increase the required recovery time after droplet ejection and thereby decrease the permissible ejection rate. The liquid is replenished at a defined time subsequent to ejection of the drop in such a manner as to minimize system disturbance. By decoupling droplet ejection from liquid replenishment, the ejection mechanism, and the dynamics of the ejection process, can effectively be optimized independent of the physical restrictions of the replenishment cycle.

Thus, velocity control of the ejected droplet can be greatly enhanced and varying dot diameters, and thus varying image resolution, can readily be achieved.

Having illustrated and described our invention,

Claims (11)

we claim: CLAIMS

1. A droplet ejector for controllably ejecting liquid droplets on demand, comprising A. a reservoir having an inlet port for admitting liquid thereto and an outlet port for ejecting droplets therefrom, B. a first transducer associated with said reservoir and operable in response to electric signals applied thereto to eject a droplet through said outlet port, and C. a second transducer associated with said reservoir and operable in response to electric signals applied thereto to regulate liquid transfer through said inlet port.

2. A droplet ejector according to claim 1 in which said second transducer restricts flow through said inlet port during ejection of said droplet.

3. A droplet ejector according to claim 2 in which said second transducer is operable to replenish liquid in said first reservoir after ejection of a droplet therefrom.

4. A droplet ejector according to claim 3 in which said transducer is positioned to block said inlet port from fluid flow therethrough when de-energized, and is operable to open said inlet port in response to signals applied thereto.

5. A droplet ejector according to claim 4 in which said transducer comprises a piezoelectric crystal.

6. A droplet ejector according to claim 1 in which said second transducer is connected to operate in synchronism with said first transducer for blocking said inlet port during ejection of liquid from said reservoir and for opening said port subsequent thereto for replenishing liquid therein.

7. A droplet ejector according to claim 6 in which said first and second transducers comprise piezoelectric crystals.

8. A droplet ejector according to claim 1 in which said second transducer comprises a piezoelectric transducer blocking said inlet port when in the quiescent state and electrically energizable to open said inlet port for fluid communication with said reservoir.

9. A droplet ejector according to claim 8 which includes means for selectively energizing said second transducer to block said inlet port during ejection of liquid from said reservoir and to open said port from replenishment of third in said reservoir subsequent to liquid ejection therefrom.

10. Apparatus for controllably ejecting liquid droplets on demand, comprising A. means defining a first reservoir having an outlet port for ejecting droplets therefrom, B. means operable to eject a droplet from said reservoir, C. means for selectively coupling said reservoir to a further source of liquid.

11. A droplet ejector substantially as described herein with reference to the accompanying drawings.