Patent Publication Number: US-6702209-B2

Title: Electrostatic fluid ejector with dynamic valve control

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned, U.S. patent applications Ser. No. 10/122,566 filed Apr. 15, 2002, in the names of Christopher N. Delametter et al., entitled DROP-ON-DEMAND LIQUID EMISSION USING INTERCONNECTED DUAL ELECTRODES AS EJECTION DEVICE; and Ser. No. 09/788,797 filed Feb. 19, 2001, in the names of Ravi Sharma et al., entitled IMPROVED DROP-ON-DEMAND INK JET PRINTING WITH CONTROLLED FLUID FLOW DURING DROP EJECTION. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to drop-on-demand liquid emission devices such as, for example, ink jet printers, and more particularly such devices which employ an electrostatic actuator for driving liquid from the device. 
     BACKGROUND OF THE INVENTION 
     Drop-on-demand (DOD) liquid emission devices with electrostatic actuators are known for ink printing systems. U.S. Pat. Nos. 4,520,375; 5,644,341 and 5,668,579 disclose such devices having electrostatic actuators composed of a diaphragm and opposed electrode. The diaphragm is distorted by application of a first voltage to the electrode. Relaxation of the diaphragm expels an ink droplet from a nozzle orifice. Other devices that operate on the principle of electrostatic attraction are disclosed in U.S. Pat. Nos. 5,739,831, 6,127,198, and 6,318,841; and in U.S. Pub. No. 2001/0023523. According to the prior art, an electrostatic attraction force is applied in a single direction, as the electrodes can only attract; repulsion being impractical. Thus, the devices must rely on the elastic memory of the diaphragm to return to an at-rest position. 
     Devices that rely on the elastic memory of the diaphragm to expel liquid drops exhibit a reduction on the liquid-expulsion force over the time period that liquid is being expelled. That is, the speed at which the diaphragm moves as it approached its at-rest position decreases. The result is a tendency for liquid to be expelled at a greater velocity at the beginning of the time period and a lesser velocity at the end of the time period. This often results in the production of undesirable satellite droplets following a main drop. 
     It is known that the force that expels drops from the emission device also causes some liquid to flow backward toward the liquid reservoir. The backward flow of liquid diverted from the nozzle orifice further reduces the velocity of the liquid being emitted from the nozzle orifice. 
     SUMMARY OF THE INVENTION 
     A drop-on-demand liquid emission device, such as for example an ink jet printer, includes a member movable through a path for driving liquid from the device, wherein the speed at which the member moves is reduced over the time period that liquid is being expelled. During that time period, a portion of the liquid flows through a passage away from the nozzle orifice. According to a feature of the present invention, a variable flow restrictor increases the resistance to flow through the passage during the time period that liquid is being expelled; thereby tending to compensate for the reduction of the liquid-expulsion force over the time period. The result is a reduction of undesirable satellite droplets following a main drop. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a drop-on-demand liquid emission device according to the present invention; 
     FIG. 2 is a cross-sectional view of a portion of drop-on-demand liquid emission device of FIG. 1; 
     FIGS. 3-5 are top plan views of alternative embodiments of a nozzle plate of the drop-on-demand liquid emission device of FIGS. 1 and 2; and 
     FIG. 6 is a cross-sectional view of the drop-on-demand liquid emission device of FIG. 2 shown in an actuation stage. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     As described in detail herein below, the present invention provides an apparatus and method of operating a drop-on-demand liquid emission device. The most familiar of such devices are used as printheads in ink jet printing systems. Many other applications are emerging which make use of devices similar to ink jet printheads, but which emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. The inventions described below provide apparatus and methods for operating drop emitters based on electrostatic actuators so as to improve energy efficiency and overall drop emission productivity. 
     FIG. 1 shows a schematic representation of a drop-on-demand liquid emission device  10 , such as an ink jet printer, which may be operated according to the present invention. The system includes a source  12  of data (say, image data) which provides signals that are interpreted by a controller  14  as being commands to emit drops. Controller  14  outputs signals to a source  16  of electrical energy pulses which are inputted to a drop-on-demand liquid emission device such as an ink jet printer  18 . 
     Drop-on-demand liquid emission device  10  includes a plurality of electrostatic drop ejection mechanisms  20 . FIG. 2 is a cross-sectional view of one of the plurality of electrostatically actuated drop ejection mechanisms  20 . A nozzle orifice  22  is formed in a nozzle plate  24  for each mechanism  20 . A wall or walls  26  that carry an electrically addressable electrode  28  bound each drop ejection mechanism  20 . The outer periphery of electrode  28  is sealingly attached to wall  26  to define a liquid chamber  30  adapted to receive the liquid, such as for example ink, to be ejected from nozzle orifice  22 . The liquid is drawn into chamber  30  through one or more ports  32  from a supply, not shown. Ports  32  are sized as discussed below. Dielectric fluid fills the region  34  on the side of electrode  28  opposed to chamber  30 . The dielectric fluid is preferably air or other dielectric gas, although a dielectric liquid may be used. Addressable electrode  28  is preferably at least partially flexible and carries a rigid piston  35 . The piston has baffle members  36  aligned with ports  32 . A ground electrode  38  is generally axially aligned with addressable electrode  28  and nozzle orifice  22 . 
     FIGS. 3-5 are top plan views of nozzle plate  24 , showing several alternative embodiments of layout patterns for the several nozzle orifices  22  of a print head. Note that in FIGS. 2 and 3, the interior surface of walls  26  are annular, while in FIG. 5, walls  26  form rectangular chambers. Other shapes are of course possible, and these drawings are merely intended to convey the understanding that alternatives are possible within the spirit and scope of the present invention. 
     Referring to FIG. 6, to eject a drop, an electrostatic charge is applied to the addressable electrode  28 , which pulls that electrode toward ground electrode  38  and away from the nozzle orifice, as indicated. Since this electrode forms a wall portion of liquid chamber  30  behind the nozzle orifice, movement of electrode  28  away from nozzle plate  24  expands the chamber, drawing liquid through ports  32 , past opening baffles  36 , and into the expanding chamber  30 . 
     Subsequently (say, several microsecond later) addressable electrode  28  is de-energized, causing addressable electrode  28  to return from the state illustrated in FIG. 6 towards its state depicted in FIG. 2 under the sole force of stored elastic potential energy in the system. This action pressurizes the liquid in chamber  30  behind the nozzle orifice, causing a drop to be ejected from the nozzle orifice with an initial velocity. A fair amount of liquid escapes from chamber  30  through ports  32 , which should be properly sized to present sufficiently low flow resistance so that filling of chamber  30  is not significantly impeded when electrode  38  is energized, and yet present sufficiently high flow resistance so that the back flow of liquid through the ports is not of serious consequence. 
     As the movement of piston  35  progresses towards nozzle orifice  22 , more and more of ports  32  are covered by moving baffles  36 . This increases the flow resistance through the ports (variably restricting flow through ports  32 ) at the same time that the force of stored elastic potential energy in the system is decreasing. Thus, a greater percentage of displaced liquid is ejected through nozzle orifice  22  rather than through ports  32 . This tends to cancel out the tendency for liquid to be expelled through nozzle orifice  22  at a greater velocity at the beginning of the time period and a lesser velocity at the end of the time period. This, in turn, inhibits the production of undesirable satellite droplets following a main drop. 
     The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modification and variations are possible and will be recognized by one skilled in the art in light of the above teachings. Such additional embodiments fall within the spirit and scope of the appended claims.