Shear mode transducer for drop-on-demand liquid ejector
This is a continuation of Ser. No. 382,866, filed May 28, 1982, now abandoned. 
    
    
     The invention relates to a drop-on-demand liquid droplet ejecting apparatus wherein a single piezoelectric transducer used in a shear mode is shared by more than one ejector. The transducer is segmented for operation in the shear mode by utilizing a series of electrodes formed on its surface. This invention can be utilized in any pressure pulse drop ejector apparatus; however, the greatest benefits are realized when the shear mode transducer concept of this invention is utilized in a drop-on-demand ink jet printing system. Accordingly, the present invention will be described in connection with such an ink jet recording system. 
     Ink jet printers are well known in the art, many commercial units being presently on the market. Generally, these ink jet printers utilize a piston-like push-pull action to eject ink drops from a small nozzle to form an image. Typically, a piezoelectric transducer is used to provide the piston-like action. A piezoelectric transducer is a device that converts electrical energy into mechanical energy. Several transducer arrangements have been proposed for drop-on-demand ink jet printers. In U.S. Pat. No. 2,512,743 to C. W. Hansell, issued June 27, 1950, an ink jet was described in which the circular piezoelectric transducer was used in an extensional mode, the extension being along the axis to drive ink. The piezoelectric transducer was arranged coaxially with a conical nozzle, the axial extension used to create pressure waves causing expression of droplets from the nozzle. 
     Another basic arrangement was disclosed in &#34;The Piezoelectric Capillary Injector--A New Hydrodynamic Method for Dot Pattern Generation&#34;, IEEE Transactions on Electron Devices, January, 1973, pp. 14-19. In the system disclosed, a bilaminar piezoelectric metallic disk is used to drive ink coaxially with the bilaminar disk. In that system, application of an electrical voltage pulse across the disk causes the disk to contract resulting in the deflection of the disk into the ink, forcing droplet ejection. U.S. Pat. No. 3,946,398, issued Mar. 23, 1976, shows a similar device; however, as disclosed in that patent, the deflection of the disk is used to eject ink through an orifice, the axis of drop ejection being perpendicular to the axis of the disk. 
     Another arrangement is shown in U.S. Pat. No. 3,857,049, issued Dec. 24, 1974. In the arrangement shown in FIG. 1 through FIG. 4 of that patent, a tubular transducer surrounds a channel containing the ink; and the transducer, when excited by application of an electrical voltage pulse, squeezes the channel to eject a droplet. As shown in FIG. 6 of that patent, there is disclosed a system in which the radial expansion of a disk in response to an electrical voltage pulse is used to compress ink in circumferential channels thereby forcing ink droplets out of a nozzle. In U.S. Pat. No. 4,243,995, issued Jan. 6, 1981, to us there is shown a drop-on demand ink jet printer in which a rectangular piezoelectric transducer is arranged abaxially over an ink-containing channel with an edge in operating relationship with the channel. 
     In each of the above examples, the excitation electrical field is applied parallel to the direction of transducer polarity. Also, in all of these examples, each individual jet has its own discrete transducer. Such structures are relatively time-consuming and expensive to manufacture. The invention as claimed is intended to provide an improved drop-on-demand ink jet printer which is relatively simple and inexpensive to manufacture. This is accomplished by utilizing a single transducer in the shear mode to provide the driving pulse for a plurality of jets. To do this, the transducer is provided with a plurality of electrode segments, each segment associated with a separate ink channel. 
     The invention can better be understood by reference to the following description particularly when taken in conjunction with the attached drawing which shows a preferred embodiment. Thicknesses and other dimensions have been exaggerated as deemed necessary for explanatory purposes. 
    
    
     FIGS. 1A and 1B show greatly exaggerated how the shear mode electrical excitation, that is, the excitation potential is applied orthogonal to the direction of polarization of the transducer, affects a piezoelectric transducer segment. 
     FIG. 2 is a side view of a larger section of a piezoelectric transducer showing greatly exaggerated how the piezoelectric transducer is deflected by the shear mode excitation of the transducer. 
     FIG. 3 is a top view of an ejector array in accordance with the present invention. 
     FIG. 4 is a cross-sectional view of the ejector array of FIG. 3 taken along line 4--4 in FIG. 3. 
     FIG. 5 is a perspective view of the piezoelectric member only showing the electrode arrangement and a schematic representation of a drive circuit for the array of FIG. 3. 
     In all of the Figures, the same parts are given the same number designations. The Figures are not drawn to scale. 
    
    
     Referring now to FIGS. 1A and 1B, there is shown a piezoelectric member 3 rectangular segment S. The piezoelectric member 3 is polarized in the direction P in this exemplary instance. Referring to FIG. 1B, application of a potential between electrodes E 1  and E 2 , in the direction or vector indicated by arrow E orthogonal to the direction of polarization P, causes internal shear within segment S causing a distortion of segment S as shown by comparing FIG. 1A with no potential applied with FIG. 1B with potential applied. This principle can be utilized to provide a deflecting member useful as a driver in a pressure pulse ejector as can be understood by reference to FIG. 2. 
     Referring now to FIG. 2, there is shown a side view of a piezoelectric member 3 in its fully deflected position with electrodes 5, 7, 9 and 11 formed thereon as shown. A more detailed description of the electroded array appears below in connection with FIGS. 3-5. In FIG. 2, electrodes 9 and 11 are made, in this exemplary instance, positive and electrodes 5 and 7, negative. The resulting electric field vector is shown as E. The piezoelectric material 3 shears in the direction of the cross product of the polarization vector P and the electric field vector E causing the piezoelectric member 3 in the vicinity of electrodes 9 and 11 to deflect in the direction shown by arrow 27 to the position depicted in FIG. 2. Although in FIG. 2, the electrodes 11 and 7 on the lower surface of piezoelectric member 3 are illustrated as excited, it has been shown that, due to the high capacitance coupling between electrodes 11 and 9 and the high capacitance coupling between 7 and 5, it is not necessary to independently excite electrodes 11 and 7 to have piezoelectric member 3 shear or deflect to the position shown in FIG. 2. 
     Referring now to FIGS. 3, 4 and 5, there is seen ejector array generally designated 1, which, in this exemplary instance, comprises three ejectors. Ejector array 1 has a single piezoelectric member 3 for driving the three ejectors. Piezoelectric member 3 has electrodes 5, 7, 9a, 9b, 9c and 11a, 11b, 11c formed on its surfaces as shown in the Figures. Piezoelectric member 3 is attached to ink jet ejector body 15 (see FIG. 4). Ejector body 15 has, in this exemplary instance, three ink channels 21 formed in it. Ink channels 21 are connected to ink channel outlet orifices 23 by reduced sections 24. A source of ink (not shown) is connected to ink channels 21 by similar reduced sections 26. Ink channels 21 and ink channel body 15 are separated from piezoelectric member 3 by an isolating layer 17 (see FIG. 4). A reaction block 25 is attached to the opposite surface of piezoelectric member 3. A shown in FIG. 5 in this exemplary embodiment, electrode 5 is connected to one side of power supply 29, and active electrodes 9 are connected by controller 19 to the other side of power supply 29. A controller 19 is provided, which responds to an input image signal representative of the image it is desired to print by closing and opening selected ones of switches 31. In order for the piezoelectric member 3 to operate as a source of driving pulses for ink contained in ink channels 21, it is necessary to first polarize the piezoelectric member 3. This is usually done by the manufacturer and entails applying a DC potential difference across the narrow dimension in direction P (see FIG. 2) of the whole of the piezoelectric member 3 between the surface on which electrode 5 is formed and the surface on which electrode 7 is formed. In order to drive individual ejectors, which is required for drop-on-demand ink jet printers, it is necessary to divide the piezoelectric member 3 into discrete deformable sections. This is accomplished by providing a series of electrodes 9 on piezoelectric member 3, each electrode 9 corresponding to an ink channel 21. Application of an electrical potential difference of the proper polarity between electrode 5 and an electrode 9 will cause piezoelectric member 3 to deform into the ink channel 21, which is located under the activated or pulsed electrode 9, causing compression of the ink contained in ink channel 21 and the resultant ejection of an ink droplet from ink channel outlet orifice 23. 
     To increase the efficiency of operation and to minimize cross-coupling, a reaction clamp block 25 may be used. The purpose of this block is to provide a strong footing against which the piezoelectric member 3 can push. Reaction clamp block 25 may conveniently be bonded to electrode 5 by insulating adhesive layer 31. Reaction clamp block 25 is shaped approximately the same as electrode 5 so as not to interfere with the deflection of piezoelectric member 3 under electrodes 9. 
     In operation, ink channels 21 are filled with ink through reduced sections 26 from an ink supply source not shown. A controller 19, which responds to an input image signal (not shown) closes the appropriate switch, which applies an electrical potential difference from power supply 29 between electrode 9 and surrounding electrode 5. Typical drive circuits for drop-on-demand ink jet ejectors are well known in the art (see, for example, U.S. Pat. No. 4,216,483, issued Aug. 5, 1980, U.S. Pat. No. 4,266,232, issued May 5, 1981, and copending commonly assigned application Ser. No. 257,699, filed Apr. 27, 1981). 
     Referring now specifically to FIG. 5, controller 19 has closed switch 31b leading to electrode 9b on the center ejector. By closing switch 31b, power supply 29 is connected such that an electrical pulse is applied between electrode 9b and surrounding electrode 5 causing piezoelectric member 3 to deflect in the direction shown by arrow 27. Deflection of piezoelectric member 3 into ink channel 21b causes a droplet (not shown) to be ejected from orifice 23b (see FIG. 3). 
     It can be seen that electrodes 7 and 11a, 11b, 11c need not be involved in the operation of the ejector. Also, it can readily be seen that the same principle of operation can apply to an array of indefinite length, the practical limiting factor being the length of piezoelectric material, which is commercially vailable. As an example, a three-jet ejector array was made from a 0.3 by 0.64 by 0.015 inch piezoelectric member 3 having nickel electrodes on both major surfaces and having been polarized by the manufacturer. Such piezoelectric members 3 are available commercially from Vernitron Piezoelectric Division, Bedford, Ohio. The piezoelectric member 3 is masked and portions of the nickel removed to form the pattern as shown in the Figures on both the upper and lower surfaces. Electrical lead-in wires 33 and 35 are then connected to electrodes 9 and 5, respectively. The entire surface on which electrodes 7 and 11 are formed is coated with an epoxy layer 17, which acts as a seal for ink channels 21 when ejector body 15 is attached to piezoelectric member 3. Ejector body 15 measures approximately 0.3 by 0.64 by 0.125 inches and is made of castable epoxy Stycast 1267, available from Emerson &amp; Cuming, Inc., Canton, Mass. The ink channels measure approximately 0.12 inches wide by 0.010 inches deep. The outlet orifice is approximately 0.002 inches in diameter. The epoxy layer is about 0.0006 inches thick. A brass block, shaped similar to electrode 5 and being about 0.125 inches thick, may, if desired, then be bonded to electrode 5 using Stycast 1267 epoxy, available from Emerson &amp; Cuming, Inc. Electrodes 9a-c and 11a-c measure about 0.08 inches by 0.22 inches. The space between electrodes 9a-c and electrode 5 is about 0.02 inches. The space between electrodes 11a-c and electrode 7 is the same. A 20-microsecond electrical potential application between electrodes 9 and 5 of about 200 volts at a frequency of up to and exceeding 6000 hertz has been found to be useful in a drop-on-demand ink jet ejector environment. 
     Although the present invention has been disclosed in connection with a preferred embodiment, it is to be understood that the invention is entitled to the protection as described in the appended claims.