Ink jet electrohydrodynamic exciter
BACKGROUND 
     This invention relates to the methods and apparatus for electrohydrodynamically (EHD) generating a continuous stream of fluid droplets. More specifically, this invention relates to an improved ink jet printing system of the type wherein selected droplets from a continuous stream of electrically conductive droplets are diverted to a printing surface or target. 
     Richard G. Sweet disclosed an ink jet printer of the present type in his U.S. Pat. No. 3,596,275. Central to the printer is the generation of droplets. The droplets are preferred to be generated at a fixed frequency with a constant velocity and mass. To achieve this goal, Sweet discloses three techniques represented in his FIGS. 1, 2 and 10. The first technique is to vibrate the nozzle emitting a column of fluid under pressure. The second technique is to excite a fluid column electrohydrodynamically with a single EHD exciter. The third technique is to impose a pressure variation on the fluid in the nozzle by means of a piezoelectric transducer or the like associated with the cavity-feeding the nozzle. This later technique is prevalent in the existing literature and is used in the IBM 66/40 Printing System, a registered trademark and tradename of the International Business Machine Corporation of Armonk, New York. This product lends itself to being characterized as an ink jet typewriter. 
     Heretofore, the EHD exciter has not been an attractive device for promoting the formation of droplets compared to the piezoelectric transducer. For one, the Sweet disclosed EHD device requires very high voltages (roughly 2000-6000 volts) and expensive transformers to obtain them. The high voltages represent an electrical complexity, high cost and safety hazard. As should be appreciated, the high voltages needed to excite or pulsate the fluid column also interfered with the subsequent droplet charging step. In contrast, piezoelectric transducers don&#39;t interfere with charging and require much lower voltages. 
     Accordingly, it is a main object of this invention to overcome the limitations of prior EHD exciters. 
     Another important object of this invention is to devise an EHD exciter capable of operating at efficient voltage levels. 
     Yet a further object of the invention here is to improve ink jet printing systems of the type using a continuous stream of fluid droplets. 
     Still another object of my invention is to optimize the efficiency of a single EHD exciter. 
     Even a further object of the invention is to employ multiple EHD exciters for each nozzle in the formation of droplets from a column of conductive fluid emitted under pressure from the nozzle. 
     Also, it is an object to space multiple EHD exciters relative to a column of conductive fluid emitted under pressure from a conductive nozzle at intervals that permit one exciter to be compressing the fluid column while another exciter is expanding the fluid column. 
     SUMMARY 
     The foregoing and other objects of the present invention are achieved with a single EHD exciter by selecting the length of the single exciter to be about one half the droplet spacing. The objects are achieved with multiple exciters by locating them relative to each other at multiples of approximately one half the droplet spacing. 
     PRIOR ART STATEMENT 
     The above Sweet U.S. Pat. No. 3,596,275 discloses the basic concept of an EHD exciter. However, this disclosure is limited to the fundamental operation and does not suggest the novel improved exciters of the instant invention including the single exciter having a length of a half droplet spacing or the multiple exciters located relative to each other at half droplet spacings. 
     The patent to Ernest Bassous, Lawrence Kuhn and Howard H. Taub, U.S. Pat. No. 3,949,410 discloses an EHD exciter integrated into a nozzle. Specifically, in connection with FIG. 4, they describe the fundamental EHD process first articulated by Sweet in his above patent. Bassous et al report the periodic swelling and non-swelling of a fluid column due to the electric field associated with the geometry at the nozzle orifice. Their disclosure is silent of and unrelated to the droplet spacing as called for in the present invention. At best, they merely state the fluid mechanics principal (at Column 9, lines 41-47) that the wavelength of the swelling (i.e. droplet separation) is given by the velocity of the fluid divided by the frequency of the swelling or perturbations. 
     The John B. Gunn U.S. Pat. No. 3,769,625 discloses a plurality of electrodes adjacent the pertubations in a fluid column prior to droplet formation. The electrodes are charging electrodes and not EHD exciters or pump electrodes as in the present invention. The piezoelectric transducer 12 shown in his FIG. 1 is the exciter for the generation of the droplets. The multiple electrodes adjacent the fluid column and droplet stream are used merely as the charging device for the droplets. The multiple electrodes are switched by a delay line or the like to keep a charging signal in sync with the moving droplet. In his FIG. 4A, the multiple electrodes 14 are shown against timing pulses that synchronize the application of the video signal with the flight of the droplets. In fact, both 4A and 4B illustrate only one electrode activated in the region prior to droplet formation. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the instant invention are apparent from the present specification and the drawings which are: 
     FIG. 1 is a side elevation, in partial section, of a conductive fluid column emitted under pressure from a nozzle, a droplet formed from the fluid column and an EHD exciter according to the present invention using three EHD or pump electrodes for droplet generation. 
     FIG. 2 is a side elevation, in partial section, of a conductive fluid column emitted under pressure from a nozzle, a droplet formed from the fluid column and an EHD exciter according to the present invention using a single EHD or pump electrode for droplet generation. 
     FIG. 3 is a graph of fluid column maximum expansion rate, i.e. velocity away from an initial nozzle orifice diameter, vs. droplet wavelength or spacing expressed in terms of π with 2π being a full cycle or droplet separation. The three curves illustrate the effects of the one, two, and three (or more) pump electrode EHD exciters of my invention. The more electrodes, the sharper and larger the response. 
     FIG. 4 is a schematic of an ink jet printing system using the three pump electrode EHD exciter of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     In the prior art exciters, the length of the exciters have been ignored. Simply put, they are either too short or too long relative to the wavelength of the droplets. The invention here includes the selection of the length t of EHD pump electrode exciters to be from about 0.2S to about 0.8S where S is the spacing between droplets, i.e. wavelength. (See FIGS. 1 and 2). 
     The preferred pump electrode length t is about one half the droplet spacing sice it is the most efficient. The exciters can be odd multiples of S/2 also. For example, a pump electrode length of (5/2)S is as effective as S/2. The reason is that the last S/2 portion of the pump electrode is what effects the desired droplet formation. 
     Another important advance of the invention is to use two or more pump electrodes for droplet formation. The key is to have the force exerted by one electrode reinforced by the force exerted by the other. This is illustrated by the following description where two electrodes are spaced at an odd multiple of S/2 and the varying voltages coupled to each are 180 degrees out of phase. 
     Any phasing may be used provided the voltages coupled to the multiple electrodes are reinforcing each others efforts. For example, if one electrode is exerting a contracting force on a fluid column, the other electrodes should be exerting a reinforcing contracting, expanding or other vector force that tends to promote rather than cancel the first mentioned contracting force. This reinforcement is achievable for any pump electrode spacing provided the phase of the driving voltages are adjusted to reflect the spacing between the two electrodes involved. It is presently understood that the most efficient phasing is either an in-phase or 180 degree out-of-phase relation as described in the following specific embodiments. 
     FIG. 1 shows a fluid Column 1 being emitted from an appropriate nozzle 2. The fluid is forced out of the nozzle by head means (not shown) under a pressure of about 20 to about 120 psi giving the fluid columns or streams having diameters of 1-10 mils a velocity of from about 300-1000 inches per second. The surface tension and other forces tend to create swells in the fluid column that ultimately result in the breakup of the stream into droplets. Sweet (in the above U.S. Pat. No. 3,596,275, the disclosure of which is hereby incorporated by reference) taught that by stimulating or exciting the fluid at or near its natural droplet formation frequency, the droplets in the resultant stream have a fixed spacing, i.e. wavelength, a fixed break-off length, length B in FIGS. 1 and 2, and a constant mass. As reported earlier, Sweet disclosed a nozzle vibrator, an EHD exciter and a piezoelectric transducer as means for periodically stimulating the fluid. As with nearly all basic ideas, however, the disclosed excitation techniques are less than perfect. Sweet&#39;s EHD device in particular requires voltage amplitudes in the range of 5000 volts. The vibrating nozzle and piezoelectric devices are subject to noise that is evident in many ways including a jitter on the breakoff length. To suppress the noise in all three types of exciters, the driving energy must be very large to yield an acceptable signal to noise ratio. In contrast, the instant invention is an EHD exciter capable of operating at low voltages--in the 100 volt range--and with a good signal to noise ratio. Consequently, the droplets streams generated using the EHD exciters of the instant invention exhibit significant stability over prior art exciters of all three types disclosed by Sweet. 
     The EHD exciter 3 includes three pump electrodes 4, 5 and 6. Each electrode is identical being a conductive metal cylinder having an inside diameter larger than the nominal fluid column diameter 7. The pump electrodes are separated by an electrical insulator members 8 and 9. By way of example, the members 8 and 9 are phenolic boards with member 8 plated with copper on two sides to a desired thickness to form pump electrodes 4 and 5 and with member 9 plated with copper on one side to form pump electrode 6. The droplet tunnel 10 is drilled through the sandwich formed when the copper coated boards 8 and 9 are abbutted as shown. The insulators prevent electrical shorting between the pump electrodes. The exciter 3, and the other exciters disclosed herein, can be fabricated from silicon wafers. The wafers can be devised to include the pump electrodes and much of the related circuitry in one integrated unit. 
     The EHD exciter is novel in that it is a multielectrode exciter. In addition, it is novel in that it has a definite geometry vis-a-vis the fluid column. The droplet to droplet spacing of the fluid stream generated from column 1 is the peak to peak spacing S of the swells in the fluid column. The distance S is the droplet wavelength which is calculated for a particular system from the fluid velocity divided by the frequency of droplet generation F. In the instant invention as exemplified by device 3, the pump electrodes 4, 5 and 6 are positioned at consecutive half-wavelength intervals. In addition, the voltage coupled to adjacent pump electrodes is 180 degrees out-of-phase. Therefore, the varying electric field established between the pump electrodes 4, 5 and 6 and the fluid column 1 are such that while electrodes 4 and 6 are causing the column to expand, electrode 5 is causing the column to contract. Since the electrodes are positioned at half-wavelength locations, the pumping action of one reinforces that of the others. The pump electrodes do not produce expansion and contraction of the fluid column immediately. But rather, the exciter electrodes 4, 5 and 6 exert a force on the fluid column that accelerates the fluid to produce the expansions and contractions. Also, the swelling is not necessarily apparent at the pump electrodes as illustrated in FIGS. 1 and 2. 
     An EHD electrode works in the following fashion. A periodic voltage of about 100 volt peak-to-peak amplitude is applied to each pump electrode. The fluid is conductive and is electrically grounded through the conductive nozzle 2 as indicated by grounding means 12. The potential difference between the pump electrode and fluid establishes an electric field that exerts a force on the electric charge near the surface of the fluid column adjacent the electrode. Since the fluid is free in space the fluid volume in the region of the electrode expands and contracts as the magnitude of the potential on the electrode varies relative to the magnitude of the potential coupled to the fluid (ground potential in the case being described). That is, the fluid is accelerated inwardly and outwardly. 
     The varying voltage applied to the pumping electrodes 4, 5 and 6 comes from the electrical source and control circuitry 13. Any suitable circuitry may be used. For example, the electrodes 4 and 6 are coupled to one end of the output coil of a transformer and the electrode 5 is coupled to the opposite end of the same coil. Since the efficiency of exciting the fluid column at halfwave intervals is effective at low voltages, the transformer may be an inexpensive component with from a 1:1 to about a 10:1 turn ratio. The electrical lead lines 14a, 14b and 14c couple the 180° phase shifted voltages to the three pump electrodes. The frequency of the voltage coupled to the pump electrode establishes the frequency of the droplets, i.e. the wavelengths. The relationship between the two is well understood. For example, when the varying voltage has a pure sine wave shape, the droplet frequency is twice that of the voltage. Other voltage wave shapes have different frequency relationships. 
     In the example of FIG. 1, the length of the pump electrodes t are all the same. The length t should be from about S/4 to about (9/20)S for maximum efficiency. The break-off length B and droplet frequency is selected using Lord Raleigh&#39;s analysis for the shortest length and optimum frequency (or wavelength) for a selected stream velocity and fluid column diameter. 
     More than three pump electrodes can be used. The three pump electrode exciter of FIG. 1 is effective. Also effective is a two electrode exciter obtained simply by removing either pump electrode 4 or 6. Where compactness is paramount, the single EHD exciter of FIG. 2 is an excellent choice. Here the nozzle 15 emits the fluid column 16 which is stimulated into a desired droplet frequency by EHD exciter 17. In this single electrode embodiment, the length of the electrode, t, is expanded to about S/2, where S is the wavelength of the droplets in the column and droplet stream 16. The half wavelength, single pump electrode 17 yields excellent results at low voltages as with the EHD exciter 3 of FIG. 1. A comparison of the effectiveness of the three EHD&#39;s recommended herein is given in FIG. 3. 
     In FIG. 3, curves 20, 21 and 22 represent the amplitude of the velocity or swelling rate of a fluid column plotted against the frequency of the droplets normalized by the transit of the fluid in the exciter. Curve 20 represents the performance of the EHD exciter 3 in FIG. 1, curve 21 the performance of a two pump electrode exciter (exciter 3 with either pump electrode 4 or 6 inactivated) and curve 22 the performance of the single pump electrode exciter 17 in FIG. 2. The increase in the number of electrodes yields an increase in response for a given input voltage. Clearly, for a larger, sharper response, four or more pump electrodes can be used. In addition, the curves 20 and 21 indicate that the response is selective near the driving frequency represented by π. Those frequencies in the 0- π/2 and 3π/2-2π region are suppressed, thereby improving the signal to noise ratio. The three pump electrode represented by curve 20, of course, exhibits better noise rejection than even the two pump electrode as represented by curve 21. 
     The single pump electrode represented by curve 22 is one that has a length of about one half the droplet spacing. Although it is not as efficient as the EHD&#39;s exciters represented by curves 20 and 21 or as suppressive of harmonics, it nonetheless, is significantly more efficient than the EHD exciter of Sweet. 
     Another alternative to the above-described one, two and three pump electrode exciters is a combination of the exciters 3 and 17. That is, a multiple pump electrode exciter of excellent efficiency is one wherein each electrode is of a length t of about one half the droplet spacing and the intervals between electrodes is at least one half a droplet spacing. The t=S/2 electrode of FIG. 2 must be spaced at least S/2 from the next electrode unless an infinitely thin insulator 8 of FIG. 1 is used to separate them. The embodiment of FIG. 1 employs a length t of between about S/4 to about (9/20)S. This allows the length t to come as close to S/2 as is practical for most insulators. The sacrifice in optimum length t is offset by the compactness of the EHD device 3. 
     When physical space is available in a given design, the pump electrodes 4, 5 and 6 of FIG. 1 (or four or more electrodes in other designs), may be separated by multiples of S/2. In fact, if the electrode 4, 5 and 6 are spaced a distance S from each other, the varying voltage coupled to them can be the same phase. The phase to those electrodes that are offset by odd multiples of S/2 must have a varying voltage coupled to them that is 180 degrees out of phase with the adjacent electrode. Simply put, the multiple electrodes must be mutually cooperative in exerting a force on the fluid column. The cooperation in terms of phasing is determined by whether a given pump electrode, relative to the others, is operating on a maxima or a minima, i.e. a peak or a valley, in the columns undulations. The spacings between electrodes may be different as long as they are all multiples of S/2. 
     FIG. 4 shows the EHD exciter 3 of FIG. 1 in an ink jet system. The system, except for EHD exciter 3 and associated circuitry, is of the type disclosed in the Sweet patent and for particulars not present here the reader is referred to that reference. A conductive fluid is supplied to nozzle 24 under pressure which emits the fluid column 25. Droplets break off from the column as indicated by droplet 26 at a charging electrode 27. The fluid column extends through the EHD exciter 3 where it is stimulated by electric fields by pump electrodes 4, 5 and 6 as shown in detail in FIG. 1. 
     The droplets, e.g. droplet 26, when uncharged, travel a substantially straight path or trajectory 30 until it impacts target 31. The target is, conventionally, plain paper. The charged droplets, again droplet 24 for example, are deflected by a deflection means including deflection electrodes 32 and 33 to some trajectory between path 30 and the extreme path 34. The exact trajectory varies according to the quantity of charge induced on the droplet at the charging electrode 27. The deflection is in the direction of path 34 for negatively charged droplets when the deflection plates are coupled to ground and +B. Typically, the +B potential is about 2000-4000 volts for plate spacings of 100 mils. 
     When a droplet 26 is charged positively it follows a path below path 30 such as the trajectory 35. This causes the droplets to go into the gutter 36. Therefore, for the system illustrated, except for small positive charge values, the zero and negatively charged droplets go to the target 31 for printing whereas the positively charged particles go to the gutter 36 for collection and circulation back to the nozzle 24. 
     The target 31 is supported for movement in a direction normal to the surface of FIG. 4. The target is propelled by feed rollers 38 and 39. This orientation is 90 degrees to that of the printing system of the IBM 66/40 printing system mentioned at the beginning of this specification. The IBM 66/40 printing system has an ink jet system having a single nozzle mounted on a traversing carriage opposite a sheet of plain paper supported on a typewriter platen. As the carriage traverses the platen, the ink jet stream is deflected vertically to compose a line of characters. The platen is incremented one line to initiate the composition of the next line of characters. 
     In the printing system of FIG. 4, the ink jet apparatus to the left of target 31 is stationary. The motor and motor control circuitry 40, under system control, causes the target to advance normal to the plane of FIG. 4 at a constant velocity. The target velocity is slow compared to the time it takes for a plurality of droplets to compose a line of dots on the target at and between the trajectories 30 and 34. A character is formed on the target in a line by line fashion as the target is fed by the rollers 38 and 39. 
     The ink jet system of FIG. 4 is expandable by cascading the apparatus side by side. In this case, one line of dots is printed by a plurality of adjacent nozzles. The extremely deflected droplets of one nozzle are one droplet (or pixel) position away from the extremely deflected droplets of nozzles on either side. Offset row configuration can also be employed to give an apparent increase in nozzle density. 
     In the plural nozzle system described, separate deflection plates, charging electrodes and gutters similar to items 32, 33, 27 and 36 are required. Also, each nozzle may have its own EHD device 3. However, a common EHD device for all the nozzles is an alternative. The common EHD device is, in one embodiment, a single (or two or more) flat conductive member above or below the ink stream. The length t parallel to the stream is the same as the dimensions t in FIGS. 1 and 2. The flat member extends normal to the dimension t some finite length to span all the other fluid columns emitted by adjacent nozzles. Consequently, the flat member excites all the parallel fluid columns simultaneously. The exciting electric field between the fluid column and single flat member electrode is not as efficient as that established by the cylindrical geometry of FIGS. 1 and 2. However, the efficiencies of the S/2 length t, the S/2 spacing and reinforcing voltage phasing are still sufficient to yield excellent droplet generation. 
     Referring back to FIG. 4, the formation of the droplets is synchronized with the charging process at the charging electrode 27. A system clock at a significant rate above the droplet generation rate is applied to a timing circuit 43. The timing circuit develops timing signals related to the droplet production rate and applies these signals to the video circuit 44, the EHD electrical energy source and control logic circuit 43 and the motor and motor control circuit 40. 
     The timing signal applied to circuit 45 establishes the frequency of the varying potentials applied to the pump electrodes 4, 5 and 6 at the exciter 3. The phase of the three potentials is depended upon the electrodes spacing which for the case shown is the same as the device of FIG. 1. The potential of electrode 5 is 180 degrees out-of-phase with the phases of the potentials coupled to electrodes 4 and 6. The timing signal coupled to the video circuit 44 gates the video input data to the charging electrode 27 at the instant a droplet 26 is formed. The timing signal coupled to motor circuit 40 synchronizes the movement of the target 31 with the sweep of the droplets between the extreme trajectories 30 and 34. The sweep of the droplets occurs at a much higher rate than the movement of the target. 
     Other embodiments and alternatives of the described exciters will be apparent to those skilled in the art based on the foregoing description and drawings. The foregoing alternates are intended to be within the scope of the present invention.