Drop Generator With Rotatable Transducer

Abstract

A drop generator in which a working fluid is forced through orifices in a flexible plate mounted on the drop generating head. The resulting filaments of working fluid break down into drops and the size and spacing of the drops are regulated by propagating a series of bending waves down the length of the orifice plate. The waves are propagated by means of an ultrasonic transducer mounted for rotation about its axis on the drop generator and directly contacting the orifice plate. Fine control over the exact point of contact between the tip of the transducer and the orifice plate is attained by forming the tip offset with respect to the axis of the transducer and rotating the transducer about its axis.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to copending patent applications TWIN ROW DROP GENERATOR, Ser. No. 189,298 and APPARATUS AND METHOD FOR GENERATION OF DROPS, Ser. No. 189,297 filed on an even date herewith and assigned to the same assignee as the present invention.

This invention relates generally to the field of fluid drop generation and the application thereof to jet drop recorders of the type shown in Sweet et al. U.S. Pat. No. 3,373,437 and Taylor et al. U.S. Patent No. 3,560,641. In recorders of this type there are one or more rows of orifices which receive an electrically conductive recording fluid, such as for instance a water base ink, from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. These recorders accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and thereafter depositing at least some of the drops on a moving web of paper or other material.

The above mentioned charging is accomplished by application of control signals to charging electrodes positioned near each of the streams. As each drop breaks off from its parent fluid filament, it carries with it a charge which is in effect a sample of the voltage present on the associated charge electrode at the instant of drop separation. Thereafter the drop passes through an electrostatic field and is deflected in the field direction a distance which is proportional to the magnitude of the drop charge. In a preferred embodiment the drops are charged binarily for print-no-print operation; some drops being uncharged and undeflected for printing, and all other drops being charged to a fixed level and deflected into a catcher.

In order to accomplish reproduction with recorders of the above described type it is necessary to control drop formation with a great deal of precision. Left to natural stimulating disturbances, the streams would break up erratically into drops of various sizes at irregular intervals to produce a recording which at best would be a poor sample of the input control voltages. Accordingly it is customary to apply a fixed frequency, constant magnitude stimulating disturbance to all of the fluid streams. This results in trains of uniformly sized and regularly spaced drops and enables reasonably good sampled data recording.

Various types of magnetostrictive and piezoelectric transducers have been proposed for fluid stream stimulation, and for multiple channel operation the transducer may be coupled to the structure of the fluid manifold as shown in the above mentioned Sweet et al patent or to a fluid supply line as shown in Taylor et al. Unfortunately these prior art systems stimulate drop formation in a phase which varies uncontrollably and unpredictably from stream to stream. This causes a timing uncertainty which may be approximately plus or minus one half of a drop repetition period in the drop charging process and a noticeable drop positional placement error equal to the paper movement distance during that period.

There is a second and more serious difficulty with the above mentioned prior art stimulation systems. This is an acoustical cancellation and reinforcement phenomenon which causes unpredictable stream-to-stream variation in stimulation energy amplitude. Such variations do not affect the size or spacing of the drops, but they markedly vary the lengths of the continuous fluid filaments which supply liquid for the drops. This difference in length may be as much as plus or minus 3 times the drop spacing distance. In high speed photographs the filament-to-filament length difference presents itself as a sort of cusping pattern.

In order to induce a proper charge in the tip of a filament it is necessary that there be some charge electrode surface in the vicinity of the drop breakoff point. Thus it can be seen that the above mentioned filament length variations result in a requirement for a very long electrode; something which is difficult to implement in tightly packed arrays of the type here concerned. Moreover these length variations produce a relatively large drop positional placement error.

This error arises from channel to channel differences in drop flight time; that is, the elapsed time from drop breakoff/charging to impact on the moving web of paper. It is somewhat analogous to a gunnery problem wherein a projectile must be aimed to hit a moving target. Here each drop is programmed to hit the paper at a precise position relative to other drops, and if it must fall a greater or lesser distance than had been anticipated, it will miss. With a web speed adjusted for slight overlap of adjacent printed dots, the above mentioned length difference of plus or minus 3 times the drop spacing distance will produce a printing error in the direction of web movement of plus or minus about 3 printed dot diameters. Such an error is unacceptably large for printing of graphic arts quality.

In the above noted, related application, APPARATUS AND METHOD FOR GENERATION OF DROPS, the stimulation of the fluid streams is attained by utilizing a transducer having a probe directly in contact with the orifice plate through which filaments of working fluid are projected.

In this type of system, as described in that application, it is important to obtain stimulating bending waves of exactly the right mode to obtain stimulation of the desired quality. One of the variables affecting the characteristics of the waves propagated is the point on the surface of the orifice plate where the transducer probe makes contact.

Under ideal circumstances the probe will contact the orifice plate along the center-line thereof and produce bending waves for stimulation as desired. However, it has been found that fully assembled coating heads with their attendant tolerance errors and other unpredictable variables often times fail to stimulate properly. Further it has been observed that this condition may be corrected by fine adjustment of the contact point between the transducer probe and the orifice plate. It is an object of this invention to provide a simple and practical means for such post-assembly adjustment.

In a drop generator in accordance with the present invention fine adjustment of the point of contact between the tip of the transducer probe and the orifice plate can be attained to compensate for any variation between the actual and theoretical contact point.

Thus, the tip of the transducer probe is formed with a point offset with respect to the axis of the transducer, and means are provided for mounting the transducer on the generator for rotation about its axis. In this way the exact point of contact will be shifted as the transducer is rotated about its axis.

FIG. 1 is an exploded perspective view of a recording head assembly;

FIG. 2 is a cross sectional through the assembly of FIG. 1;

FIG. 3 is a cross sectional view through a portion of the assembly of FIG. 1;

FIG. 4 is a view of the transducer probe; and

FIG. 5 is a bottom view of the probe of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of this invention is illustrated in exploded pictorial form in FIG. 1 together with other elements comprising a complete multiple channel recording head assembly 10. As shown in the figure, the various elements of the head are assembled for support by a support bar 12. Assembly thereto is accomplished by attaching the elements by means of machine screws (not shown) to a clamp bar 14 which is in turn connected to the support bar 12 by means of clamp rods 16.

The recording head comprises an orifice plate 18 soldered, welded or otherwise bonded to fluid supply manifold 20 with a pair of wedge-shaped acoustical dampers 22 therebetween. Orifice plate 18 is preferably formed of a relatively stiff material such as stainless steel or nickel coated beryllium-copper but is relatively thin to provide the required flexibility. Preferably dampers 22 are cast in place by pouring polyurethane rubber or other suitable damping material through openings 24 while tilting manifold 20 (orifice plate 18 being attached) at an appropriate angle from the vertical. This is a two step operation as dampers 22 require tilting in opposite directions.

Orifice plate 18 preferably contains two rows of orifices 26 and is stimulated by a stimulator 28 which is mounted on the recording head and carries a probe 30 through the manifold 20 and into direct contact with plate 18. The exact construction of the stimulator and related elements is discussed in detail below. Orifice plate 18, manifold 20, and clamp bar 14 together with a filter plate 32 and O-rings 34, 36, and 38 (see also FIG. 2) comprise a clean package which may be preassembled and kept closed to prevent dirt or foreign material from reaching and clogging orifices 26. Conduit 40 may be provided for flushing of the clean package. Service connections for the recording head include a coating fluid supply tube 42, air exhaust and inlet tubes 44 and 46, and a tube 48 for connection to a pressure transducer (not shown).

Other major elements comprising the recording head are a charge ring plate 50, an electrically conductive deflection ribbon 52, and a pair of catchers 54. Catchers 54 are supported by holders 56 which are fastened directly to fluid supply manifold 20. Spacers 58 and 60 reach through apertures 62 and 64, respectively, in charge ring plate 50 to support holders 56 without stressing or constraining charge ring plate 50. Deflection ribbon 52 is also supported by holders 56 and is stretched tightly therebetween by means of tightening block 66. Ribbon 52 extends between catchers 54 as best shown in FIG. 2.

Catchers 54 are laterally adjustable relative to ribbon 52. This adjustability is accomplished by assembling the head with catchers 54 resting in slots 68 of holders 56, and urging them mutually inward with a pair of elastic bands 70. Adjusting blocks 72 are inserted upwardly through recessed 74 and 76 to bear against faces 78 of catchers 54, and adjusting screws 80 are provided to drive adjusting blocks 72 and catchers 54 outwardly against elastic bands 70. Holders 56 are made of insulative material which may be any available reinforced plastic board.

The fully assembled recording head is shown in cross section in FIG. 2. As therein illustrated coating fluid 82 flows downwardly through orifices 26 forming two rows of streams which break up into drops 84. Drops 84 then pass through two rows of charge rings 86 in charge ring plate 50 and thence into one of the catchers 54 or onto the moving web of paper 88. Switching of drops between "catch" and "deposit" trajectories is accomplished by electrostatic charging and deflection as hereinafter described. Coordinated printing capability is achieved by staggering the two rows of streams in accordance with the teaching of Taylor et al. U.S. Pat. No. 3,560,640. As taught in that patent, the drops in the forward row of steams (i.e. the row most advanced in the direction of web movement) are switched in a time reference frame delayed from that of the rear row by a time d/V where d is the row spacing and V is the web speed. This produces a coherence such that the two rows of streams function as a single row with an effective stream spacing equal to half the actual spacing in either of the real rows.

Formation of drops 84 is closely controlled by application of a constant frequency, controlled amplitude, stimulating disturbance to each of the fluid streams emanating from orifice plate 18. Disturbances for this purpose are set up by operating stimulator 28 to vibrate probe 30 at constant amplitude and frequency against plate 18. As described in detail in the above noted, related application, APPARATUS AND METHOD FOR GENERATION OF DROPS, this causes a continuing series of bending waves to travel the length of plate 18; each wave producing a drop stimulating disturbance each time it passes one of the orifices 26. Dampers 22 prevent reflection and repropagation of these waves. Accordingly each stream comprises an unbroken fluid filament and a series of uniformly sized and regularly spaced drops all in accordance with the well known Rayleigh jet break-up phenomenon.

As each drop 84 if formed it is exposed to the charging influence of one of the charge rings 86. If the drop is to be deflected and caught, an electrical charge is applied to the associated charge ring 86 during the instant of drop formation. This causes an electrical charge to be induced in the tip of the fluid filament and carried away by the drop. A static electrical field is set up between deflection ribbon 52 and the faces of each of the catchers 54 (by opposite polarity electrical charging thereof), and when the drop traverses this field it is deflected to strike the face of the appropriate catcher. Thereafter the drop runs down the face of the catcher, is ingested, and carried off. Drop ingestion may be promoted by application of a suitable vacuum to the ends 90 of catchers 54. When drops which are to deposit on the web 88, no electrical charge is applied to the associated charge rings.

Appropriate charges for accomplishment of the above mentioned drop charging are induced by setting up an electrical potential difference between orifice plate 18 (or any other conductive structure in electrical contact with the coating fluid supply) and each appropriate charge ring 86. These potential differences are created by grounding plate 18 and applying appropriately timed voltage pulses to wires 92 in connectors 94 (only one connector illustrated). Connectors 94 are plugged into receptacles 96 at the edge of charge ring plate 50 and deliver the mentioned voltage pulses over printed circuit lines 98 to charge rings 86. Charge ring plate 50 is fabricated of insulative material and charge rings 86 are merely coatings of conductive material lining the surfaces of orifices in the charge ring plate. Voltage pulses for the above purpose may be generated by circuits of the type disclosed in Taylor et al, and wires 92 receiving these pulses may be matched with charge rings 86 on a one-to-one basis. Alternatively the voltage pulses may be multiplexed to decrease the number of wires and connectors. For such an alternative embodiment solid state demultiplexing circuits may be employed to demultiplex the signals and route the pulses to the proper charge rings. Such solid state circuits may be manufactured by known methods as a permanent part of charge ring plate 50.

The printing head as above described is adapted to be employed in combination with another such head further in accordance with the teachings of Taylor et al. Such a combination will produce solid printing coverage with the streams in each row on 16 mil centers, which is within the state of the art for current orifice plate and charge ring plate manufacturing techniques. The effective stream spacing for the equivalent single row is 4 mils, and this will produce solid printing coverage if each drop makes a printed dot in the order of about 5 mils. Suitable drops for such printed dots may be produced with 1.5 mil orifices, a fluid pressure of about 11 p.s.i. and a stimulation frequency of about 60 KHz. To achieve similar solid coverage in the direction of web travel the speed of web 88 should be set at about 1,200 ft. per sec.

Unexpectedly it has been found that solid printing coverage may be obtained by operating a single printing head as above described but at a reduced web travel speed. In particular, a web speed of about 450 ft. per sec. has been found to be satisfactory. This reduction in web speed results in a decreased longitudinal (i.e. web movement direction) spacing of drop deposit points. In fact when two consecutive drops in one stream are both deposited they tend to pile up and spread in all directions. They behave much like one drop of larger volume, and they fill the laterally adjacent marking cell left empty by omission of the second recording head. This, of course, degrades the resolution of the resulting "print", but a recording head has been saved. For operation in such a mode it is necessary to slow down the rate of the input drop switching data for avoidance of dimensional scaling distortion in the longitudinal direction. Thus a signal which would cause catching of (or permit deposition of) one drop in the faster two head system is stretched to catch on the average about 2.7 drops in the single head system. Catching or deposition of a single drop is not meaningful for the above mentioned single head recorder unless it is desired to make gray scale reproductions as taught for instance in Sweet et al. U.S. Pat. No. 3,373,437.

As noted above it is desirable to be able to provide for precise adjustment of the point of contact between the probe 30 and the surface of the orifice plate 18. Such adjustment is provided in accordance with the practice of this invention by utilizing a probe with an off-center point and a rotatable mounting, the details of which are described below. Initial placement of the probe point for approximately correct contact is accomplished by mounting stimulator 28 in support bar 12 as illustrated. The nominal contact area will ordinarily be along the center-line of orifice plate 18.

Thus, as best seen in FIG. 3 of the drawings, the transducer 130 is received in an elongated hollow casing 100 which in turn is press fitted into a cap 102. Cap 102 has integrally formed therewith a reduced portion 104, threaded on its exterior surface, as at 106. Transducer 130 converts electrical energy to mechanical vibration energy and may be of conventional design including piezoelectric elements, a load mass, and a horn structure, none of which are shown.

The transducer 130 is received in the casing 100 in a manner such that it is free to rotate within the casing about the longitudinal axis thereof. A transducer probe 108 is attached to the transducer by means of a thread 110 formed on the upper end of the probe and received in a complementarily threaded socket 112 in the main portion of the transducer. The probe 108 extends downwardly through the reduced portion 104 and terminates in a point 114, which may be of conical configuration as best seen in FIG. 4 of the drawings.

A ring 116 is fixed in the end of a casing 100 by means of a pin or the like 118 and a coil spring 120 is interposed between the ring 116 and the upper end of the transducer 130, thereby urging the point 114 of the probe into contact with the surface of the orifice plate 18, as best seen in FIG. 3 of the drawings.

Leads 112 are provided for energizing the piezoelectric elements or other driving means in the main body portion of the transducer 130 and these leads may be threaded outwardly through the coil spring 120 and retaining ring 116 to any suitable source of power. The threaded portion 106 of the end cap engages complementary threads on the clamp bar 14 to fix the casing with respect to the drop generator while the transducer 130 and attached probe 108 are free to rotate with respect to the remaining elements of the generator.

As best seen in FIGS. 3 through 5 of the drawings, the point 114 is offset with respect to the longitudinal axis of both the probe 108 and the casing 100. Thus, the transducer may be manually engaged through the openings 124 in the casing 100 and rotated about is longitudinal axis. This will result in a shifting of the point of contact between the point 114 on the probe and the upper surface of the orifice plate.

As a result, small variations between the actual and optimum contact point may be compensated for after the unit has been assembled. In this way bending waves of the desired character may be propagated to provide the stimulation of the filaments of the working fluid.

While the form of apparatus herein described constitutes a preferred embodiments of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention.

Claims

1. Apparatus of the type described: a. an orifice plate having a plurality of orifices formed therethrough, b. means for supplying a working fluid to said orifices, c. an ultrasonic transducer having a body portion and a tip portion, d. said tip portion being offset with respect to the axis of said body portion, and e. means mounting said transducer for rotation about said axis thereof with

2. The apparatus of claim 1 wherein: a. said supplying means includes a support bar, b. said transducer mounting means includes a transducer casing attached to said support bar, and c. said transducer is rotatably received in said casing and extends

3. The apparatus of claim 2 further comprising: a. openings defined through a wall of said casing, b. said transducer being accessible through said openings for rotation

4. The apparatus of claim 2 further comprising: a. spring means received in said casing and urging said transducer into

5. The apparatus of claim 4 further comprising: a. a cap mounted in one end of said casing in spaced relationship to said transducer, b. said spring means being received in said casing between said cap and

6. The apparatus of claim 1 wherein: a. said tip portion comprises a rod member attached at one end to and extending from said transducer body portion, b. said rod member having an opposite end terminating in a point in contact with said orifice plate,

7. The apparatus of claim 6 wherein:

8. Apparatus of the type described comprising: a. an orifice plate having a plurality of regularly spaced orifices formed therethrough, b. a manifold communicating with said orifices, c. a support bar supporting said manifold and said orifice plate, d. means mounted on said support bar for supplying a working fluid to said manifold, e. an elongated hollow transducer casing mounted on said support bar and extending substantially perpendicularly with respect to said orifice plate, f. an ultrasonic transducer rotatably received in said elongated transducer casing for rotation about the longitudinal axis thereof, g. an elongated rod member attached to said transducer and extending outwardly of sad casing in coaxial relationship thereto, h. said rod member terminating in a point offset with respect to the longitudinal axis thereof, i. said rod member extending through said manifold in noncontacting relationship thereto and contacting said orifice plate with said point, j. a cap member received on an outer end of said casing, k. a compression spring received in said casing intermediate said cap member and said transducer and urging said point into contact with said orifice plate, and l. openings formed in a sidewall of said casing to permit said transducer to be engaged for rotation thereof about the axis of said casing to shift the point of contact between said rod member point and said orifice plate.