05/096083

04/11/1972

12/08/1970

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The Mead Corporation (Dayton, OH)

347/76

361/226, 209/3, 347/1, 239/690, 209/127.100

B41J2/105; B41J2/07; G01D15/18

346/1,75,140 239/3,15 209/127R,127C,3 178/6.6 317/3

Hartary, Joseph W.

What is claimed is

1. In a particle sorting apparatus comprising means for producing a progression of uniformly sized and regularly spaced particles, means for coding the progression of particles by selective impressment of a predetermined electrical charge on only some of said particles, and means for switching the charged particles into a diverted trajectory; the improvement wherein said last named means comprises electrically conductive surface means of laterally non-symmetrical configuration with respect to the initial trajectory of said particles.

2. The improvement of claim 1 further comprising means to catch the diverted particles and means to produce a visual trace of the non-diverted particles.

3. The improvement of claim 1 said electrically conductive surface being tilted in the direction of the diverted trajectory.

4. The improvement of claim 1 said electrically conductive surface means being configured and positioned to carry in response to an electrical charge impressed on a particle as aforesaid a sheet of induced electrical charge of sufficient magnitude to attract said particle away from its initial trajectory with a lateral acceleration of at least 350 meters per sec. per sec.

5. Recording apparatus comprising particle sorting means and means to produce a visual record of particles sorted by said sorting means; said sorting means comprising:

6. means for projecting a series of particles in a spaced progression,

7. means for applying an electrical charge to selected particles in said progression, and

8. an electrically conductive surface positioned near the path of said progression and configured to enable nearby charged particles to induce thereon sheets of electrical surface charge of distribution and strength for displacing the inducing particles into laterally sorted trajectories.

9. Fluid drop marking apparatus comprising:

10. Apparatus according to claim 6 said means for generating a stream of uniformly sized and regularly spaced fluid marking drops comprising:

11. Apparatus according to claim 7 said electrically conductive surface being tilted away from the path of said stream in the direction of drop deflection.

12. Apparatus according to claim 7 said electrically conductive surface extending upwardly into the region where said fluid filament breaks up into drops, and said means for creating an electrical field comprising means for establishing an electric potential difference between the conductive surface and the fluid filament.

13. Apparatus according to claim 9 said electrically conductive surface being a surface of revolution with its axis parallel to the path of said stream and offset therefrom.

14. Apparatus according to claim 7 said deflection surface being at the same electric potential as the fluid filament and said means for creating an electrical field comprising:

15. Apparatus for fluid drop recording without a steady state deflection field comprising:

16. Fluid drop marking apparatus according to claim 12 said means for producing a laterally deflecting self-induced electrical field for each charged drop comprising a common electrically conductive surface positioned near all of said streams and configured whereby the sheets of electrical charge induced thereon in response to the charges on nearby drops each produce a resultant lateral electrical field in the region of the sheet inducing drop.

17. Fluid drop marking apparatus according to claim 13 said nozzles being arranged along a straight line.

18. Fluid drop marking apparatus according to claim 14 the induced sheets of electrical charge each being of sufficient magnitude to attract the inducing particle away from its initial trajectory with a lateral acceleration in the order of about 5,600 meters per sec. per sec.

19. In a particle sorting process comprising the steps of:

20. projecting a series of particles in a spaced progression,

21. applying an electrical charge to selected particles in said progression, and

22. deflecting the charged particles laterally by the action of an electrical field;

23. Method of recording an image comprising the steps of:

24. producing a bi-level control signal representative in time of the information content of an image to be recorded,

25. producing a progression of uniformly sized and regularly spaced particles of marking material,

26. applying an electrical charge of predetermined magnitude to all of said particles produced while said control signal is at one of its levels; all particles produced while the control signal is at the other of its levels being given no electrical charge,

27. directing all of said particles through a region of influence of an electrically conductive surface for deflection of charged particles by self induced deflection fields,

28. catching the particles deflected as aforesaid, and

29. moving a recording surface through the path of the uncharged and undeflected particles for deposition thereon of said uncharged particles.

30. Method according to claim 17 said step of producing a progression of uniformly sized and regularly spaced particles of marking material comprising the further steps of:

31. producing a filament of fluid marking material, and

32. generating said particles by applying a constant frequency disturbance to said filament.

33. Method according to claim 18 said step of applying an electrical charge to said particles comprising the step of establishing an electric field of predetermined strength in the region of the tip of said filament when the control signal is at its first mentioned level.

34. Method according to claim 19 said self-induced deflection fields being of strength for initial lateral acceleration of the inducing drops in the order of about 5,600 meters per sec. per sec.

BACKGROUND OF THE INVENTION

This invention relates generally to particle sorting apparatus of the type wherein the particles are projected through space in a spaced progression and are selectively displaced laterally in accordance with electric charges impressed thereon. Typical prior art devices of this type are shown in Fulwyler U.S. Pat. No. 3,380,584 and Sweet et al. U.S. Pat. No. 3,373,547; the former sorting particles in accordance with distinctive particle characteristics and the latter sorting in accordance with variations in a corresponding input intelligence signal. Both of these prior art devices work with liquid particles and both accomplish lateral displacement of the particle by creating a steady state electrical field. The lateral accelerating force in these systems is proportional to the magnitude of the applied field and to the magnitude of the charge impressed upon the particles.

In the above mentioned prior art devices it is necessary to isolate the particle (drop) forming region from the deflection field in order to avoid inducing spurious charges in the drops. This isolation may be provided by space along in a relatively large sorting device or by a separate guard electrode. As an alternative one may provide compensation in lieu of isolation. In either event the remedy becomes more difficult with decreasing size and with an increasing number of channels.

Another disadvantage of the prior art which is present in multiple channel configurations is that of crosstalk between channels. Consider for example a sorting device having two channels each of which operates in a maximum-charge/zero-charge binary mode. If a particle in one channel is supposed to be uncharged but actually receives say 10 percent of a maximum charge because of crosstalk from the adjacent channel, then the spuriously charged particle will be deflected 10 percent of the full scale "catch" deflection. This is an error which could not be tolerated in a precision recording process.

SUMMARY OF THE INVENTION

This invention overcomes the disadvantages of the prior art by eliminating the steady state deflection field and accomplishing particle deflection by a self-induced deflection field. Each charged particle sets up its own deflection field and is enabled to do so by an electrically conductive surface placed near the particle path. Depending upon the configuration and position of the conducting surface, there is set up thereon a sheet of electrical charge having a distribution which may be determined by using known field theory techniques. This sheet of induced charge is opposite in sign to that of the inducing charge and has an associated electrical field which attracts the charged particle toward the conductive surface. In this regard it is essential that the self-induced electrical field have a net resultant lateral component along the base non-perturbed particle path. The consequence of this requirement is a corollary requirement that the configuration of all conducting surface area influencing the charged particles have no plane of symmetry passing through the base particle path. In other words, if the conductive surface is a plane parallel to the base path, there may be no similar plane equidistant on the other side of the base path; otherwise there will be a cancellation effect and no net resultant induced deflecting field. Similarly, if the conductive surface is a cylinder, it may have its axis parallel to the base particle path but not colinear therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a single channel particle sorting apparatus which charges liquid particles on a binary basis and employs self-induced electrical fields to segregate charged particles from uncharged particles.

FIG. 2 is a diagrammatic illustration of the distribution of actual and image charges induced in a conductive wall due to the presence of a nearby charged particle.

FIG. 3 is a schematic illustration of a typical prior art fluid drop recording apparatus.

FIG. 4 is a schematic representation, partially in section, of a printing head using a common conductive surface for self-induced deflection of a linear array of charged drops.

FIG. 5 is a schematic representation of a particle sorting apparatus employing self-induced deflection for fluid particles generated in a circular array.

DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiment of the present invention is shown schematically in FIG. 1 wherein a conductive fluid 1 flows through an orifice 2 in an electrically conductive plate 3 forming a filament 4 and thereafter breaks up into drops 5, 5a; some of which 5a are caught by catcher 6. The fluid is forced through orifice 2 under pressure and is stimulated to break up into uniformly sized and regularly spaced drops by a constant frequency oscillating transducer (not shown) in communication with the orifice assembly or fluid supply. An electrically conductive charge strip 7 is positioned adjacent filament 4 and is connected to one side of a source of electric potential 8. The other side of source 8 is connected to plate 3 for control of the potential of filament 4. Alternatively, in the case where the orifice plate 3 may be non-conductive, source 8 may communicate directly with fluid 1. In either event source 8 produces a potential difference between strip 7 and filament 4 in response to a control signal applied to input terminals 9. As is well known in the art, this potential difference causes electrical charges to appear on filament 4, and some of this charge is carried away by the drops when they separate. Drops 5a carry such a charge and follow a curved trajectory 10 as shown. Drops 5 are uncharged due to a zero magnitude signal having been present at input terminals 9 at the respective instants when those drops separated from filament 4. Drops 5 follow a straight trajectory 11.

Drops 5a are deflected to follow trajectory 10 by self-induced attraction to conductive deflection strip 12. Normally strip 12 will be grounded and mounted on an insulated wall 13. A vacuum source (not shown) is connected to porous plate 14 and draws off drops 5a as they are caught by catcher 6.

As is subsequently explained, the attractive force operating on any drop 5a is inversely proportional to the square of the distance between the drop and strip 12. Therefore, it is desirable to position strip 12 very close to filament 4. At the same time, however, it is desirable that trajectory 10 curve back far enough to guarantee a clean catch for drops 5a. This means that the lower portion of strip 12 ought to be positioned relatively far away from trajectory 11. These somewhat conflicting requirements can both be satisfied by angling or tilting strip 12 back away from the drop stream as illustrated in FIG. 1. Typically, the tilt angle may be about 2.5°. Associated with this tilt angle would be a drop mass of about 6.2 × 10 ¹² kg., a drop diameter of about 23 microns, a drop velocity of about 9 meters per sec., a deflection strip length of about 1.25 mm, and a total distance of about 2.5 mm from the orifice plate to the catcher. The distance between filament 4 and charge strip 7 would be about 25 microns and the potential difference about 200 volts. This in turn will produce a drop charge of about 10 ^-13 coul. and a drop deflection of about 56 microns or about 2.5 drop diameters. Thus, it is seen that this invention finds utility primarily in very small devices.

The operation of the deflection strip is best understood by reference to FIG. 2 wherein a charged drop 15 is falling past a conductive wall 16. Drop 15 is assumed to carry a negative charge which distributes itself about the drop surface as at 17. In general the negative charge distribution will be somewhat more dense on the side of the drop facing wall 16.

It is well known that when an electrical wall is placed near a conductive surface, there is induced on that surface a distributed charge of opposite sign. Thus positive charges 18 are distributed along the surface of wall 16 as shown, with the charge density greatest in the area nearest drop 15. Lines 19 represent the electric field set up between drop surface charges 17 and wall surface charges 18. This field intersects the wall surface everywhere at right angles. The wall 16, being a conductor, is a volume of constant potential, and therefore there is no electrical field within the wall interior. The total electric field as illustrated in FIG. 2 is the sum of the applied field of the charges 17 and the induced field of the charges 18.

This invention depends for its operation upon the force exerted against drop 15 by the induced field of charges 18. This force can be calculated by assuming charges 17 to be concentrated at a point within drop 15 and summing the incremental forces exerted against this concentrated charge by the distributed charges on each elemental area of the surface of wall 16. Unfortunately the resulting surface integral cannot be solved without knowledge of the distribution of charges 18. In the general case (including conducting walls of various shapes) this distribution can be determined only by solving a very involved boundary value problem. It can be shown, however, that distributed charges 18 may be replaced (for mathematical purposes) by image charges 21 arranged around the surface of an image drop 20. See for instance a discussion on the theory of images in the book "Electromagnetic Theory" by J. A. Stratton, McGraw-Hill Book Company, Inc., 1941 at Sec. 3.18. Making this replacement and applying Coulomb's law to the resulting charge configuration, one finds the force on drop 15 to be given approximately by the equation:

F =(1/4πε _o ) (Q ² /(2d) ² )

where Q is the total charge on drop 15, ε _o is the permittivity of air and d is the distance from drop 15 to the surface of wall 16.

The operation of this invention is to be contrasted to the operation of a prior art system as illustrated in FIG. 3. There a supply of conductive fluid 22 is forced under pressure through a nozzle 23 to form a filament 24. Vibrator 25 stimulates filament 24 to break up into uniform regularly spaced drops 26,26a which are either caught by catcher 27 or deposited on receiving member 28. Drops 26 are uncharged while drops 26a are charged due to an appropriately timed potential difference applied between charge tunnel 29 and filament 24. The required potential difference is produced by source 30 under control of a signal applied at terminals 31.

The force which the illustrated prior art system requires to sort charged drops 26a from uncharged drops 26 is provided by a pair of deflection plates 32. Fixed potential source 33 is connected to create a potential difference between plates 32 and this in turn produces a force exerting electric deflection field. Comparing this with the present invention, it will be appreciated that each charged drop 26a will see an oppositely charged image drop in each plate 32 which may or may not provide a non-negligible self-induced deflecting force, depending upon the distance from the drop to the plate. Such a force would be in addition to the force exerted by the steady state deflection field. However, any such self-induced deflection force is balanced out, at least initially, by a similar force attracting the drop to the other plate. This cancellation of self-induced deflection forces will occur in any case wherein the configuration of electrically conductive surfaces influencing the drops is symmetrical with respect to the base or non-diverted particle path. In the practice of the instant invention the electrically conductive surface means has a non-symmetrical configuration with respect to the initial or non-diverted particle path, and there is no cancellation of the self-induced deflection forces. Thus a particle sorting device made in accordance with the present invention requires no steady state electrical field for deflection of the charged particles.

A printing head 34 embodying the present invention is illustrated in schematic cut-away form in FIG. 4. This embodiment has a row of orifices 43 in an orifice plate 35 bonded to the under side of an ink supply manifold 36. Bonded to the lower side of the orifice plate are insulative support blocks 37 and 38. A catching blade 40 is bonded to a porous plate 39, and the porous plate in turn is bonded to support block 37. A series of charge strips 41 are arranged to charge the ink filaments which issue from orifices 43, and a common conductive deflection plate 42 enables self-induced deflection of all charged drops. The required pressure source, vacuum source, stimulation transducer, and electrical potential source are all connected as previously described.

The drawing of FIG. 4 is not to scale as the distance from each orifice 43 to its associated charge strip 41 is in actuality much less than the distance between adjacent orifices. Typically the orifice-to-orifice distance may be about 0.1 mm while the orifice-to-charge strip distance may be about 0.025 mm. Associated with these dimensions may be an orifice diameter of about 0.013 mm and a charge strip width of about 0.05 mm. Other system parameters may be as mentioned above in the discussion for the single channel illustrated in FIG. 1. Deflection plate 42 and charge strips 41 may be easily fabricated by well known printed circuit techniques. Head 34 may be used in combination with three similar heads to provide solid printing coverage. In such a combination the heads are arranged in a staggered fashion and time coordinated in the manner described in pending U.S. Pat. application Ser. No. 768,790, now U.S. Pat. No. 3,560,641. Such an arrangement will provide a highly precise printing capability with the orifices located only about a quarter centimeter away from the paper.

The system as above described with reference to FIGS. 1 and 3 achieves an initial lateral drop acceleration of about 5,600 meters per sec. ² which is sufficient to displace the drops a distance of about 2.5 diameters (56 microns) during a vertical fall of only 1.25 mm. For particle sorting applications wherein vertical distances need not be so short, then the electrically conductive deflection surface may be vertically extended to a length of about 5 mm or so. In such a case the same 2.5 diameter drop deflection may be achieved with a lateral acceleration of only about 350 meters per sec. ² . This reduction in the acceleration requirement enables reduction of the self-induced deflecting force by a factor of 16 and a 75 percent reduction of charging potential from 200 volts to 50 volts.

From the foregoing it is apparent that the total deflection of particles in an array built in accordance with the present invention is proportional to Q ² . However, due to the presence of adjacent channels it is more accurate to state that the total particle deflection is proportional to (Q + δ) ² where δ is a crosstalk charge. In a typical closely packed array δ may be 10 to 20 percent as large as Q. Now for an array adjusted to have the same sensitivity but built to have its particles sorted by prior art methods, the total deflection of a particle is proportional (with the same proportionality constant) to the factor Q (Q + δ) where Q is the nominal maximum value for Q. For a particle sorting process where all nominally charged particles are caught and only the nominally uncharged particles are recorded, the only error of interest is that associated with zero values of Q. In such a case the present invention has a crosstalk error proportional only to δ ² whereas the prior art sorting method produces an error proportional to Q δ. Thus this invention achieves a substantial reduction in crosstalk errors.

An alternative embodiment of this invention is shown in FIG. 5 wherein a group of orifices 45 are circularly arranged in an orifice plate 44 and discharge a set of fluid filaments 46 through a conductive cylindrical tunnel 47. Tunnel 47 is electrically insulated from plate 44 and a source of pulsed electrical potential (not shown) is connected therebetween. During periods of time when the input potential is zero all drops breaking off from the ends of filaments 46 are uncharged and follow straight trajectories passing through a circular aperture in base plate 49. However during periods of non zero potential (typically 200 volts) all newly forming drops are charged. Illustrated drops 48 were all formed during periods of non-zero charging potential. As these drops fall away from their filaments they induce sheets of electrical charge on the inside surface of tunnel 47. Since none of the filaments 46 is located along the axis of tunnel 47, each drop 48 sees an electrically conductive surface which is laterally non-symmetrical with respect to its initial trajectory. Accordingly each drop 48 is accelerated laterally outward toward the inner wall of tunnel 47, but it leaves the end of the tunnel before hitting the tunnel wall. Drops 48 maintain the lateral velocity which they achieve before leaving tunnel 47 and this lateral velocity carries them outward beyond upstanding lip 50 on base plate 49. Having thus been caught, they may be drawn off by a suitable source of vacuum.

It is interesting to note that in this embodiment a single conductive surface performs both the charging and deflection functions. As each drop forms it sees the electrical charging field set up between tunnel 47 and the parent filament 46. However, this field is largely confined to the upper end of tunnel 47, and the only really significant field which the drop sees for most of its journey down tunnel 47 is that which is self induced by the drop. It should be clear that dual function operation is not peculiar to a cylindrically configured conductive surface. In the embodiment of FIG. 1, for instance, charging strip 7 could be extended to meet deflection strip 12 without affecting the operation of the single illustrated channel.