US3560956A

US3560956A - Image forming apparatus

Info

Publication number: US3560956A
Application number: US651240A
Authority: US
Inventors: Richard C Sinnott
Original assignee: SINNOTT CO
Current assignee: SINNOTT CO
Priority date: 1967-07-05
Filing date: 1967-07-05
Publication date: 1971-02-02
Anticipated expiration: 1988-02-02
Also published as: GB1247561A; NL6809544A; DE1766684A1; FR1580418A

Abstract

AN IMAGE FORMING DEVICE SUCH AS A DISPLAY DEVICE INCLUDES A PARTICLE ACCELERATOR WHICH FORMS A TRANSPARENT CLOUD OF PARTICLES ADJACENT A DISPLAY SURFACE. THE PARTICLES ARE ATTRACTED FROM THE CLOUD TO THE DISPLAY SURFACE IN A DESIRED PATTERN BY A UNIDIRECTIONAL FIELD OF FORCE. THE PARTICLE ACCELERATOR INCLUDES AN ALTERNATING ELECTROMAGNET AND A SCREEN DIVIDING THE ENCLOSURE INTO TWO CHAMBERS. PARTICLES TOO LARGE TO PASS THROUGH THE SCREEN ARE RETAINED IN THE CHAMBER AWAY FROM THE DISPLAY SURFACE, AND ARE ACCELERATED BY THE ALTERNATING FIELD. SMALLER PARTICLES CLING TO THE LARGER ONES AND ARE JARRED LOSE WHEN THE LARGER PARTICLES STRIKE THE SCREEN. THE SMALLER PARTICLES THEN PASS THROUGH THE SCREEN AT A HIGH VELOCITY TO FORM THE TRANSPARENT CLOUD ADJACENT THE DISPLAY SURFACE. IN ONE EMBODIMENT THE UNIDIERCTIONAL FIELD IS CREATED BY D.C. ELECTROMAGNETS. IN ANOTHER EMBODIMENT THE PARTICLES ARE ELECTROSTICALLY CHARGED AND THE DISPLAY SURFACE IS OPPOSITELY CHARGED BY AN ELECTRON GUN TO ATTRACT THE PARTICLES TO THE SURFACE IN THE DESIRED PATTERN.

Description

Feb. 2, 1971 A a. c. QINNOTT 3,560,956

IMAGE FGRMING APPARATUS Filed July 5. 19s? 7 Sheets-Sheet 1 INVENTOR. PICA/4P0 C S/N/VOTT Feb. 2; 1971 R. c. SINNOTT IMAGE FQRMING APPARATUS 7 Sheets-Sheet 3 Filed July" 5 1967 INVENTOR. AICWAED C. SIN/V077 BY F'OWA 5A3 IVA/055E e GAME/9614 ,4 r ram/5V5.

Feb; 2, 1971 Q Q, slN 3560,95

IMAGE FORMING APPARATUS INVENTOR. P/CH/ZPD C SYN/V077 FOWL 5e; M0555 5 60/145195 R. C. SINNOTT IMAGE FORMING APPARATUS Feb. 2, 1911 7 Sheets-Sheet 6 Filed July 5. 1967 INVENTOR. PICA/1W0 C SINNOTT 0% E W m e 0 0 m mm MM Feb. 2, 1971 R. c. SINNOTT IMAGE. FORMING APPARATUS 7 Sheets-Sheet 7 Filed July 5, 1967 INVENTOR. IPIC'HAQD 6'. SYN/V077 .mNN

. GSGG 29R WSQQuSQ Fan L52, 0/0555 a MAI? re/vs ATTOBNE/S".

3,560,956 IMAGE FORMING APPARATUS Richard C. Sinnott, Menlo Park, Calif., assignor to Sinnott Company, San Mateo, Calif., a California limited partnership t ontinuation-in-part of application Ser. No. 565,353, July 11, 1966, which is a continuation-in-part of application Ser. No. 511,707, Dec. 6, 1965, both now abandoned. This application July 5, 1967, Ser. No. 651,240

. Int. Cl. G09b 9/30 US. Cl. 340-324 I 33 Claims ABSTRACT OF THE DISCLOSURE An image forming device such as a display device in-- cludes a particle accelerator which forms a transparent cloud of particles adjacent a display surface. The particles are attracted from the cloud to the display surface in a desired pattern by a unidirectional field of force. The particle accelerator inludes an alternating electromagnet and a screen dividing the enclosure into two chambers. Particles too large to pass through the screen are retained in the chamber away from the display surface, and .are accelerated by the alternating field. Smaller particles cling to the larger ones and are jarred loose when the larger particles strike the sceen. The smaller particles then pass through the screen at a high velocity to form the transparent cloud adjacent the display surface.

In one embodiment the unidirectional field is created by DC electromagnets. In another embodiment the particles are electrostatically charged and the display surface is oppositely charged by an electron gun to attract the particles to the surface in the desired pattern.

RELATED APPLICATIONS This application is a continuation-in-part of applicants co-pending application Ser. No. 565,353, filed July 11, 1966, which is a continuation-impart of applicants prior application Ser. No. 511,707, filed December 6, 1965, both now abandoned.

BACKGROUND OF THE INVENTION This invention relates to devices for forming characters or the like, and more particularly, relates to such devices which use a force field to attract particles of force field responsive material to a surface in a desired pattern,

Changeable devices which operate by translating an electrical signal into a selected image have been used for many years. However, in recent years the scope and extent of their use has increased tremendously. Athletic Scoreboards, stock quotation boards, airline arrival-departure displays, and time and temperature advertising displays are but a few of the more common uses. Even more significant, perhaps, is the large demand for digital display devices in the electronics industry, e.g. for direct readout of test instruments. It appears that the market for such display devices has barely been tapped and expansion to a multitude of uses can be foreseen.

Although the operational requirements for image formihg devices vary in accordance with the particular application, generally it is necessary that the characters or symbols formed by the device be easily readable. Thus, the displayed lines should be sharp, and the contrast between the lines and the background should be pronounced under all lighting conditions in which the device is apt to be used.

Further, a satisfactory display device must be reliable. This means that, in addition to beingI-a'ccurate under optimum operating conditions, the display device should not be adversely afiected by the vibrations, shocks, and other environmental conditions which it might encounter in use. Reliability is closely related to long-life, since a display device generally is not satisfactory if it requires frequent servicing and replacement of parts in order to keep it operating reliably.

In addition, a satisfactory display device desirably should be simple and inexpensive to manufacture and operate. The control logic for selecting. the display pattern is a major factor in the cost of manufacturing and operating the device. Since the control logic generally includes transistorized circuits itis particularly desirable that the 1 power and voltage requirements of the display device be low in order to easilymatch the characteristics of inexpensive transistors.

For many uses, memory, i.e. retention of an image after the image-creating signal is removed,*is a further desirable characteristic. Thus, for 'example if an image is to be unchanged for a period ofminutes, or hours, the signal can be discontinued, thereby increasing the life of the control circuit components. t a

On the other hand, if rapid changes in the image are reqiured, speed of response in forming and erasing the image is an important criterion.

Prior changeable image forming devices have been seriously deficient with' 'respect to one or more of these characteristics, so there is a great demand for an improved device. This invention i s a device calculated to satisfy that demand. It incorpofates a decidedly advantageous combination of the desirable qualities discussed above. 5

SUMMARY OF THE INVENTION This invention uses an alternating force field to accelthe particles become practically invisible and form a transparent cloud in a hollow enclosure. A unidirectional force field is applied selectively to the cloud to attract the particles to a surface of the enclosure in a desired pattern.

In the exemplary embodiment the high particle velocity required to form the transparent cloud is achieved by the inclusion of both large and small particles of magnetic material in the enclosure. The smaller image particles are capable of passing through a filter which divides the enclosure into an upper chamber in front of the surface or display wall and a lower chamber below the display wall, The larger propulsion particles, however, will not pass through the filter and are contained in the lower chamber. An electrical coil encompassing the lower chamber creates an alternating magnetomotive' force which agitates both the large and small particles, so that the large particles contact the smaller ones and increase their energy level to assist them in attaining the desired velocity. As the particles strike the filter, the larger ones are retained in the lower chamber while the smaller particles pass through to form the transparent cloud in the upper chamber.

The smaller image particles which pass through the filter include particles in two distinct size ranges in an exemplary embodiment. Thus, small particles of one size range form the major portion of the display and very fine particles of even smaller size range fill in the gaps in the image pattern to sharpen the lines. In that exemplary em bodiment a third larger size which will not pass through the filter is still used to accelerate the particles of both smaller size ranges.

In one exemplary embodiment of the invention, the unidirectional force is created by a plurality of electromagnets disposed behind the display wall with their poles arranged to attract the particles in various patterns, when selected ones of the electromagnets are energized with direct current. The magnetomotive forces created by the electromagnets cause the particles from the cloud to accumulate opposite the poles of the electromagnets in patterns having clearly defined outlines. The particles visually contrast with the display board so that the pattern is readily visible.

Another exemplary embodiment uses image particles which are both electrically insulative and magnetic. The particles in the cloud are given a positive electrostatic charge and the unidirectional field of force is created by an electron gun which scans the display surface with an electron beam and negatively charges the display surface in a desired pattern. The positively charged image particles are attracted to the negatively charged portions of the display surface in the desired visual pattern.

Thus, in this aspect the invention includes an enclosure,

an alternating field of force for accelerating a plurality" of electrostatically charged particles to form a trans parent cloud within the enclosure, and means for charging a surface'of the enclosure to attract the particles to it in a selected pattern.

The combination of the particle gun with the electron gun eliminates the need for mechanical devices to position the charged particles adjacent the image surface and provides a rapid response time as well as a relatively uniform distribution of the particles over the image pattern.

These and other features and objects of this invention will be apparent from the following detailed description when read with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded perspective view of a display device constructed in accordance with this invention, with portions of the display magnet support removed for clarity;

FIG. 2 is a vertical section through the assembled display device. illustrated in FIG. 1;

FIG. 3 is a partial elevation view taken generally along lines 33 of FIG. 2, showing a portion of the front of the display magnet assembly with portions removed for clarity;,.

FIG. 4 is a front elevation view showing a plurality of display devices mounted on a display panel, with portions removed for clarity; I

FIG. 5 is a perspective schematic representation illustrating the electrical controls and magnetic fields of the display device;

FIG. 6 is a vertical section similar to FIG. 2, illustrating a modified embodiment of the display device;

FIG. 7 is an exploded perspective view illustrating the display, wa'll and.a mod ified display magnet assembly, with portions of the support member removed for clarity;

FIG. 8 is a vertical section through one of the electromagnets illustrated, in FIG. 7; 7

FIG. 9 is a perspective view of another modified embodiment of the display magnet;

FIG. 10 is a vertical section through still another modifled embodiment of the di play m g e FIG. 11 is a vertical section through an assembled display device including display magnets of the type illustrated in FIG. 10, and a modified embodiment of the particle gun;

FIG. 12 is a partial elevation view taken generally along lines 1212 of FIG. 11, showing a portion of the front of the display magnet assembly with portions removed for clarity;

FIG. 13 is a perspective view of another embodiment of the invention utilizing an electrostatic field;

FIG. 14 is a longitudinal section through the display device of FIG. 13; and

FIG. 15 is an enlarged schematic in which the relative size of the particles is exaggerated to illustrate the operation of the particle accelerator.

Referring to FIGS. 1 and 2, the display device illus trated included an enclosure having a transparent window unit 10 and a display surface or wall 12 visible through the window 10. A particle gun 14 applies an alternating field of magnetomotive force to a plurality of particles 16 of magnetic material contained within the hollow enclosure to cause them to form a transparent cloud in front of the display wall. A display magnet assembly 20 mounted behind the display wall selectively applies a unidirectional field of magnetomotive force to the cloud of particles 16 to attract them to the display wall in a predetermined image or pattern, for example the numeral 2 as illustrated. An erase coil 22 mounted behind the display wall selectively creates alternating magnetomotive force for removing the particles 16 from the display wall 12 and returning them to the transparent cloud in front of the display wall.

The transparent window unit 10 includes a transparent plate, the major portion of which is flat to form the front wall 24 of the enclosure, and the upper part of which is curved rearwardly to form a top 26. Two transparent plates 28 extend away from the front 24 and top 26 =window surfaces to form sides of the enclosure. An offset partial rear wall 30 and a horizontal shoulder 32 extend between the lower portion of the two sides 28 to define a rectangular rear opening 34 opposite the front window surface 24. An outwardly extending flange 36 at the bottom completes the transparent window unit, leaving ,the central portion 38 of the bottom open.

The display wall 12 is a flat, generally rectangular panel which fits tightly in the rear opening 34', joining the top 26, sides 28, and shoulder 32 of the window unit 10 in tight relationship, to form the upper rear wall of the enclosure.. Thus, the transparent window 10 extends beyond the boundaries of the display wall 12 in both height and width so that the display is easily visible.

The bottom flange 36 of the window unit seats on the upper flange 40 of a generally rectangular particle gun bobbin 42. A rectangular floor plate 44 is supported horizontally, on an internal shoulder 46 at about the midpoint of the bobbin interior wall 48. Thus, a sealed enclosure is formed by the, floor 44, the upper half of the bobbin 42, the transparent window unit 10 and the display wall 12. Preferably all the interior surfaces of the enclosure are made of non-magnetic material, e.g. Teflon being suitable for the display -wall, floor and bobbin.

A filter or screen 50 of non-magnetic material is supported on an inturned shoulder 52 in the upper part of the particle gun bobbin 42 to divide the interior of the enclosure into upper 54 and lower 56 chamber portions, the upper chamber portion being mostly in front of the display wall 12, and the lower chamber portion being interior of the particle gun bobbin 42.

The particle gun 14 includes an electric coil 58 of insulated wire wound in one direction around the bobbin 42 to create a magnetic field parallel to the display wall when energized with current. Thus, the coil 58 of the particle gun encompasses the lower chamber 56 of they similar to a transformer core, are mounted below the floor 44 Within the electric coil 58 to concentrate the flux lines and increase the magnetic field generating efficiency of the coil.

The air may be partially evacuated from the enclosure to reduce air resistance to particle movement. In those instances, all points of the enclosure are suitably sealed, for example by an appropriate resin.

The plurality of small particles 16 of magnetic material contained within the enclosure are preferably small permanent magnets made of such material as, for example, finelypdivided barium ferrite or various hard magnetic iron and nickel alloys suspended in a plastic binder and cut, for example, into small elongate slivers, cubes or spheres.

The small image particles 16 of magnetic material may include particles in two separate size ranges which will pass through the apertures of the screen 50 and are free to move between the two

chamber portions

54, 56. Larger propulsion particles 62 of the same material which are not capable of passing through the mesh of the screen 50 are contained in the lower chamber 56. As an example, the smaller image particles might include fine particles of about .005 to about .008 inch across and very fine particles of about .001 to about .003 inch across, while the larger particles are on the order of about .100 inch across. Only a few exemplary particles are schematically illustrated in the drawings.

Referring now to FIG. 3, as well as FIGS. 1 and 2, the exemplary display magnet assembly illustrated includes seven electromagnets 64 each having an electric coil 65 wound on a non-magnetic bobbin 66 through which a fiat core 68 extends. The core is made of suitable magnetic material such as soft iron. The forward end of each of the cores terminates in an elongate fiat edge 70 which forms one pole of the electromagnet 64. The oppo site end 72 of each of the magnetic cores is bent to form an L-shape.

The seven electromagnets 64 are mounted on a rectangular, molded, non-magnetic support member 74 with their forwardly extending poles 70 arranged to form a desired display, e.g. a rectangular figure 8 bar matrix pattern as illustrated. The support member 74 is inserted in the rectangular opening 34 of the window unit 10 behind the display wall 12 with the poles 70 of the magnets extending through the support member so as to be immediately behind the rear side of the display wall.

A plate 76 of magnetic material, such as soft iron, is countersunk in the forward surface of the support member 74 within the upper loop of the figure 8. The plate '76 is in the form of a polygon, a square in the example illustrated, with each of its edges 78 parallel to one of the four

elongate poles

70a, 70b, 70c, 70d which form the upper loop. The plate is slightly spaced from the four poles by the non-magnetic support member leaving four linear non-magnetic gaps 79 forming a square loop. An elongate rod 80 of magnetic material, e.g. soft iron, extends rearwardly from the magnetic plate 76 to the rear leg 72a of one of the upper electromagnet cores. The rear legs 72 of the four upper cores are stacked in abutting relationship, so that the square magnetic plate 76, through the rod 80, forms a common pole for the upper four electromagnets 64ad. A bolt 82 extends through the magnetic plate 76, rod 80, and the four legs 72 to mount the electromagnets 64 and rod 80 on the support member 74 A. second magnetic plate 84 is countersunk in the lower loop of the figure 8 to form a common pole for the lower three electromagnets 64e-g. This plate 84 is mounted in the same way as the upper plate 76 and differs only in that a trapezoidal portion is removed from the upper part of the plate 84. Thus, its upper edge 86 is not adjacent the elongate pole 70b of the central electromagnet 64b, since the lower plate does not form a second pole for that magnet. As the lower assembly includes only three magnets while the upper assembly includes four, an annular 6 magnetic spacer 88 is inserted on the rear of the lower bolt 82. A metallic plate 90 threaded on the two bolts 82 interconnects the magnetic circuits.

A terminal board 92 mounted behind the display mag net assembly 26 includes fourteen electrical terminals 94 to which the two ends of each of the seven electromagnet coils 65 are connected for receiving direct current from a source not shown in FIGS. 1-3.

The erase coil 22 is wound in one direction on a large non-metallic, generally rectangular bobbin 96 to create a magnetic field perpendicular to the display wall 12 when energized. The erase coil bobbin 96 encompasses the display magnet assembly 20 so that the magnetic field generated by the coil 22 extends completely across the display wall 12.

The particle gun 14 and enclosure are mounted on a vertical panel 98 having a rectangular opening 100 into which the gun and enclosure extend. The device is mounted with the display wall 12 and the axis 102 of the particle gun coil 58 vertical, and the axis 104 of the erase coil 22 horizontal. The mounting maybe accomplished using any suitable means such as a bracket 106 extending around the erase coil 22 and display magnet assembly 20 and fastened to the panel by bolts 108. A housing 110, attached to the panel by the lower bolt 108, encloses the portions of the particle gun coil 58 which would otherwise be exposed on the forward side of the panel 98.

One mode of operation will be described with particular reference to FIG. 5. The particle gun coil 58 is connected to a source of alternating current 112 through a normally closed switch 114, and the erase coil 22 is connected to the source of alternating current 112 through a normally open :switch 116. Each of the display magnet coi s 65, only one being shown in FIG. 5 for purposes of illustration, are connected to a source of direct current 118 by a separate, normally open switch 120.

With the particle gun switch 114 closed, the particle gun coil 58 is energized with alternating current and creates a field of magnetomotive force alternating in polarity, schematically indicated by the double-headed arrows 122, which is generally vertically disposed, and preferably fairly well confined to the lower chamber area 56. This alternating force 122 agitates the

magnetic particles

16, 62, and causes them to move violently and randomly within the confines of the enclosure. The randomly oriented

particles

16, 62, each of which is a permanet magnet, interact with each other to further enhance the violent movement.

The energy or velocity which a particle gains from the magnetic field increases with particle size, so that the larger propulsion particles 62 tend to gain more energy than the smaller image particles 16. However, the smaller image magnet particles, including both the fine size and the very fine size contact and collect on the larger magnet particles and travel with them thereby increasing the energy level of the smaller particles. As is shown in FIG. 15, as the particles move randomly, the larger ones 62 strike the non-magnetic screen 50 and return to the lower chamber 56 since they cannot pass through the mesh of the screen. The smaller image particles 16, however, are dislodged from the larger particles upon impact with the screen 50 and are propelled upward through the screen into the upper chamber 54 at velocities exceeding those they would achieve without the assistance of the large propulsion particles. In the upper chamber they continue their violent random movement until they lose their velocity and fall through the screen to the lower chamber where they are re-accelerated.

The size of these smaller particles 16 in relation to the strength of the particle gun field is selected so that the smaller particles, including both the fines and very fines, will move at sufficient velocity to become practically invisible, thereby forming a transparent cloud in front of the display surface 12. Thus, the display surface is visible through the transparent window 10 and the transparent cloud. The magnitude of the particle gun alternating field is selected to be great enough to overcome the attraction of the large particles to each other and not so large as to demagnetize the particles.

When one of the display magnet switches 120 is then closed to energize one of the direct current display electromagnets 64, a horizontal unidirectional field of magnetomotive force, schematically represented by the singleheaded arrows 124, is created through the transparent cloud. This unidirectional magnetomotive force 124 attracts the small particles out of the transparent cloud to the display wall to form a thin line on the wall immediately opposite the gap 79 between the two poles of the energized electromagnet. The fine particles form the basic display shape and the very fine particles fill in any gaps or irregularities to provide a sharp line image.

By selectively energizing various combinations of the display magnets, various patterns, e.g. any selected numeral -9, may be displayed on the wall. The color or texture of the display wall is selected to visually contrast with that of the small particles so that the pattern of particles 16 is readily visible on the display wall 12.

In one embodiment the display board is thin enough that the particles are attracted to the energized display electromagnets but thick enough that the magnet particles are not attracted to the de-energized electromagnets. Thus, the display magnets are energized for so long as the display is to remain. If it is desired to leave the displayed numeral on the display board for a substantial period of time, the particle gun may be de-energize'd by opening the particle gun switch 114 until the display is to be removed. When it is desired to remove the display, the display magnet switches 120 are opened de-energizing the display magnets, the erase coil switch 116 is closed, energizing the erase coil 22 with alternating current, and the particle gun switch 114 is closed, if it had been opened. The alternating erase field or magnetomotive force, represented schematically by the double-headed arrows 126, moves the displayed particles away from the display wall and returns them to the transparent cloud, the alternating erase field assisting the particle gun in forming the cloud. The strength of the erase coil alternating field is selected so as to overcome the attraction of the small particles for each other without demagnetizing them.

A sharper line of particles on the display wall is obtained by constructing the

various coils

22, 58, 65 to so balance the magnetic fields that the erase coil 22 and particle gun coil 58 may be energized continuously, with formation and removal of a display controlled by operation of the display switches 120 to energize and de-energize the display electromagnets. In such an arrangement, the alternating field of the erase coil assures that particles adhere to the display surface only in a fine line immediately between the energized poles 70 of the display electromagnets. Further, in such an arrangement, the agitation of the particles in the cloud is increased if the erase coil 22 and particle gun coil 58 are connected in phase so that at any particular instant both have similar poles toward the cloud.

In another embodiment, the thickness of the display wall 12 may be so selected that, when the erase coil, particle gun and direct current display electromagnets 64 are de-energized, the "attraction of the permanent magnet particles to the magnetic poles 70 of the electromagnet is sufiicient to hold the particles 16 in the previously formed display pattern against the pull of gravity, until the erase coil 22 is re-energized.

Referring now to FIG. 4, a large panel 128 having a plurality of the display devices mounted side-by-side is illustrated. For such a display, a plurality of the separate enclosures are mounted in side-by-side relationship on the large panel 128. The particle gun and erase coil are preferably modified, however, by substituting a large particle gun coil 130 and a large erase coil 132 each of which encompasses all of the enclosures. The large particle gun coil creates a verticle magnetic field in all of the enclosures mounted on the panel and the large erase coil 132 creates a horizontal alternating field across all the enclosures for erasing the display. The individual enclosures are preferably separated from each other by upper 134 and lower 136 side walls so that the

particles

16, 62 cannot drift from one enclosure to another causing uneven distribution of the particles across the width of the display.

The display device illustrated in FIG. 4 also has a plurality of circular direct current electromagnets 138 mounted behind the display wall to form decimal points 140 in the serial of numbers.

Referring to FIG. 6, in a modified embodiment of the display device the top 16' and shoulder 32' of the window unit 10' are canted, and the sides 28' are cut at an angle so that the display wall 12 is mounted in a plane oblique to the axis 102' and field of the particle gun coil 58'. Similarly, the mounting panel is angled with its lower portion 142 parallel to the window front 24' and its upper portion 144 parallel to the display wall 12'. The mounting bracket 106' is modified accordingly as is evident from the drawing.

As in the embodiment of FIGS. 1 and 2, the axis 104 and field of the erase magnet coil 22, as well as of the display magnet coils 64 are perpendicular to the display wall 12 and intersect the axis 102 of the particle gun coil. All other parts of the embodiment of FIG. 6 are similar to those of FIGS. l-5, and will not be re-described in detail.

The modified'embodiment illustrated in FIG. 6 is preferably mounted with the axis 102' of the particle gun coil 58 vertical, although it is sometimes desirable to instead tilt the gun 14' and mount the display wall 12' vertical. In any event, the response time in forming the display pattern is decreased by the obliquity since the display Wall is directly in the trajectory of the particles.

Referring now to FIG. 7, a modified form of electromagnet display assembly includes, for example, three display electromagnets 146 mounted on the support member 74- behind the display wall 12. Each display electromagnet 146 includes a non-magnetic bobbin 148 with an electric coil 150 wound around it, and a magnetic core 152 extending through the coil. A magnetic plate 154 extends transversely from each end of the core 152 forwardly into the support member 74. A plurality of laminated plates of alternating magnetic 156 and nonmagnetic 158 material are stacked in an abutting series between the two forwardly extending magnetic plates 154. The display electromagnets are mounted on the support member 74 with the

plates

154, 156, 158 disposed substantially perpendicular to the display wall 12 and behind it. A portion 160 of the edges 162 of the plates extends through the support member 74 to form a pattern, eg, the word GO immediately behind the display wall.

When one of the coils 150 is energized with direct current, a magnetic field is established in the end plates 154 by the field in the core 152, and in turn, a field is induced in each intermediate magnetic plate 156 so that opposite surfaces of each intermediate magnetic plate 156 are of opposite polarity. The magnetic particles 16 from the transparent cloud are attracted by the magnetomotive force from the plates to form the word G0 on the display wall.

Another modified embodiment of the display electromagnet is illustrated in FIG. 9. This embodiment is similar to that illustrated in FIGS. 7 and 8 in that it includes an electric coil 164 wound on a non-magnetic bobbin 166, a magnetic core 168 extending through the coil, and a plurality of alternatng magnetic 170 and nonmagnetic 172 plates. However, each magnetic plate 170 is connected directly to the core 168 by a rearwardly extending arm 174 of magnetic material fixed to the core 168 so that a plurality of alternating magnetic 170 and non-magnetic 172 ones of the plates 170 are connected to opposite ends of the core so that each magnetic plate 170 is a single pole and alternate magnetic plates are of opposite polarity. When the coil is energized with direct current the small particles 16 are attracted from the transparent cloud to the display device in a pattern, such as the word GO in the example shown.

Referring now to FIG. 10, another modified embodiment of the display magnet 20" includes a coil 176 and bobbin 178 similar to those illustrated in FIGS. 1-6. The core includes a U-shaped plate 180 one leg 182 of which extends through the coil 176. Another plate 184 extends alongside the U-plate 180 from within the coil 176 forward beyond the U-shaped plate and through the support member 74" to form an elongate pole 186 immediately behind the display wall 12". A third plate 188 extends rearwardly from within the coil alongside the U-plate and is reverse bent to extend forwardly through the support member 74 and form a second elongate pole 190. The forward portions of the two

pole plates

184, 188 are each curved in an S-shape toward the other within the support member 74" and at their forward ends are only slightly spaced, the gap 192 being filled by the non-magnetic support member 74". As with the other embodiments, the support member may be molded with the display magnets in place.

The display magnet embodiment of FIG. 10 is particularly effective in achieving a sharp line image when the display device is operated with the erase coil continuously n. Apparently the alternating field of the erase coil is collected by the

display magnet plates

184 and 188 thereby concentrating the alternating field immediately adjacent the display magnet poles causing the magnet particles 16 to accumulate only in a distinct line between the

poles

186 and 190 so as to avoid ragged edges.

Referring now to FIG. 11 a modified embodiment of the display device utilizes a plurality of the independent pole display magnets 20 cantilevered from the support member 74". A modified embodiment of the particle gun 14" includes an electric coil 58 disposed directly below the erase coil 22" with its axis horizontal. A core 200 of laminated soft magnetic plates extends through the bobbin 48" of the particle gun coil 58 and is turned upward at its forward end to a position immediately beneath the non-magnetic floor 44". Because of the blend in the plates of the core 200, the alternating field of the particle gun 14" is oriented vertically in the upper and lower chambers of the enclosure. The transparent window 10" extends down below the particle gun and then back to the lower mounting panel 142" where it is fastened by bolts 108". This arrangement eliminates the protruding front housing 110 illustrated in FIG. 2. As is apparent from the drawings certain other details such as the shape of the rear bracket 106 are modified to accommodate the particle gun 14".

As shown in FIG. 12 in the exemplary embodiment, the

poles

186 and 190 of the display electromagnets are positioned so that the group of display magnets 20" form an inclined figure eight display rather than an upright eight. This arrangement is preferred for many uses of the display device although it should be understood that the display magnets 2 also lend themselves to arrangement in an upright figure eight.

The mode of operation of the embodiment of FIGS. 11 and 12 is similar to that described earlier with reference to the other embodiments. The erase coil 22" and particle gun 14" create horizontal and vertical alternating fields in the enclosure to form a transparent cloud of both sizes of the smaller particles 16". The display magnets 20" are selectively actuated to attract particles from the cloud in the desired display pattern.

Still another embodiment of the invention is illustrated in FIGS. 13 and 14. That embodiment includes a particle gun 202 similar to that described above with reference to FIG. 11. The direct current display magnets are replaced, however, with an evacuated envelope 204, and an electron 10 gun 206 generally similar to that described in Pat. No. 3,109,062, issued Oct. 29, 1963', to C. K. Claur et al.

The forward Wall of the envelope is an electrical insulating member 208 which forms the display surface of the particle enclosure 210. A generally circular transparent window 212 having a circular flange 214 circumscribing the insulating member 208, a floor 216 and'a rear wall 218 complete the enclosure. An erase or image refining coil 220 is disposed around the forward end of the envelope 204 behind the insulating member 208.

A cathode 222 and grid 224 at the read end of the envelope generate a beam of electrons directed at the insulating member 208. Modulating signals areapplied to the cathode and the grid to regulate the intensity of the electron beam in accordance with the input information signal 225. A deflection yoke 226 disposed around the envelope between the electron gun 206 and the insulating member 208 is energized by a source 228 of sweep voltage signals, such as a sweep generator, to sweep the modulated electron beam across the surface of the insulating member 208 and place an electrostatic charge on the insulating member in a desired distribution pattern.

In accordance with this embodiment of the invention, the small image particles which form the transparent cloud are good electrical insulators as well as being magnetic. The surfaces and materials other than the particles interior of the enclosure 210 are of a dissimilar insulating material having a dielectric constant lower than that of the display particles. As the particles are accelerated by the alternating magnetic force field of the particle gun 202 they come into contact with the dissimilar insulating materials and acquire a positive electrostatic charge. This charging can be enhanced by the use of large charging particles of material having even a lower dielectric constant than the interior surfaces of the enclosure.

In operation, the electron beam is activated and a selected information signal is fed to the electron gun 206 while the beam is scanned across the surface of the insulating member 208 by the sweep generator 228. A selected information signal 225 modulates the electron beam so that a negative electrostatic image in a desired pattern is applied to the insulating member 208. After the electrostatic image is written on the display surface 208, the'particle gun 202 and erase coil 220 are activated with alternating current to generate intersecting alternating magnetic fields which accelerate the image particles in the manner described above with reference to the previous embodiments to form a transparent cloud of the image particles in the upper chamber of the enclosure 210 above the screen 229. Since the magnetic image particles are electrostatically charged, they are attracted from the cloud by the opposite charge on the insulating member 208 to form a visible image in the desired pattern. When the image particles have been sufiiciently distributed over the insulating member surface, the particle gun coil and erase coil 220 are de-energized until it is desired to remove the display. The erase coil provides an alternating magnetic force field sufficient to propel the display particles from portions of the surface on which the charge is less then the amount required for an image. Thus, this field prohibits image particles from adhering to the surface where a random weak charge might occur.

When it is desired to remove the display, an erase electron gun 230 may be energized to flood the insulating member with electrons producing a secondary emission ratio greater than unity for electrons striking the target surface. As pointed out in Pat. No. 3,109,062, this bombardment by the flood beam causes the electrons forming the electrostatic charge on the insulating member 208 to be removed from the surface, thus dissipating the negative electrostatic charge pattern and preparing the surface for receiving another charge pattern from the modulated electron beam. The erase coil 220 is also activated to propel the particles from the display surface.

It has been found that if ferrite particles are used as the material for the image particles, an image can be maintained for about 2:4 to 30 hours on a Lucite display surface. If longer image retention time is desired, the ferrite particles may be coated with an appropriate glass or plastic having resistivity on the order of to 10 ohm centimeters. Since charge migration is proportional to resistivity, materials with high resistivity will maintain their charge longer, and thus have better memory characterstics.

The above-described embodiments are intended to be exemplary only and this invention should not be limited except in accordance with the following claims:

1 claim:

1. The combination comprising:

a hollow enclosure;

a plurality of force field responsive particles within the enclosure;

means for applying an alternating force field to the particles to accelerate them and form a transparent cloud of free particles within the enclosure;

and means for applying a unidirectional force field to the cloud of particles to accumulate particles from the cloud on a surface in a desired pattern.

2. A device in accordance with claim 1 wherein:

the particles are responsive to a magnetic force field;

and

the means for applying an alternating field of force to the particles includes at least one electromagnet for applying an alternating magnetic field.

3. -A device in accordance with claim 2 further including means for creating at least a second alternating force field for intersecting the particle accelerating field, one of said fields being generally perpendicular to the display surface.

4. A device in accordance with claim 3 wherein the two intersecting alternating force fields are magnetic fields and are in phase with each other so that at any particular instant both fields have the same pole toward the transparent cloud of particles.-

5. A device in accordance with claim 2 wherein:

the particles of materials are permanent magnets;

and the materials forming interior surfaces of the enclosure are non-magnetic.

6. A device in accordance with claim 2 wherein:

the particles are electrically insulative and are electrostatical ly charged in the cloud; and

the means for applying a unidirectional force field to the cloud of particles includes means for scanning the surface with an electron beam in a selected pattern.

7. A device in accordance with claim 2 wherein:

the means for applying a unidirectional force field to the cloud of particles includes at least one electromagnet for applying a unidirectional magnetomotive force to the particles.

8. The combination comprising:

a hollow' enclosure;

a plurality of force field responsive particles within the enclosure;

means for applying an alternating force field to the particles to accelerate them and form a transparent cloud of particles within the enclosure;

means for applying a unidirectional force field to the cloud of particles to accumulate particles from the cloud on a surface of the enclosure in a desired pattern;

a filter dividing the interior of the enclosure into a first chamber adjacent said surface and a second chamber removed from said surface, the particles which form the transparent cloud adjacent said surface being capable of passing through the filter to form the cloud in the first chamber;

and a plurality of larger particles of materials which will not pass through the filter, said larger particles being contained in the second chamber for agitation by the'alternating force field to contact the smaller particles and increase their energy level.

9. A device in accordance with claim 8 wherein the smaller particles include particles of two separate size ranges.

10. An image forming device comprising an enclosure including a transparent window and a wall having a display area visible through the window;

a filter dividing the interior of the enclosure into a first chamber in front of the display area and a second chamber removed from the display area;

a plurality of propulsion particles of magnetic material contained in the second chamber, said particles being too large to pass through the filter;

a plurality of smaller image particles of magnetic material contained in the enclosure, said smaller particles being capable of passing through the filter;

alternating magnet means for agitating "the magnetic particles to form a transparent cloud of the smaller particles in front of the display area; and

means for applying a unidirectional force field for attracting the smaller particles from the cloud to the display area in a selected pattern.

11. An image forming device in accordance with claim 10 wherein the alternating magnet means comprises means for creating a first alternating magnetic field within the enclosure generally perpendicular to the display wall;

and means for creating a second generally vertically disposed alternating magnetic field within the enclosure, the second alternating field intersecting the first alternating field.

12. An image forming device in accordance with claim 10 wherein the alternating magnet means further comprises a first electric coil disposed behind the display wall with its axis generally perpendicular thereto for creating a first alternating magnetic field;

a second electric coil disposed beneath the first with its axis generally parallel to that of the first for creating a second alternating magnetic field;

and a soft magnetic core extending into the second coil and turned up at one end for conducting the magnetic field created by said second coil so as to be oriented generally parallel to the display wall in the enclosure.

13. An image forming device in accordance with claim '10 wherein the alternating magnet means further c0mprises a first electric coil disposed behind the display wall with its axis generally perpendicular thereto for creating a first alternating magnetic field;

and a second electric coil having its axis substantially veitical for creating a second alternating magnetic fie d.

14. An image forming device in accordance with claim 10 wherein the alternating magnet means comprises an electric coil mounted with its axis oblique to the display wall.

15. An image forming device in accordance with claim 10 wherein a plurality of the enclosures are mounted with their display walls side-by-side;

and the alternating magnetic means includes an electric coil for creating a magnetic field in all the enclosures, and means for supplying alternating current to the coil.

16. An image forming device in accordance with claim -10 wherein a plurality of the enclosures are mounted with their display walls side-by-side;

and the alternating magnet means includes two electric coils for creating intersecting alternating magnetic fields in all the enclosures, one of said fieldsbeing generally perpendicular to the display wall, and means for supplying alternating current to the coils.

17. An image forming device in accordance with claim 13 ll) wherein the unidirectional force field means comprises a plurality of electromagnets behind the display wall each having two elongate poles slightly spaced-apart to define a linear gap, the poles of each electromagnet being; independent of the poles of the others; and means for selectively energizing the electromagnets with direct current to form various patterns of the particles on the display wall. 1 8. An image forming device in accordance with claim wherein the unidirectional force field means'comprises a plurality of electromagnets having a common first pole defining a polygon behind the display wall, and each of the plurality of electromagnets having a sec ond elongate pole behind the display wall adjacent a side of the common first pole;

and means for selectively energizing the electromagnets with direct current'ito form various patterns of the particles on the display wall. 19. An image forming device in accordance with claim 10 wherein the unidirectional force field means comprises an electromagnet means having a plurality of abutting plates disposed substantially perpendicular to the display wall, alternate ones of said plates being of mag- .netic materials and non-magnetic materials to form jlinear magnetic poles and non-magnetic gaps respectively, a portion of the edges of said plates forming a patterned surface behind the display wall;

and means for selectively energizing the electromagnet means with direct current to form a pattern of the particles on the display wall.

20. An image forming device in accordance with claim 1ft wherein the air is partially evacuated from the interior of the enclosure.

21. An image forming device in accordance with claim 10 wherein:

the image particles are electrically insulative;

the enclosure includes electrically insulative material for contacting the display particles to electrostatically charge them; and

the means for applying a unidirectional force field includes means for applying an electrical charge to the display. area in a selected pattern, the display area charge being of opposite polarity from-the display particle charge.

22. An image forming device in accordance with claim 21 wherein the means for charging the display area comprises an electron gun for scanning the display surface with a beam of electrons modulated in intensity to produce an electrostatic charge on the display surface in the selected pattern.

23. An image forming device in accordance with claim 21 wherein the display particles are magnetic particles coated with electrically insulative material.

24. An image forming device in accordance with claim 23 wherein the coating material has resistivity of at least 10 ohm-centimeters.

25. An image forming device in accordance with claim 21 wherein the materials forming interior surfaces of the enclosure are electrically insulative, non-magnetic materials of a lower dielectric constant than the display particles, so as to form at least part of the means for charging the display particles.

26. An image forming device in accordance with claim 25 wherein the electrically insulative means for charging the particles further comprises charging particles of an electrically insulative material dissimilar from the display particles, said charging particles being contained in the second chamber and being too large to pass through the filter.

27. Apparatus for forming an image comprising:

an enclosure;

a plurality of force field responsive particles within the enclosure;

means for applying an alternating force field to the i4 particles to cause them to form a transparent cloud of free particles within the enclosure; means for electrostatically charging the particles; and means for charging a surface of the enclosure to attract the particles to the surface in a selected pattern;

28. Apparatus for forming an image in accordance :vith claim 27 wherein the surface is electrically insuative.

29. A display device for presenting rapidly changeable visual displays of letters or numerals comprising a sealed enclosure including a flat generally upright display surface of non-magnetic material, a transparent window opposite said display surface extending at least to the boundaries of the, display surface, and a floor of non-magnetic material;

a screen dividing the interior of the enclosure into an upper chamber disposed between the transparent window and the display surface, and a lower chamber disposed below the display surface, said screen having apertures of a preselected size;

a first plurality of permanent magnet particles contained in the lower chamber, said particles being larger than the screen apertures;

a second plurality of permanent magnet particlesfcontained in the enclosure, the particles of said second plurality being smaller than said screen apertures and free to pass back and forth between the upper and lower" chambers;

an erase electromagnet behind the display surface for creating an alternating magnetic field perpendicular to the display surface;

a particle gun electromagnet for creating a generally vertical alternating magnetic field in said upper and lower chambers to agitate the permanent magnet .particles and form a transparent cloud of the particles of the second plurality in front of the display surface;

and means disposed behind the display surface for creating a unidirectional force field to attract particles from the cloud to the display surface in a selected pattern, :flle portion of the display surface to which the particles are attracted contrasting visually with the particles in the cloud.

30. A display device in accordance with clairri" 29 wherein I the transparent window joins the display surface along the top and side edges thereof; the erase electromagnet circumscribes the display surface;

the particle gun electromagnet includes a coil disposed with its axis generally horizontal and a core of laminated soft magnetic plates extending out of the coil and upwardly beneath the" floor of the enclosure to orient the particle gun field vertically in the enclosure.

'31. A display device in accordance with claim 29 wherein the means for creating a unidirectional force field comprises:

a plurality of display electromagnets each having two elongate poles spaced-apart to define a linear gap, the poles of each electromagnet being independent of the poles of the others and the display electromagnets being disposed so that their gaps define a figure eight.

32. A display device in accordance withclaim 29 wherein the second plurality of permanent magnet particles are electrically insulative;

the interior surfaces of the enclosure are primarily electrically insulative materials of a dielectric constant lower than the second plurality Qof magnet'particles for positively charging said particles; the means for applying a unidirectional force field includes means for sweeping a modulated electron 1 5 beam across the display surface to negatively charge that surface in the desired pattern; and the erase electromagnet provides an alternating magnetic force field sufiicient to propel the particles from only those portions of the surface on which the charge is less than the amount required for an image. 33. The combination comprising a hollow enclosure, a plurality of magnetic field responsive particles within the enclosure; means for applying an alternating magnetic field to the particles to accelerates them and form a cloud of particles within the enclosure, means providing a surface exposed to said particles, and means for supplying magnetic fields at said surface to accumulate particles from said cloud onto said surface in a desired pattern.

References Cited UNITED STATES PATENTS 1Q THOMAS B. HABECKER, Primary Examiner D. L. TRAFTON, Assistant Examiner U.S. Cl. X.R.