This invention generally relates to cathode ray tubes and, particularly, to means for damping the resonant mask vibrations in tension mask color cathode ray tubes. 5 As is known in the art, a color cathode ray tube generally is constructed with a glass envelope having a color phosphor screen or layer formed on the inner surface of a panel of the glass envelope. A color selecting electrode is located within the envelope

10 opposing the phosphor screen. An electron beam is emitted .from an electron gun located within a neck por¬ tion of the envelope, the electron beam being scanned by an electromagnetic deflecting device for impingement on a desired phosphor or phosphors of the phosphor screen.

15 Xn conventional color cathode ray tubes having two-dimensionally curved color selecting electrodes or shadow masks, the curvature of the mask and its thick¬ ness causes it to be structurally self-supporting. Another type of commercial shadow mask is tensed on .a

20. cylindrical ..support frame and is not self-supporting as is the two-dimensionally curved type. It is used in conjunction with, a cylindrically configured phosphor screen. A new type of shadow mask tube has a perfectly flat faceplate and an associated perfectly flat shadow

25 mask. The shadow mask is a very thin foil maintained at a tension of tens of thousands of pounds per square inch. The afore-described cylindrical and flat tension shadow mask configurations are prone to vibrations, as may be

- 2 - caused by external pulses, or by a speaker in an as¬ sociated television receiver, for example. The resonant frequency of vibration of the mask will vary depending on the mechanical parameters of and tension in the mask. Any vibration of the mask will cause electron beam landings to be out of registry with their respec¬ tively associated phosphor elements, causing color impurities in the reproduced images.

Various means have been suggested for damping the resonant vibrations described above. For instance, in U.S. Patent No. 3,638,063 a damping wire or rod is stretched across grid elements of the tube. With such an arrangement, the grid elements are resiliently pressed by the damping rod and, therefore, are not likely to be caused to vibrate by external mechanical shocks or elec¬ tron beam bombardment. In U.S. Patent No. 4,504,764 resonant vibrations are damped by making the resonant frequency of at least one aperture grid element of the color selecting aperture grill so as to be different 0 from that of another grid element inihe vicinity thereof." It should be noted that with, such prior art systems, HI the'grids or grills are cylindrically curyed, rather than being flat, and (2). the grill in¬ cludes, a number of parallel band-shaped grid elements. 5 Therefore, with, the system of U.S. Patent No. *α63, the damping rod can be held against the grid elements be¬ cause of the curved nature of the cathode ray tube screen. I /U.S.'...Patent.No. ^Λ .764, the grid elements themselves: can be selected of different resonant fre- ^Q quencies. Such, solutions to the problem of resonant vibrations are not appropriate with color cathode ray tubes using apertured shadow masks- which are flat and in high, tension. A damping rod or wire cannot be held in engagement with a flat shadow mask. 5 More particularly, a tension shadow mask is a rectangular membrane suspended in a high, vacuum within the cathode ray tube envelope under high, mechanical tension. The shadow mask is flat and, therefore, is

capable of vibrating in so-called "membrane modes," i.e., the two-dimensional equivalent of the vibrations of a stretched string. This type of vibration is defined by _ _> _.., _c , the fact that the restoring force due to stiffness is 5 negligible compared to that due to tension. The most prominent membrane mode is the fundamental one, with maximum amplitude in the center of the shadow maste. Elsewhere, the amplitude is a sinusoidal function of position. It is. readily apparent that prior art mask

10 damping devices, such as damping wires stretched in engagement with, a cylindrically curved grill, are in¬ effective for use with a flat tension shadow mask. This invention is directed to providing a solution to the problem of damping resonant vibrations in a flat tension

15 shadow ^* mask and thus avoiding a deterioration of picture quality caused by external vibrations.

The present invention therefor provides a vibration damping apparatus for a color selection electrode adapted or mounting in tension, on the -. ace-?

20 plate of a color cathode .ray tube by> support means associated with said faceplate, said electrode having a central apertured portion and a peripheral portion lo¬ cated between said apertured portion and the junction of said electrode with, said support means, said electrode

25 being susceptible to vibration independently of said support means, said faceplate having a target area and said vibration damping means being located outside the target area and secured to said peripheral portion of said electrode for damping vibrations in said electrode.

30. ^' One of the advantages of vibration damping means in accordance with, the invention is that it main¬ tains its effectiveness in spite of significant changes in the resonant frequency of the color selection electrode which may result from heating and cooling of the elec-

35 trode.

Another feature of such vibration damping means is that it does not occupy any portion of the

scanned active area of the electrode and therefore casts no shadow on the picture area of the screen.

Further, vibration damping means in accordance with, the invention are low in cost, easy to install and 5 not apt to damage a fragile foil electrode.

Preferably, the vibration damping means of the invention are able to withstand the high temperatures encountered during tube processing and are compatible - with, the vacuum environment within a cathode ray tube. 10. Further features and advantages- of the pres¬ ent invention may best be unders-tood by reference to the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings, in the figures of which like reference numerals 15 identify like elements, and in which.:

Figure 1 is a -cut-away view in perspective of a cabinet that houses a cathode ray tube and showing major components, relevant to the disclosure;.

Figure 2 is a side view in perspective of the 20 color cathode ray tube of Figure 1 showing another view of the components depicted in Figure 1 together with cut-away sections that indicate the location of vibration damping means of the invention;

Figure 3 is a plan view- schematically showing 25 one location of the vibration damping means to the inner surface of the cathode ray tube faceplate shown by Fig¬ ure 2;

Figure 3A is a schematic showing of another location of the vibration damping means; 30. ^* Figure 4 is aview in elevation of a section of the front assembly and associated tube funnel;

Figure 5 is a schematic illustration of a vibration mode of a flat color selection electrode;

Figure 6 is a perspective view of one form of 35 device for carrying out the concepts of the invention; Figure 7 is a schematic illustration of how the yibration damping■means, of the invention functions; and.

Figures 8 - 16 are perspective views of other forms of devices for carrying out the concepts of the invention.

With reference to Figure 1, there is shown a

- video monitor 10 that houses a color cathode ray tube

12. The tube could as well be contained in a television console of the home entertainment type. The tube 12 shown is notable for the substantially flat imaging area 14 that makes possible the display of images in ₀ undistorted form. Imaging area 14 also offers a more efficient use of the screen area as the corners are relatively square in contrast to the rounded corners of the present-day home entertainment cathode ray tube.

With reference also to Figures 2, 3 and 4, a

3_5 front assembly 15 is depicted and includes a glass faceplate 16 noted as being flat, or alternately, "sub¬ stantially flat," in that it may have finite horizontal and vertical radii, for example. Faceplate 16, depicted as being flangeless, is indicated as having on its inner

2o surface 17 a centrally disposed phosphor screen 18 on which is deposited an electrically conductive film (.not shown)., typically composed of aluminum. The phosphor screen 18. and the conductive film comprise the electron beam target area.

25 Screen 18 is shown as being surrounded by a peripheral sealing area 21 adapted to be mated with a funnel 22. Sealing area 21 has, by way of example three cavities: 26A, 26B and 26C therein. The cavities provide, in conjunction with complementary rounded in-

30 dexing means, for indexing faceplate 16 with funnel 22. Funnel 22 has a funnel-sealing area 28 with second indexing elements 30A, 3OB and 30C therein in orientation alike to indexing elements 26A, 26B. and 26C. Indexing elements 30 and 30B" are depicted in Figure 4

35 as being V' -grooves in facing adjacency with, the

- 6 - cavities 26A and 26B. (.Indexing elements 30C and 26C are similarly located.) The V-grooves of indexing elements 30A, 30B and 30C are preferably radially oriented, and the indexing elements are preferably located at 120 degree intervals in ihe funnel-sealing area 28. Ball means 32A, 32B- and 32C, which provide the afore-described complementary rounded indexing means, are conjugate with, the indexing elements 26A, 26B and 26C, and 32A, 32B. and 32C for registering faceplate 16 and funnel 22.

Front assembly 15 includes a tension foil shadow mask support structure 34, noted as being in the form of a frame secured to the inner surface 17 of faceplate 16 between the centrally disposed screen 18 and the peripheral sealing area 21 of faceplate 16, and enclosing screen 18. The shadow mask support struc¬ ture 34 is preferably- composed of sheet metal, and is secured to the inner surface 17 on opposed sides of screen 18, as indicated by Figure 4. A foil shadow mask 35 is secured in tension on structure 34 at the locations indicated by asterisks in Figure 4.

As seen in Figures 1 and 2, a neck 36 extend¬ ing from funnel 22 is represented as enclosing an elec¬ tron gun 38 which is portrayed as emitting three elec- tron beams 40, 42 and 44. The three beams serve to selectively excite to luminescence the phosphor deposits on the screen 18 after passing through the parallax barrier formed by shadow mask 35.

Funnel 22 is indicated as having an internal electrically conductive funnel coating 43 adapted to receive a high electrical potential. The potential is depicted as being applied through an anode button 45 attached to a conductor 47 which conducts a high elec¬ trical potential to the anode button 45, which projects through the wall of funnel 22. The source of the poten¬ tial is a high-voltage power supply (not shown) . The potential may be, for example, in the range of 18 to 30 kilovolts, depending upon the type and size of cathode ray tube. Means for providing an electrical connection

between the sheet metal frame 34 and the funnel coating 43 may comprise spring means 46, as depicted in Figure 2. An internal magnetic shield 48 provides shielding for the electron beam excursion area and the front as- se.nbly 15 from the influence of stray magnetic fields. A yoke 50 is shown as encircling tube 12 in the region of the junction between funnel 22 and neck 36. Yoke 50 provides for the electromagnetic scanning of beams 40, 42 and 44 across the screen 18. The center axis 52 of tube 12 is indicated by the broken line. Items designated as "radially extending " extend radially outwardly from this axis.

The above description of the video monitor, color cathode ray tube and shadow mask has been pre- sented for exemplary purposes to illustrate one applica¬ tion of the vibration damping means of the invention. Ho-vever, it should be understood that the invention is readily applicable for any color selection electrode other than a "shadow mask." In essence, a shadow mask of a color cathode ray tube, or other color selection electrode of the type ^• with which, this invention is concerned comprises a rectangular membrane suspended in high vacuum under high mechanical tension. The mask therefore is capable of vibrating in "membrane modes" which are the two- dimensional equivalent of the vibrations of a stretched string. As illustrated in Figure 5, a color selection electrode 56 is suspended under high mechanical tension between surrounding support rails 58. The rails are fixed to a glass faceplate 16* which is part of the glass envelope for the color cathode ray tube. A color phosphor screen or layer is formed on the inner surface of panel 16', as at 60. The electron beam emitted from the electron gun of the cathode ray tube passes through color selection electrode 56 for impingement upon phosphor screen 60.

Figure 5 illustrates, in dotted lines, the

resonant vibration of color selection electrode 56 as observed along a horizontal or vertical center line. It is apparent that the most prominent membrane mode is the fundamental one shown in Figure 5, with maximum 5 amplitude in the center of the electrode. Such vibra¬ tion causes incorrect electron beam interception by the electrode. The resulting "landing errors" are most prominent at two points on the horizontal center line located approximately 55% of the distance from the center

10. to the edge of both sides of the electrode, as indicated by lines 61. For instance, for a mask deflection of one mil at the center of mask 10, the landing errors at the two worst points are approximately .26 mils. In high resolution color cathode ray tubes, such resulting land-

15 ing errors are not acceptable. Because of the absence of damping in high, vacuum, the electrode, once excited by any kind of shock, may vibrate for a period of one minute or longer, corresponding to a "Q" in the order of 130,00.0.

20 The amplitude of vibration of the electrode at points other than the center of the mask is a sinusoidal function of position. There may also be a problem with one of the first overtones. For instance, the frequency of the fundamental mode may be approximately 500Hz.

25 Tie first horizontal overtone C ith a vertical nodal line! may be at approximately 750Hz.

Figures 3 and 3A show schematic locations for the electrode vibration damping means of the invention. In Figure 3, the damping means are shown located inter-

30 mediate the ends of the "long" sides of the color sel¬ ection electrode, as at "X", the region of maximum peripheral motion for the fundamental mode and the first vertical overtone. Figure 3A shows the location of the damping means intermediate the ends of the"short" sides

35 of the color selection electrode, as at "Y", the region of maximum peripheral motion for the first horizontal overtone.

- 9 - Briefly, the invention contemplates in one preferred embodiment an improved color selection elec¬ trode damping system incorporating a dynamic vibration damper which, avoids frequency tracking problems by using the electrode tension to determine the resonant frequency not only of .the. electrode but also of the damper device. Th.e damper ^" includes rigid means secured to the edge of the tensed electrode and dissipative or resistive means connected to the rigid means and spaced from the tensed

~-~ electrode. In this preferred embodiment, resistive load¬ ing of the rigid means is achieved by lossy flexural means. In essence, the system involves the use of coupled resonators.

More particularly. Figure 6 shows one possible

1 construction of the coupled resonator vibration damping means, generally designated 62, of. the invention which includes a channel-shaped elongated member in the form of a bar 64 for amplifying the vibration in the electrode 56. Bar 64 is secured to a bracket 66 which, in turn,

~ ~- is. secured to tensed color selection electrode 56 on the marginal portion of the electrode, immediately in¬ side supporting rail 58. Bracket 66 is in the form of an angle-bracket to provide rigid support for rigid channel-shaped bar 64. The bracket preferably is fab-

25 riσated of relatively heavy metal material, such, as 0.020 inch steel, so as not to flex. Bar 64 is made of thinner material such as .015 inch, steel in order to reduce its moment of inertia, but it is channel-shaped to op¬ timize its flexural rigidity. The bracket 66 may be spot

³⁰ - welded to electrode 56, with the bar 64 spot welded to the bracket, or a one-piece construction may be provided. The two-piece construction shown may be preferred be¬ cause the projecting bar may make handling of the elec¬ trode during photoscreening of the cathode ray tube more

- ^■ - ^■ difficult. Making bar 64 and bracket 66 rigid, i.e., keeping their compliance negligible compared to the compliance of electrode 56 to which, the means 62 is

- 10 - secured, ensures frequency tracking when the electrode tension changes, as will now be described.

It can be noted in Figure 6 that bracket 66 and the attached channel-shaped bar 64 are angled rεla- 5 tive to the faceplate in order to accommodate a magnetic shield which will be mounted on rail 58 over the color selection electrode. The angle must not be too great so as not to interfere with the electron beams as they are scanned to the edge of the screen. In addition, it can 0 be seen that bracket 66 has a low profile versus the higher bar 64. The bracket is deliberately kept low in profile because it is attached to the electrode before it goes through, the screen exposure process steps. A tall bracket could catch on an operator's clothing or 5 otherwise cause interference. Therefore, the bar is welded to the bracket after all screening operations are completed. In this manner, amplification of vibration is achieved without having a high bracket throughout the screening processing. .

20. Figure 7 illustrates schematically the con¬ dition when a moment is applied to rigid means 62 (here shown as bracket" 66 and bar 64) . The bar and bracket remain rigid and rotate together about axis of rotation 68, while electrode 56 stretches to permit such rotation.

25 The angular s-tiffness, defined as the applied moment divided by the angular displacement, is a function of the size and shape of the bracket support area (i.e., the area defined by the spot welds between the bracket and the electrode)., and also is proportional to the tension

30. in electrode 56. The resonant frequency of angular vi¬ bration of bar 64 and bracket 66 about axis 68 is there¬ fore proportional to the square root of the electrode tension. This same relationship, however, is true for the resonant frequency of the electrode itself. Con-

35 sequently, as the tension relaxes when the electrode is heated by the electron beam during tube operation, the

resonant frequencies of the electrode and the bar de¬ crease at the same rate, and frequency tracking is en¬ sured.

How the bracket-bar assembly 62 functions as a dynamic vibration damper for a selected resonant mode, e.g., the fundamental membrane mode of electrode 56, will now be explained.

If assembly 62 were held in a fixed position, vibration of the electrode would result in the portion of the electrode adjacent to bracket edge 70 (Fig. 7) moving up and down while turning about edge 70 as its axis. Since the electrode is under tension, it would exert an alternating force upon assembly 62, attempting to set it into angular vibration. Conversely, if the electrode were held in a fixed position at its center, angular vibration of as¬ sembly 62 would displace edge 70 up and down, attempting to set electrode 56 into vibration. Electrode 56 and assembiy 62 thus represent two coupled resonators. As previously stated, their resonant frequencies are made substantially alike. As is well-known, a pair of coupled resonators exhibits two new resonant frequencies; for an experimental structure consisting of electrode 56 and bar-bracket assembly 62 as described, each, separately resonant at 470 Hz, the two coupled resonances were ob¬ served to occur at 447 Hz and 494 Hz.

In a system of two coupled resonators, energy originally present in one resonator is rapidly trans¬ mitted to the other, and the entire system can be damped by applying damping to just one of the resonators. As¬ sembly 62 functions to extract vibratory energy from electrode 56 and render it accessible to resistive means 72 (Fig. 6) wherein it may be dissipated.

In the preferred embodiment, the resistive means 72 includes flexural means for applying resistive damping to bar 64. The flexural means is capable of

- 12 - propagating energy in the form of flexural waves. In an environment where viscous liquids or eddy current damping devices cannot be used, such, as in the vacuum environment of a color cathode ray tube, it is difficult

5 to produce a well-defined mechanical resistance. How¬ ever, the invention illustrates various forms of suit¬ able flexural means such as that shown in Figure 6. More particularly, a flexural wave transmission line 74, such as a wire or a thin, flat strip, is connected be- 0 tween bar 64 and a support 76. The wire preferably may be stranded in order to provide increased flexibility as well as. internal frictional resistance. The propaga¬ tion velocity of flexural waves in a given wire or strip is proportional to the square root of frequency, and it 5 decreases as flexibility increases. Low propagation velocity is desirable because, to obtain sufficient damping, the transmission line should be approximately

₄ 2-4 wavelengths long. To allow convenient placement of the line, inside a cathode ray tube, the wavelength

20. should therefore not exceed 2-3 inches. At 500 Hz this requires a maximum propagation velocity of 1,000- 1,530- inches per second. In practice, a stainless steel wire rope which is stranded with, seven strands of 0..0.11 inch, wire has been used successfully. The wire

25 is attached to the top of bar 64 by a small flexible clip made of 0.005 inch, thick, steel. Its measured propagation velocity at 470 Hz is approximately 25 meters (.1,00-0. inches), per second.

It has been found that if wire 75 is made

30. approximately 40 inches long, its natural losses (.pre¬ sumably friction between strands) suffice to provide the desired resistive behavior: A flexural wave at 400- 50.0. Hz, launched at one end and reflected from the other, is sufficiently attenuated upon its return to the launch-

35 ing end to make the mechanical impedance of the line substantially resistive, equal to its characteristic

iu tsu iict! wu-icn. xs cne proαuct or πexurai wave velocity and mass per unit length. However, the same effect can be obtained with a six-inch wire (approximately three wavelengths long) by loosely stringing light objects upon - the wire. When the wire vibrates in flexure, these objects rattle and thereby extract energy from the vibration, converting it to random vibrations and eventually into heat, resulting in damping the bar 64 and electrode 56 vibrationally coupled thereto. 0 Figure 6 shows one embodiment wherein steel bushings 78 are strung on wire 74, with some clearance between the bushings so that they can vibrate freely. The resulting damping action has been found to be in¬ distinguishable from that observed when the wire was _5 loosely wrapped with sound-absorbent textile or paper- based material which., of course, cannot be used in a cathode ray tube. When the electrode is caused to vibrate in its lowest frequency mode by a brief driving pulse, the time constant of amplitude decay is on the order of o -20 milliseconds. In actual practice, 23 steel bushings, 1/4 inch long, having 0.040 inch I.D. and 0.078 inch O.D. were strung on the stranded wire 74.

Figure 8 illustrates another embodiment wherein a coil spring 80 is positioned in loose surrounding re- 5 lationship about wire 74. Such a spring can also be used for vibration damping and may have advantages, from a manufacturing standpoint, over multiple small parts such as bushings 78.

There may be instances wherein it is i prac- 0 tical to place a supporting bar 76 at a corner of the cathode ray tube envelope. Figure 9 shows an alternate form of the invention wherein wire 74' is doubled-back toward means 62 whereby one end 82 of the flexural transmission line is secured to the top of bar 64, and 5 an opposite end 84 of the line is secured to bracket 66. The line is folded back onto itself, as at 86. Again, loose objects, such as bushings 78, are strung

along both portions of the line which may be shaped as a triangle, as shown. The transmission line thereby becomes self-supporting.

Figure 10. shows another embodiment of a coupled _c resonator system of the invention wherein, instead of using a lossy flexural transmission line, the vibration' damping means comprises a flexurally resonant stranded wire. Two s±randed wires 88 are shown secured to op¬ posite sides of bar 64. As is known, stranded wire is •J_ _Q much, more flexible than solid wire of the same cross- section. When stranded wire flexes, th individual strands: slide against each other, causing friction which extracts, vibratory energy, and thereby provides damping. Dimensioning the wire to be at least approximately resonant ■ _j e increases its amplitude and facilitates energy loss. Alternatively, a lossy fibrous mass may be attached to bar 64 to provide damping.

Figure 11 shows another embodiment of the _Λ invention wherein a plurality of resonators ^" are provided 20 which, resonate at different frequencies with the range of frequencies at which electrode 56 is expected to resonate as it heats up during tube operation. Specifical¬ ly, a plurality of compliant reeds 90 are secured to bracket 66. As a reed blends as it vibrates, the bending 25 of the lossy material extracts energy from the system. The compliance ^" of the reeds, in combination with the compliance provided by the electrode, establish different resonant frequencies for the different reeds. The reeds can be of different lengths, as shown, and/or of dif- _3Q; ferent thicknesses to resonate at different frequencies. The reeds should be at least somewhat lossy. For ex¬ ample, they may be made of pure magnesium which is known to have vibration-damping properties.

Whereas the embodiment of Figure 6 provides 35 a self-tracking system, as described, with, excellent damping regardless of frequency. Figure 12 shows a

version which will not track electrode resonance changes, but is simple and employs a single lossy, compliant reed resonator 90 ' . This version offers the advantages of low cost and easy execution.

5 It should be noted that the resonant frequency of reed 90 * is determined by the effective mass of the reed in combination with its total compliance, i.e., the sumof the compliances of the reed itself and the compliance prevailing at bracket 66 on which the reed is

1 mounted. The latter compliance varies inversely with the tension of electrode 56. Therefore, the resonant fre¬ quency of reed 90*, while unable to track the temperature- engendered variations of the resonant frequency of electrode 56 completely, it follows these variations at

15 least in part.

Figure 13 shows an embodiment of the invention wherein, instead of using a mechanical transmission line- a lossy reed or the like, a: ^" form of "friction brake" 92 is used to extract energy from the system by a

23 rubbing action. The friction brake must be detuned, i.e., it does not resonate with bracket 66 and bar 64. The brake is secured to rail _. 12, as at 94, and includes a torsional spring portion 96. Friction between bar 64 and brake 92 is controlled by the torsional spring por-

25 tion and will extract energy from the system.

Figure 14 shows another embodiment of the invention, again using the coupled resonator principles. A relatively massive rod or wire 98 is welded to the peripheral portion near the apertured area of the elec-

30. trode. Wire 98 provides mass to the vibration damping means the same as bracket 66 described above. By properly selecting the mass of the wire, the wire can be set into resonance at the same resonant frequency as electrode 56. Since the electrode tension provides the compliance

35 for both the electrode resonance and the resonance of the wire, this system also will have the frequency track¬ ing feature. To extract energy from the system, an overlaid braid 100 is provided. The braid is not secured

to the wire but vibrates or "rattles" against it.

The braid can be welded to the electrode near the weld line of the electrode to rail 12.

Figure 15 shows an embodiment of the invention wherein electrode 56 is coupled to a lossy reed"resonator 102 by means of a weak, bent leaf spring 104. The reed is not mounted on electrode 56 but on rail 12, as shown at 106. Operation of this embodiment is analogous to ^" that described in connection with Figure 12, except that the resonant frequency of reed 102 does not track that of electrode 56 even in part.

Lastly, Figure 16 shows a simple embodiment of the invention wherein a simple energy absorber 108 is secured along the peripheral portion of electrode 56 to damp vibrations in the electrode. The energy absorber can be of braided material, for instance.

It will be appreciated that numerous modifica¬ tions in t e described embodiments of the invention will be apparent to those skilled in the art without depart- ing from its true spirit and scope. For example, damping of a resonator by resonant stranded wires (Figure 10), friction (Figure 13) or contact with a braid (Figure 14) may b used in embodiments other than those where it is illustrated.