US2980814A

US2980814A - Combined magnetic shielding and beam centering assembly for cathode-ray tubes or the like

Info

Publication number: US2980814A
Application number: US755270A
Authority: US
Inventors: Henry S Vasilevskis
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1958-08-15
Filing date: 1958-08-15
Publication date: 1961-04-18
Anticipated expiration: 1978-04-18

Description

April 18, 1961 H. S. VASILEVSKIS COMBINED MAGNETIC SHIELDING AND BEAM CENTERING ASSEMBLY FOR CATHODE-RAY TUBES OR THE LIKE Filed Aug. 15, 1958 2 Sheets-Sheet 1 NflE/Vff 1 05/ 77 ON 19f GAEE 5 INVENTOR. HEN/F) .5. VHS/LEVf/f/ April 1961 H s. VASILEVSKIS 2,980,814

COMBINED MAGNET I0 SHIELDING AND BEAM CENTERING ASSEMBLY FOR CATHODE-RAY TUBES OR THE LIKE Filed Aug. 15, 1958 2 Sheets-Sheet 2 1N VENTOR. HAW/9) 0'. mm EVjK/S United States Patent COMBINED MAGNETIC SHIELDING AND BEAM CENTERING ASSEMBLY FOR CATHODE-RAY v TUBES OR THE LIKE Pa., assignor to Phileo Philadelphia, Pa., a corporation of Penn- The present invention relates to television display systems and more particularly to improvements in beam shielding and centering devices for such systems.

The present trend in television systems is to reduce the over-all length of the television picture tube, first by increasing the maximum deflection angle of the beam, thereby to reduce the length of the bulb for a given screen size, and secondly by reducing the length of the neck of the cathode-ray tube as much as possible. In reducing the length of the neck it becomes necessary to move the cathode and first and second grids of the electron gun closer to the deflection yoke. This has introduced the need for means for shielding these elements of the electron gun from the effects of the fringing field of the deflection yoke.

The reduction in the length of the neck of the cathoderay tube has also made it impossible to locate conventional magnetic centering devices on the neck of the cathode-ray tube between the electron gun and the deflection yoke. The removal of the centering devices from the neck of the cathode-ray tube has made it necessary to find other means for positioning the point of impingement of the undeflected cathode-ray beam at the desired spot on the picture tube screen. Attempts have been made to increase the precision of manufacture of cathode-ray tubes so that no centering device is required. This greatly increases the unit cost of acceptable picture tubes. Other attempts have been made to rely on electrical corrective circuits associated with the yoke to provide centering. Another alternative which has been tried is to provide some form of magnetic centering devices within the yoke itself. Each of these last two alternatives has its disadvantages and generally adds to the cost of the picture tube and the deflection yoke.

The copending application of Thomas V. Di Paolo et 211., Serial No. 744,121, filed June 24, 1958, now US.

Patent No. 2,926,272, discloses a laminated shielding and centering member which may be placed on the neck of the cathode-ray tube at the end of the deflection yoke. This novel laminated shielding member, while requiring very little space on the neck of the cathode-ray tube, effectively reduces the fringing flux and provides centering in one direction.

It is an object of the present invention to provide an improved laminated shielding member which requires very little space on the neck of the cathode-ray tube and which performs the dual function of magnetic shielding and beam centering in two directions.

it is a further object of the present invention to provide a novel on-the-neck beam centering means which is located within the electrostatic focusing region and which controls the position of the beam in two directions.

Still another object of the present invention is to provide a novel two-direction beam centering arrangement which is external of the deflection yoke and yet does not require any substantial amount of additional space on the neck of the cathode-ray tube.

These and other objects of the present invention are achieved by providing a laminated magnetic shielding assembly comprising a highly conductive non-magnetic sheet-like member and two spaced sheet-like members of high permeability secured thereto. Each high permeability member is formed with a gap therein dividing that high permeability member into two magnetically separate portions. The gaps in the two high permeability members are disposed approximately at right angles to one another. An adjustable magnet is secured to said laminated magnetic shelding member in a position to bridge the gap between the two magnetically separate portions of the first high permeability member. A second adjustable magnet is secured to said laminated magnetic shielding assembly in a position to bridge the gap between the two separate portions of the second high permeability member. Control of beam centering is accomplished by positioning the magnets to vary the strength of the magnetic fields impressed between the two magnetically separate portions of each of said two members of high permeability.

For a better understanding of the present invention together with other and further objects thereof reference should now be made to the following detailed description which is to be read in conjunction with the accompanying drawings in which:

Fig. 1 is a view partially in section of a cathode-ray tube and deflection yoke assembly incorporating the novel centering device of the invention;

Fig. 2 is a graph showing the reduction in fringing flux which may be accomplished through the use of the novel shielding and centering device of the present invention;

Fig. 3 is a view of the combined shielding member and centering device of the present invention taken in a (iirection parallel to the axis of the cathode-ray tube;

Fig. 4 is a cross-sectional view of the combined shield ing member and centering device of Fig. 3 taken along the line 44 of Fig. 3;

Fig. 5 is an exploded view of the shielding device of Figs. 3 and 4; and

Fig. 6 is a plot showing the relationship between the position of the centering magnets and the static deflection of the cathode-ray beam.

Turning now to Fig. 1, member 10 is the neck portion of a cathode-ray tube on which the deflection yoke assembly incorporating the present invention is mounted. The neck flare of the cathode-ray tube is shown at 12. The end turns of one of the horizontal deflection coils are shown at 14 and 14a. One vertical deflection coil is shown at 16. An insulating coil form 18 separates the

horizontal deflection coils

14 and 16 and serves to maintain these coils in their proper position on the neck 10 of the cathode-ray tube. Member 20 represents the magneticyoke which serves as the external magnetic path for .the flux set up by the horizontal and vertical deflection coils.

In the embodiment of the invention shown in Fig. 1 the shielding and beam centering assembly 28 includes an insulating housing 363 which provides mechanical support for the shielding and beam centering elements. As will be shown presently, housing 30 also serves as a nonmagnetic spacing member between two magnetic members of the centering device. Housing 30 may also act as an enclosing means for the terminals of the deflection coils (not shown) and for this purpose it is provided with an annular flange 30a.

Shielding of the high frequency horizontal deflection field is provided by a highly conductive disc 32 which is disposed adjacent the end turns of the horizontal deflection coils 14a. Shielding of the lower frequency vertical deflection field is provided by a high permeability member 34 which is disposed between highly conductive member 32 and housing 30. Centering in the vertical direction is accomplished by providing an adjustable magnet 36 which bridges a gap that separates the high permeability shielding member 34 into two separate magnetic portions. In the embodiment shown in Fig. 1, magnet 36 is secured to the shielding and centering assembly 28 by means of a rivet 38. Magnet 36 is rotatably mounted on rivet 38 thereby providing means for adjusting the vertical centering field.

Centering in the horizontal direction is accomplished by providing a second sheet-like high permeability memher 4% and a second adjustable magnet 42. High permeability member 4% is spaced from the high permeability member 34 by the non-magnetic housing 39. Magnet 42 is pivotally mounted in a position bridging the gap e4 between the two sections of high permeability member 40. Support for magnet #32 is provided by rivet 46. As shown more clearly in Figures 3, 4 and 5, sheet-like memer 4! is so formed that it does not come in contact with magnet 36. Also, high permeability member 34 is shaped so as to reduce the shunting effectof this member on the field induced by magnet 42.

A dielectric member 43 may be interposed between the end turns Ma of the deflection coils and the highly conductive member 32. Dielectric member 48 prevents accidental electrical contact between the highly conductive member 32 and the end terminals or lead wires from the deflection coil circuits. It also provides a slight spacing between the horizontal deflection coil 14a and conductive shielding member 32. This spacing reduces the shunting elfect of the shield member on the high frequency horizontal deflection field.

Turning now to Figs. 3, 4 and for a more complete description of the novel shielding and centering assembly 28 shown in Fig. 1, it will be seen that the conductive disc 32 which is disposed closest to the deflection coils is a circular disc provided with an aperture 50 for receiving the neck portion of the cathode-ray tube. Conductive disc 32 is provided with a number of small holes 52 for receiving

rivets

38 and 46 which support

vmagnets

36 and 42, respectively, and to receive additional rivets which are employed in one commercial embodiment of the invention to secure the

elements

32, 34 and 40 to housing 30.

The high permeability member 34 is formed of two

half discs

34a and 34b which are separated by a nonmagnetic gap 56. The central region 58 of the gap is enlarged to provide an aperture for receiving the neck portion 10 of the cathode-ray tube. The half discs 34a and 34bare formed. with holes 6% which register with the holes 52 in conductive member 32 to receive the rivets mentioned above. an aperture 62 which is located adjacent magnet 42 on member 49. The outline of aperture 62 is shown by the broken line 62 in Fig. 3. The purpose of aperture 62 is to minimize the shunting efiect of half disc 34 on the gap 44 in member 40. In other embodiments of the invention the aperture 62 may extend to the outer edge of member 34a and thus form a notch in this outer edge. Although no aperture or notch is required in member 34b for proper operation of the invention, member 34b may be shaped the same as member 34a in order to reduce the cost of construction of the assembly 28.

As shown in Fig. 3, magnet 36 is pivoted so that it bridges the gap

Edbetween half discs

34a and 34b. Magnet 36 is magnetizedalong a diameter as indicated by the letters N and S in Figs. 3 and 5. The letters N and S designate the north and south poles of the magnet 36. It will be seen that rotation of magnet 36 changes the net magnetic flux impressed between the two

halves

34a and 34b of shielding and centering member 34. Obviously other equivalent means may be provided for supporting magnet 36 and magnet 36 may have shapes other than circular and still accomplish the same result. For example, magnet as may take the form of a simple bar magnet which is held in place by its own magnetism. If a more permanent attachment is required the magnet can Half disc 34:: is formed With,

be adjusted until the proper centering is achieved and then secured in place by a suitable adhesive.

Housing 36 is provided with an aperture 66 for receiving the neck portion til of the cathode-ray tube. A portion of housing 3% is also cut away as shown at 8 to receive the magnet 36. Since magnet 36 projects through the cut-away portion 68 of housing 3%, it may be readily adjusted without demounting the shielding and centering assembly of the cathode-ray tube. Again the holes 70 formed in housing 30 are provided for receiving the rivets.

As shown in Figs. 3 and 5, high permeability member 40 is formed of two similar portions 4% and 49b. Members 40a and till; are shown as having the some shape. Although this is not essential to the proper operation of the invention, it is desirable from a manufacturing standpoint since it minimizes the number of parts of different shapes that are required. Members and itlb are formed with a generally circular edge The edge portion 74 of member dim is so formed that her 4% does not overlie the magnet 36. The finger-like extensions 76 and '73 of members 40:! and 40b, respectively, encircle the neck portion 13 of the cathode-ray tube and terminate in a gap 30 which may be somewhat larger than the gap 4-4. The reduced cross-sectional of finger-

like portions

76 and 78 minimizes the shunting effect of member 49* on the gap 56 in member 3%. As shown in Figs. 3 and 4, sheet-like members and 3617 are secured to the rearward surface of housing 3t. Therefore member 49 is spaced from the member 34 by the thickness of non-magmetic housing 3%.

As mentioned in the description of Fig. 1, magnet 32 is so mounted that it bridges the gap between magnetically separate members 4012 and Magnet 42 is also magnetized along a diameter as represented by the letters N and S which designate the north and south poles, respectively, I" the magnet. As mentioned in connection with the description of magnet 35, magnet may have a shape other than circular and be held in position by means other than rivet 46.

The laminated shielding and centering assembly shown in the drawings prevents fringing oi the horizontal and vertical deflection fields in the following manner. The eddy currents set up in the highly conductive memoer 32 by theapproximately 15 kilocycle horizontal scening field tend to confine the flux from the end regions of. the horizontal deflection coils 14a to the region of neck portion it to the left of the assembly 23 as shown in Fig. 1. The shielding enect or" member 32 prevents the high frequency horizontal deflection flux from reaching the

high permeability members

34 and 4%. Therefore the novel laminated shield of the present invention shunts much less of the high frequency horizontal deflection field around the neck of the tube than would be shunted by a single layer shield of high permeability material at the same location. Member 32 also aids in reducing the hysteresis losses in member 34 by reducing the concentration of high frequency magnetic flux in this high permeability member. The reduction of the shunting effect and the reduction of the hysteresis loss both tend to minimize the loss of horizontal deflection power in the shielding assembly 28.

The eddy currents induced in conductive disc 32 as a result of the relatively low frequency vertical scanning field are usually not suificient to confine the vertical field to the region to theleft of disc 32. Therefore some of the fringing flux passes through conductive disc 32 to the

low'reluctance members

34a and 34b. This fringing flux is conducted around the market the cathoderay tube through

members

34a and 34b. The amount of this shunted fiux may be partially controlled by properly selecting the size of the gap 56 in the otherwise low reluctance path afiorded by

members

34a and 34b. In general, the size of this gap 56 will represent a compromise between the amount of defocusing which can be:

tolerated at the edges of the picture tube as a result of residual fringing flux and the permissible loss of vertical deflection power resulting from the shunting effect of the shielding means. In one typical embodiment of the invention gap 56 has a width of It has been determined experimentally that the shunting effect of the laminated shield member 28 on the low frequency deflection field can be changed measurably by including gap 56. This is so even though there are other relatively large air gaps in the path of the vertical deflection flux.

Fig. 2 is a plot of the field strength of the deflection flux along the axis of neck portion it? of the cathode-ray tube. The horizontal scale is in units of distance from the end plane of the deflection coils. The position of the control grid of the cathode-ray tube of Fig.1 is shown at 84 as a convenient reference point in evaluating the curves. Curve 86 represents the field strength without the assembly 28 in place. Curve 88 represents the field strength with the assembly 28 in place. It should be noted that the amount of fringing flux at the control grid region 84 has been reduced by a factor of approximately 3. At the same time the field strength at the end of the deflection coils is reduced only slightly.

Fig. 6 is a plot of the change in the static beam position which may be achieved by rotating magnet 36. The zero or reference direction is taken with the north-south axis of magnet 36 aligned with the gap 56, that is, with a diameter of disc 34. Rotating the magnet 36 from this reference position places the north-south axis across the gap 56. As a result there is a steady flux which flows through

half discs

34a and 34b and bridges the gap between the adjacent edges of these half discs. Some of this steady flux also fringes between the arcuate edges of the aperture 58, that is, across the region occupied by the neck portion 10 of the cathode-ray tube. This flux which fringes across the region occupied by the neck portion 10 of the cathode-ray tube causes a deflection of the cathode-ray beam in the vertical direction. The amount of deflection in the vertical direction will increase as the angle between the north-south axis of magnet 36 and the gap 56 increases. The maximum deflection, as represented by the peak 92 in Fig. 4, is reached when the north-south axis is perpendicular to the reference direction, that is perpendicular to the gap 56. Further rotation of the magnet 36 will reduce the amount of magnetic flux induced in

half discs

34a and 34b. This reduction in flux will reduce the vertical deflection of the beam. The deflection of the beam from its normal rest position will again be a minimum when the north-south axis of magnet 36 again moves into alignment with the gap 56. If the magnet is rotated so that the north pole is now on the opposite side of the gap from the position just described, the deflection will again be in the vertical direction but in the opposite sense as represented by the negative peak 94 in the curve of Fig. 4. That is, if the beam is directed downwardly with the north pole of magnet 36 in contact with half disc 34a, the beam will be deflected upwardly if the north pole is moved into contact with the half disc 3-4:). The misalignment of the beam-spot on the screen of the cathode-ray tube is usually caused by misali nments between the gun assembly and the neck of the cathode-ray tube rather than by misalignments in the gun assembly itself. Therefore centering of the spot on the screen on the cathode-ray tube may require an intentional misalignment of the beam with the axis of the gun. For this reason it is desirable to locate the region in which centering occurs as far forward on the electron gun assembly as possible. This location of the centering region minimizes the linear distance by which the beam is displaced from the axis of the focusing assembly by the centering means. Too great a displacement of the beam may place it in a region of poor focus and may cause the beam to strike elements of the gun or the neck of the cathode-ray tube. If the beam strikes an element of the gun or the neck of the cathode-ray tube it will cause shadowing of the picture on the screen. The novel shielding and centering device of the present invention places the centering region much further forward than is possible with conventional on-the-neck centering devices.

Since the only flux that is effective in positioning the beam is the flux that fringes across the neck portion 10 of the cathode-ray tube, the width of gap 56 must be selected with reference to the strength of magnet 36 to provide the necessary flux across the neck region of the cathode ray tube. a

The effective width of gap 56 is affected somewhat by the presence of the finger-

like portions

76 and 78 of member 40. Finger-

like members

76 and 78 of sheet-like member 40 are spaced from members 34a and 3411 by the non-magnetic housing 30. This reduces the shunting effect of these finger-

like portions

76 and 78 on the gap 56. Nevertheless it is desirable to make the width of

portions

76 and 78 as small as possible consistent with proper centering in a horizontal direction in order to still further minimize the shunting action of the sheetlike members 40a and 40b. Proper centering in the horizontal direction will usually require a cross-sectional area of finger-

like portions

76 and 78 which is large enough to prevent saturation of these portions by the flux from magnet 46.

The presence of the aperture 62 in disc-like member 34a-minimizes the magnetic flux induced in member 340 by the magnet .42. Furthermore any flux induced in sheet-like member 34a by magnet 42 will tend to fringe cross the neck portion 10 in a vertical direction as viewed in Fig. 3, rather than in a horizontal direction. Therefore this flux from magnet 42 will have very little effect on the vertical centering of the cathode-ray tube. Any effect introduced by the adjustment of magnet 42 can be compensated for by minor readjustment of the magnet 36.

Control of the position of the spot in the horizontal direction is achieved by adjusting the position of magnet -42. Rotating magnet 42 about its support 46 will cause the beam to change in position in the horizontal direction in a manner similar to that shown in Fig. 6. The actual shape of the curve may vary somewhat from that shown in Fig. 6 owing to the differences in the shape of

sheetlike members

40a and 34a, for example. The shape of the curve will also be affected by the larger gap at fail.

Nevertheless the curve will have the same general shape,

increasing to a maximum in one direction as the axis of magnet 42 is rotated to a position perpendicular to the gap 44, then decreasing to zero and increasing to a maximum in the opposite direction as magnet 42. is rotated through 180. The relatively large cross-sectional area of members 40a and 40b to the left of the figure as shown in Fig. 3 provides an adequate area of contact between members 4041 and 46b and magnet 42. This enlarged portion also permits the supporting rivets to be relatively widely spaced. This tends to reduce the variation in the size of gap 80 which might result from minor changes in location of the holes which receive the supporting rivets. Gap S0 is made relatively large so that any small variation in the position of the ends of finger-

like portions

76 and 78 which might occur during mass production assembly do not make an appreciable percentage change in the length of the air gap 80. increasing air gap 80 also tends to increase the amount of fringing flux which passes through the region occupied by the neck portion 10 of the cathode-ray tube. The shape of finger-like portions 76 and 73 and the length of the air gap 8t} will control in some degree the distribution of the fringing flux across the area occupied by the neck portion 10 of the cathode-ray tube. The shapes of finger-

like portions

76 and 78 and the length of gap 86 may be selected so as to tend to provide a uniform distribution of the flux from magnet 42 which fringes across the area occupied by the neck region 10 of the cathoderay tube; If there is a uniform distribution ofthis flux across the neck' region occupied by the neck portion 16, the presence of magnet 42 and the sheet-like members 4th: and 4th; will control the position of the spot but will have no airect on the linearity of the travel of the spot across the screen of the cathode-ray tube. Alternatively, the shape of finger-like portions 76 and 78'and the size of 86) may be purposely selected to provide a nonuniform distribution ofsteady magnetic field across the region occupied by the neck portion 10: This intentionally introduced nonlinearity may be employed to correct for nonlinearities in the horizontal sweep voltage generation circuit.

Again adjustment of magnet 36 shouldhave little or no etfect on the operation of the horizontal centering means represented by magnet 42 and sheet-likemembers itia and 49b. However if the magnet does have a slight eifect on the adjustment of the horizontal position of the beam, this change may becompensated for by a slight readjustment of the magnet 42.

Various changes and modifications may be made in the preferred embodiment shown which are within the scope of the present invention. For example the aperture 53 in member 34 may be given a shape other than circular in order to control the distribution of the flux from magnet 36 which fringes across the region occupied by the neck of the cathode-ray tube; Theshape of gap 56 may be changed for the same reason.

Members

32, 34 and 4% have been shown as fiat plates. It-lies'within the scope of the present invention to dish these members slightly so that they conform more closely to the end of the defiection yoke.

While the invention has beendescribed with reference to a single embodiment thereof, it will be apparent that various modifications and other embodiments thereof will occur to those skilled in the art within the scope of the invention. Accordingly I desire the scope of my invention to be limited only by the appended claims.

What is claimed is:

1. In combination with a cathode-ray tube system having magnetic deflection coils for creating a relatively high frequency magnetic deflection field in a first plane and a relatively low magnetic deflection field in a second plane, a laminated magnetic shielding and beam centering assembly comprising a sheet-like member formed of a highly conductive material, said sheet-like member being apertured to receive the neck of said cathode-ray tube and disposed in adjacency with said defiection coils, a first pair of semicircular coplanar plates of high pero is:

meability material disposed in juxtaposition with said sheet-like member on the-side of said sheet-like member remote from said coils; selected portions-ofthe diametrical edge of one of said first pair of plates being disposed in spaced juxtaposition to corresponding portions of the other plate of said first pair thereby to define agap pair of plates and an integrally formed, arcuate, finger like portion of relatively small radial dimension extending partially around the neck of said cathode-ray tube, a selected portion of an edge of said main portion of one of said plates in said second pair being disposed in spaced juxtaposition to a corresponding portion of the other plate in said second pair thereby to define a second gap extending transversely to said first gap, a second permanent magnet bridging said second gap between said second pair of said coplanar plates, said second magnet being so oriented as to establish a magnetic field between said second pair of said coplanar plates and across the neck of said cathode ray tube, one of said plates of said first pair being formed with an aperture therein underlying said gap between said second pair of plates, saidsecond magnet extending through said last-mentioned aperture in a non-contacting relationship.

2. A laminated magnetic shielding and beam centering assembly as in claim 1, wherein the ends of said arcuate, finger-like portions define a third gap wider than said second gap and in line therewith.

References Cited in the file of this patent UNITED STATES PATENTS 2,634,381 Kafka Apr. 7, 1953 2,653,262 Bowman Sept. 22, 1953 2,717,323 Clay Sept. 6, 1955 2,761,989 Barkow Sept. 4, 1956 2,813,212 Grundmann Nov. 12, 1957 2,817,782 Over Dec. 24, 1957 2,860,329 Reiches Nov. 11, 1958