AU600158B2 - Magnetic shunt for deflection yokes - Google Patents

Description

s 0 0 15 Ref: 65519 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int Class Complete Specification Lodged: Accepted: Published: Priority: Related Art: LIw U Mu1IKwet ,oflK44 Lb aM da-ants moadt uds Section t I -Y l -+C-0~l Name and Address of Applicant: Address for Service: International Business Machines Corporation Armonk New York 10504 UNITED STATES OF AMERICA Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: Magnetic Shunt for Deflection Yokes The following statement is a full description of this invention, including the best method of performing it known to me/us 5845/3 MAGNETIC SHUNT FOR DEFLECTION YOKES BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to display apparatus, and more particularly relates to apparatus for reducing unwanted magnetic radiation external to a cathode ray tube display device, in front of the screen thereof.

Background Art Cathode Ray Tubes ("CRTs") generally have associated coils, or yokes, to provide a varying magnetic field for electron beam deflection, for example for raster scan. In addition to manifestihg itself within the CRT, for beam deflection, this magnetic field also extends outside of the CRT, and even in front of the screen. This external magnetic field serves no useful purpose and an effort is frequently made to reduce this part of the yoke magnetic field.

o Means to provide this reduction have been proposed in the prior art. For example, one such proposal is the i 20 provision of. Helmholtz coils disposed "on top of", or radially away from and adjacent to the saddle-shaped deflection yoke. The coils are coupled to the deflection )o coils and the EMF is induced therein, giving rise to a magnetic field which tends to cancel the residual magnetic field in front of the screen. However, this is a relatively expensive and bulky solution to the problem.

KI9-87-005 2 Another proposed solution is the placement of shielding all around the CRT, which results in magnetic radiation reduction from the eddy currents induced in the shielding. However, this Is also an expensive solution to the problem, and results in only minimal reduction in the magnetic field in front of the screen.

Accordingly, there is a need for means to reduce to acceptable levels the residual magnetic field in front of the cathode ray tube display device that provides an inexpensive and compact solution to the problem.

SUMMARY OF THE INVENTION In accordance with the present invention there is disclosed a cathode ray tube display apparatus comprising a screen for viewing, means for producing a charged particle beam directed at said screen from the rear thereof and aligned with a central axis, a coil yoke having first coil portions aligned axially and second coil portions aligned circumferentially relative to said central axis for producing magnetic field compo;v'nts forming a desired magnetic field to deflect said beam and giving rise to an undesirable magnetic field in front of said screen, and a substantially complete ring having high magnetic permeability, substantially centered on said central axis and disposed near said coil yoke between said coil yoke *29'0 and said screen, said ring having configuration, magnetic permeability and position relative to said coil selected to minimize said undesirable magnetic field.

The present invention finds application in a cathode ray tube display apparatus compricing a screen for viewing, means for producing a charged particle beam directed at the screen from the rear thereof and aligned with the central axis, and a coil yoke having first coll portions aligned ogoo' axla!ly and second coil portions aligned circumferentially relative to the central axis for producing magnetic field components forming a desired magnetic field to deflect the beam and giving rise to an undesirable magnetic field in front of the screen. In order to reduce the undesirable magnetic field, the display apparatus further comprises a substantially complete ring having high magnetic permeability, substantially centered on the central axis and disposed near the coil yoke between the coil yoke and the screen, The ring has its configuration, magnetic permeability and IAD/96lo 00* 2A position relative to the coil selected to minimize the undesirable magnetic field.

The invention may be embodied in forms which are made of relatively inexpensive linear ferrite materials os 00o 0-o IAD/9610 configured in shapes that are inexpensive to provide, such as a flat ring or the like. As such, it permits a relatively inexpensive solution to the problem. In addition, in tested embodiments the present invention has demonstrated dramatic reductions in the unwanted radiation in front of CRTs to which it has been applied.

The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS C, Ci C C) C 151 C, i Q)

C

o C C Cq Fig. 1 is a diagram showing pertinent portions of an integrated yoke tube component.

Fig. 2 is a simplified diagram of one winding each from the "pper and lower deflection coils of the integrated yoke tube component shown in Fig. 1.

0 09 C 0 Fig. 3 is a along the Z axis shown in Fig. 1.

plot showing the magnetic field intensity for a typical deflection yoke such as is ,n 20 Fig. 4 is a thereto a ring embodiment of the o os 0 AI Fig. 5 is a thereto a ring embodiment of the figure like that of Fig.

50 in accordance with present invention.

diagram like that of Fig.

50 in accordance with present invention.

1, having added the preferred 2, having added the preferred KI9-87-005 -3- 1 Fig. 6 is a plot showing the effective mu vs. actual mu for the ring depicted in Fiqs. 4 and Fig. 7 is a set of curves, on the same set of axes as in Fig. 3, showing the effect on the net field A of ring Fig. 8 is a set of curves showing the effect of ring on the end turn field shown in Fig. 3.

Fig.9 is an expanded view of the portion of the curve shown in Fig. 7 beyond approximately 2.5 centimeters.

Fig. 10 is a plot like that of Fig. 9, wherein ring is a slightly different distance from the yoke.

Fig. 11 is a diagram like Fig. 9, in which the inner diameter radius of ring 50 is slightly different from that of Fig. 9.

SEig. 12 is a curve like that of Fig, 9 but wherein the distance of the ring 50 from the end of the yoke is different from that of Fig. 9 and Fig. Fig. 13 is a diagram of a further embodiment, which includes a lip portion 62.

Fig. 14 shows a still further embodiment in which a ring is provided having two portions.

Fig. 15 shows a still further embodiment made by injection molding techniques of a material such as nylon impregnated with ferrite particles.

X19-87-005 I Fig. 16 is a cross-sectional diagram through a portion of a still further embodiment of ring, made with conventional mu metal laminates.

Fig. 17 shows a further embodiment, having a hexagonal shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows the pertinent portions of an integrated yoke tube component 10 wh ch includes a CRT 12, having a front screen 14, and upper and lower horizontal deflection coils 16, 18. The deflection coils 16, 18 generate a varying magnetic field between them, inside CRT 12, to deflect the electron beam within the tube 12 for horizontal sweeping across the face of the screen 14, as is well known in the art.

15 Fig. 2 is a simplified diagram of one winding each from the upper and lower deflection coils 16, 18, of Fig. 1.

Thus, loop 20 is a single loop from coil 16, while loop 22 is a single loop from coil 18. As illustrated, a current i flows through each of the coils so as to generate the above described varying magnetic field for horizontal deflection of the electron beam. The useful portion of the loops 22 are the axially aligned portions thereof 24, 26, 28, o which produce the main deflection field.

The circumferentially aligned portions of the loop (end turns) 32, 34, 36, 38 serve only to complete the circuit of each of the respective loops 20, 22, and are otherwise unnecessary for the operation of the deflection coils 16, KI9-87-005 ii 18. These circumferentially aligned coil portions 32, 34, 36, 38 contribute the major portion of the residual distributed magnetic field that extends in front of the screen 14 (Fig. 1) which is to be reduced. In effect, the residual field is the vector sum of the main deflection field and the end turn field. The resulting sum will fc.-.low the polarity of the end turn field, since the end turn component is the larger, and both decay at the same rate with distance.

.In Fig. X, Y, and Z axes are depicted, having their origin in the plane of circumferential coil portions 34, 38 and centrally located between them. The Z axis coincides with the central axis of CRT 12 (Fig. 1) Note f that the upper a, lower halves 20, 22 are symmetrical about the x-z and y-z planes.

o In actual operation the upper and lower loops 20, 22 are interconnected to produce a dipole field on the Z axis, a, as is known. From the known coil shape and current, the B field is given by: B dl O4 i4 a^ d a.

where J is the current, R is the direction and R is the o o distance to a point of interest T on the Z axis.

A plot of the B field distribution of a typical horizontal deflection coil, such as is shown in Fig. 1, h=ga high permeability material, like forrite, is shown in Fig. 3. The actual B field is a directional field, and the plot shown in Fig. 3 shows only the magnitu'de, or intensity, of such magnetic field along the Z axis. The units depicted on the horizontal axis are centimeters, while KI9-87-005 -6- .?.A4

I

the units in the vertical axis are gauss. The curve reflects a typical coil having current flowing so as to produce a field which deflects a 20 kilovolt electron beam to an angle of about 40 degrees.

Curves A, B, and C of Fig. 3 represent the total field, the partial field from the axial wir 's and the partial field from the end turns, respectively. Curve A is t:he magnitude of the vector sum of the fields represented by curves B and C. In typical uncompensated yokes, at 55 centimeters in front of the yoke the field can be in range of approximately 1,000 2,000 nano-Tesla. Clearly, this is not a very large magnetic field. However, in accordance with the present invention this field can be reduced to an even smaller quantity. In actual experiments using the preferred embodiment described below, reductions to below 200 nano-tesla at 55 centimeters was measured.

Fig. 4 shows the ITC 10 of Fig. 1 having added thereto a ring 50 of linear ferrite operating as a magnetic shunt, in accordance with the E aferred embodiment of the present invention.

Fig. 5 shows the loops 20, 22 of Fig. 2, with the ferrite ring 50 disposed in front of it, to illustrate the relative shape and position of ring Rirg 50, as mentioned above, is a linear ferrite.

25 Linear ferrite is a well known material commonly used in transformer and yoke production. According to the preferred embodiment the ring 50 has a relatively high magnetic permeability, or mu. It also has a high volume resistivity, or rho, for example 1 Meg Ohm or more per cubic centimeter The high rho value keeps eddy currents at a minimum.

K19-87-005 -7i ~C: 1 Otherwise the loading effects on the yoke would result in a need for more energy to drive the yoke. While embodiments could be constructed, for example out of conventional mu metal laminates, having this loading effect, and be in accordance with the present invention, it was deemed desirable to keep the eddy currents low, and avoid this loading effect in the preferred embodiment. The cross section of the ring 50 is large enough to avoid saturation.

Referring to Fig. 6 a plot is shown of the variation of effective mu, mu versus actual mu, mua, for a ring such as ring 50 positioned in front of coils 20, 22, as shown in Fig. 5. It can be seen that the effective mu rises abruptly for very low values of mu a and then reaches some point where it remains relatively constant in spite of ever increasiqn 15 mu A value of 1,000 represents a point such as point 52 o o" for a linear ferrite ring having dimensions typical for the ,,application described herein. If a mu value of, for c example, 10 were selected, it would be in the sloping area o. 53 of the curve shown in Fig. 6. Such a material would be highly susceptible to variations in manufacturing tolerances, temperature of operation, and the like, and would therefore provide erratic performance depending upon the variation of these factors. By selecting the permeability to be in the flat, horizontal area of the curve 43 25 of Fig. 6, the above described undesirable variations in performance are substantially avoided. However, the material cost considerations will tend to kee F the o.o permeability of the material low within the range of acceptable permeability for providing this preferred stability.

Fig. 7 is a set of curves, on the same set of axes as these of Fig. 3, showing the effect on the net field A shown KI9-87-005 -8- 99 in Fig, 3 of a flat ring, such as ring 50 in Fig. 4, In accordance with the preferred embodiment of the present invention, Curve A in Fig. 7 is the same as curve A in Fig. 3. Curve D in Fig. 7 represents the field contribution from the magnetization effect of the ring 50, while curve E rep'resents the resultant curve from the combination of curves A and D, For a better understanding of the effect of the field represented by curve D on the overall magnetic field represented by curve A, Fig, 8 shows a set of curves including curve C representing the end turn magnetic field component. Curve C is the same curve C as is shown in Fig. 3, Curve F is a curve representing the resultant field from the combination of curves D and C. Note that in Fig, 8 the horizontal axis is the same as that in Figs. 3 and 7 while the vertical scale has been expanded, to aid In clarity, As mentioned above, curve D is the theoretical field of the ring alone. This is an Intrinsic field which is created by the magnetization force of the end turn field. It should be noted that the presence of the ring attenuates the end turn field, The degree of attenuation Is controlled by the variables such as ring dimensions and ring yoke separation, as is discussed In more detail below, It should be further noted that the end turn field combines with the main deflection field, and 0-20 the area In front of the CRT screen, to form the net measurable residual field whose reduction is an object of this Invention, At optimum attenuation, the modified end turn field F is equal in magnitude but opposite in direction to the main deflection field, resulting in a zero vector sum, As a practical matter, the net measurable residual field In Front of the CRT screen can never be IAD/961o L. 1~ :1 ii i reduced to zero. However, by application of the principles of the present invention afs disclosed herein, this field can be reduced to very small levels.

The portion of Fig. 7 beyond approximately centimeters to the right thereof is shown in Fig. 9. In order to see clearly the curve behavior in.that region, the scale is expanded in the vertical direction as compared with Fig. 7. Curves A and E are as described in Fig. 7. Curve D is not shown in -ths Fig, 1 in the interest of providing more clarity for curves A and E. Note that Curve E is very nearly at a zero field magnitude at approximately centimeters.

The compensated curve E for a typical CRT-yoke configuration, having a ring 50 of ferrite with a o 15 permeability of 1,000 3,000, and a rho of 1 meg ohm per cubic centimeter or more, and having an inner dimension of 4 centimeters, a thickness of .2 centimeters, a width of 1 Scentimeter, placed at a distance of .4 centimeters from the end of the yoke. As used herein, the width of the ring refers to its radial extent from inner diameter to outer Sdiameter.

Figs. 10-12 are plots like the plot shown in Fig. 9, for slightly different ring configurations from the I configuration producing the curves of Fig. 9. Thus, in Fig.

,i o, 25 10 all of the parameters for the ring are the same as those corresponding to Fig. 9, except the distance of the ring from the end of the yoke. In Fig. 10 the curves correspond to a configuration in which this dimension is .3 centimeters. It will be appreciated that this reveals Kt9-87-005 i j -C rl -Li :i over-compensation, as the curve E' is sligbi 1 farther frcm the horizontal ax~is, for example at 9.5 cent s.

The curves of Fig. 11 are for a configuration in which the dimensions are the same as those Ls-rresponding to Fig.

9, but wherein the inner diameter radius is 5 centimeters, instead of 4 centimeters. It can be seen that significantly less compensation is provided, as curve E" is here below the ho ri zontal axis.

Fig. 12 shows a curve for a configu~ration wherein the dimensions are as in Fig. 9, but whernin the distance of the ring from the end of the yoke is .6 centimeters, instead of .4 centimeters, it can be seen that slightly less compensation is provided, causing curve, 41 to crois the horizontal axis -at 9.5 centimeters. This was deemed to represent optimum compensation.

while curves are not provide(' showin~g the effe~ct of change of width of the ring on the compensation effectt in general, decreasing the width will tend to reduce the compensating effect# while increasing- the width will tend to inc~rease the effect.

Thuc, from the above Figs. 9-1.2, it Will be nppreciateA how changing the various aimensional piarameters Of the preferred embodiment of the presoent invention aff40ects the performance of the ring in compensating by cancelling the magnetic field componenta on the Z axi~s in front of the sc~reon due to yoke winding components$ Through an undorstanding of these effects, one practicing the present Invention can provide the adjustmenta deemed dlesira1e to optimize the canicellation effect.

4' -7-Q ilL

I

In an actual prototype experiment, in conjunction with an ITC manufactured by Matsushita Company having a series number of M34JDJOOX01, a ferrite ring of ordinary linear ferrite was provided, having a mu of approximately 1,000 3,000 and a rho of greater than 1 meg ohm per cc, ring dimensions of: an inner dimension of a width of and a thickness .of This ring was found to produce excellent cancellation effects when it was placed against tho\ circumferential wire portions of the yoke 0o provided with this ITC with spacing resulting only from the insulation of the yoke wires.

It should be noted that other configurations in accordance with the present invention may be used. For example, as shown in Fig. 13, a ring 60 having a lip portion 62, may be employed to advantage, the lip 62 being believed to serve to enhance the cancellation of the undesired field.

However, the additional machining required to make the configuration shown in Fig. 13 results in a more costly Sarticle than ring Another alternative configuration is that of a rirp C formed from two portions, such is is shown in Fig. 14.

Further, using injection molding techniques, for example with nylon impregnated with ferrite particles, a ring configuration having a cross section, such as is shown f 25 in Fig. 15 is also possible. It is believed that this 1 13 configuration also provides beneficial cancellation field shaping characteristics. However, it also represents a more costly article than a simple flat ferrite ring such as described above.

1I19-87-005 -12- Fig. 16 shows a hexagonally shaped ring, representing a still further embodiment for use with, for example, a hexagonally configured yoke.

Finally, embodiments may be made with conventional mu metal laminates, yielding rings having a cross-section as shown in Fig. While the invention has been described herein with respect to the preferred and various other embodiments, it will be understood by those skilled in this art that still other modifications and variations may readily be conceived by one of ordinary skill in the art to which it pertains, without departing from the spirit and scope of the invention as set forth herein. It is contemplated that all such variations, modifications and embodiments are encompassed within the scope of the appended claims.

0 .j 4 4 0 0 0 9C n .s KI9-87-005 -13-

Claims (1)

14- The claims defining the invention are as follows: 1. A cathode ray tube display apparatus comprising a screen for viewing, means for producing a charged particle beam directed at said screen from the rear thereof and aligned with a central axis, a coil yoke having first coil portions aligned axially and second coil portions aligned circumferentially relative to said central axis for producing magnetic field components forming a desired magnetic field to deflect said beam and giving rise to an undesirable magnetic field in front of said screen, and a substantially complete ring having high magnetic permeability, substantially centered on said central axis and disposed near said coil yoke between said coil yoke and said screen, said ring having its configuration, magnetic permeability and position relative to said coil selected to minimize said undesirable magnetic field. 2. The display apparatus according to claim 1 wherein said ring is a ring of ferrite having magnetic permeability of 1,00. or more. 3, The display apparatus according to claim 1 or 2, wherein said coil yoke is a saddle yoke and said ring is spaced from said yoke with no portion thereof under said yoke but all portions forward of said yoke toward said screen. 4. The display apparatus according to claim 1, 2 or 3 wherein said ring comprises multiple ferrite portions forming a ring. The display apparatus according to any one of the preceding claims wherein said ring comprises a pair of semi-circular ferrite portions separated by narrow gaps of non-ferrite material. 6, A cathode tube display apparatus substantially as described with reference to Figs, 4 to 12, or any one of Figs. 13 to 17 of the drawings. iAFED this FIFTEENTH day of MAY 1990 International Business Machines Corporation Patent Attorneys for the Applicant SPRUSO' FERGUSON IAD/961o .LI ILI-- M