Deflection unit having a ring with field correction elements, and cathode ray tube provided with said unit

Abstract: An electromagnetic deflection unit (5) comprising a coil holder (9) with a flange (8) whose inner side supports coils (10a, 10b) for (line) deflection of electron beams. At the flange side of the coil holder (magnetized) preformed elements (14) are arranged within these coils so as to influence the magnetic field of these coils (10a, 10b), thereby reducing spreading errors, caused during manufacture, throughout the display screen. For the purpose of automatic mounting of the preformed elements, an annular support having predetermined locations (in particular from 12 to 36) for accommodating preformed elements is used. The preformed elements (14) are accommodated at a plurality of these predetermined locations and, if made of a permanent magnetic material, produce magnetic fields having magnetic strengths measured at a distance of some mm above the heart of the elements in the range between 1 and 1000 µT. In a special embodiment the elements of permanent magnetic material belong to a limited collection as regards their magnetic field strength.

Description: The invention relates to an electromagnetic deflection unit for a cathode ray tube, comprising a hollow coil holder which supports a set of coils for electron beam deflection, said coils having parts extending substantially in the longitudinal direction of the coil holder at its inner side and field correction elements at the re-entrant side of said coils, the deflection unit further comprising, arranged within the set of coils, an annular support comprising predetermined locations for accommodating field correction elements, at a plurality of said predetermined locations field correction elements being accommodated. The invention also relates to a colour display tube provided with such a deflection unit. Such a deflection unit and a cathode ray tube (particularly a colour display tube) provided with such a unit are commercially available. "Predetermined locations" is understood to mean that one type of annular support with fixed corrector locations is used for a series of display tubes of the same type (for example colour monitor tubes having a fixed screen diagonal). The increasingly stringent requirements which are imposed on display systems with cathode ray tubes and deflection coils lead to improved designs. However, errors always remain. These may be linearity errors, raster distortion errors (in monochrome display tubes) or convergence errors (in colour display tubes). Some of these errors are produced during manufacture due to spreading of the manufacturing process. When the tube and the coil are assembled, it is attempted in practice to correct a part of the errors arisen during manufacture by adding field correction elements which are generally in the form of preformed elements sometimes referred to as "spoilers". These elements may be made of magnetic material or of electrically conducting material (for example Al or Cu). The addition is realised locally with reference to errors found by an operator. This results in a local improvement of quality. To be able to generate a maximum possible field strength at a minimum possible power, the coils are arranged at the inner side of the coil holder in such a way that - when the deflection unit is mounted on a display tube - the coils are located as close as possible to the electron beams in the display tube. As a result of this deflection unit structure, the greater part of the preformed elements must generally be glued against the inner surface of the coils. The rather non-flat surface of the coils may bring about a poor adhesion so that the preformed elements come loose. It is conventional practice to secure soft-magnetic preformed elements by means of adhesive tape and fix them with glue. The (manual) correct positioning and fixation of the preformed elements, which may have mutually different shapes and sizes, is time-consuming and hence expensive so that generally a relatively small number, for example, four or six is used. Generally this results only in a local improvement of the picture displayed on the screen. The results of this known method of optimization are therefore inadequate if more stringent requirements are imposed. It is an object of the method proposed in this application to improve the quality of the picture throughout the screen. It is a further object of the invention to provide an electromagnetic deflection unit having a construction with which the field correction elements can be positioned rapidly and preferably automatically. It is a still further object of the invention to neutralize the errors of individual deflection units. In a deflection unit of the type described in the opening paragraph according to the invention this object is achieved in that the field correction elements are of the permanent magnetic type. The use of correctors of the permanent magnetic type, or magnetized preformed elements is preferred, and will hereinafter be described by way of example, but the invention is not limited thereto. Differences may reside in the strength (magnetic induction) and the direction of magnetization, dependent on the correction required for a given combination of display tube and deflection unit. Generally, corrections with magnetic fields having a field strength in the range between 1 and 1000 µT, may be sufficient, and in most cases corrections between 5 and 500 µT are sufficient. For providing these fields, correctors are used in the form of planar elements which produce these field strengths at a certain distance above their centre, which distance is associated with the distance from the electron beams. In US-A-4,535,313 an electromagnetic deflection unit is described having a conical coil holder with a flange which carries coils for the deflection of electron beams. The coils have segments extending substantially in the longitudinal direction of the coil holder and near these segments soft-magnetic correction elements are arranged in an annular support. The exact magnetic strength and polarity of the corrector needed for each corrector location can be determined with reference to the measured error pattern followed by individual magnetization of each corrector at the desired strength (and polarity). An alternative is characterized in that the magnetized preformed elements have magnetic strengths which, while taking a given tolerance into account, have a limited number of fixed values. (For example, values which are a multiple of a given unit strength). In accordance with a further embodiment the preformed elements have an elongated (particularly rectangular) shape and have an in-plane direction of magnetization which is parallel to their short or their long axis. This provides the possibility of magnetizing all preformed elements in the same direction, while placing in a first position or in a 180° rotated position provides a choice of two opposite polarities. The strength and the polarity of the correctors to be placed in the ring can be determined by means of a computer program with reference to the known effects of a reference corrector and the error pattern measured (for example, at 25 points on the screen). Each corrector has its own influence on the convergence pattern. The correctors can be positioned rapidly and possibly automatically by using an annular corrector support having, for example a plurality of compartments arranged at fixed locations along a circular circumference for accommodating correctors. The larger the number of correctors, the greater the improvement which is achieved. It has particularly been found that the Q factor (i.e. a measure of the average weighted convergence error at a large number of (for example, 25) measuring points throughout the screen) can be reduced by at least 10% or 20%, and even by at least 50% if a sufficient number of correctors is used. The dimensions of the correctors are chosen to be such that the required (possibly large) number of correctors can be mounted in an annular support having given dimensions (dictated by the deflection unit with which it must cooperate). Placing the annular support with the correctors at a position within the deflection coil system is also important, particularly at a position between the centre thereof and the front side. In the case of saddle-type line deflection coils preferably proximate to the front transversal connection portions which interconnect the parts extending in the longitudinal direction of the coil holder. The latter notably means that the convergence of the electron beams may be influenced at an instant when the electron beams have already been deflected, which is very effective. The performance of each deflection unit can be measured on a standard display tube and each unit can be provided with a ring having correctors of the strength required for correcting errors caused by spreading. In a suitable measuring method the convergence is measured at, for example, 25 points on the display screen. An alternative possibility is to measure each deflection unit on the display tube with which it must form a combination, to assemble a ring with correctors on the basis of the measured data, to remove the deflection unit from the display tube so as to place the ring and to put the deflection unit into position again. It has been found that the operation of matching a display tube with a deflection unit having a ring with correctors according to the invention is easier and hence more rapid than matching a display tube with a deflection unit without such a ring. This is a result of the fact that the error pattern is more regular. (During the matching operation it is attempted, by way of shifting and/or tilting the deflection unit, to correct errors of the combination as much as possible.) The annular support may be placed on the (glass) envelope of the display tube, i.e. separate from the deflection unit. In an embodiment the annular support is, however, secured to the coil holder. This facilitates handling of the deflection unit. The support may be secured in different manners. The annular support may have a very thin wall of the order of 1 mm, while compartments having a bottom thickness of between 0. 1 and 0.5 mm may have been recessed in this support. The correctors may have the shape of very thin flat discs (comprising, for example a permanent magnetic ferrite material) arranged (for example, glued or clamped) in the compartments in the annular wall and do not, or hardly, extend outside the wall. Since the annular support replaces separately provided preformed elements of similar thickness, the use of the annular support does not involve any or hardly any larger distance between the inner deflection coils and a colour display tube extending within the deflection unit. Dependent on the required correction, the support may be provided with correctors at a plurality of locations or with correctors at all available locations. An advantage of the use of preformed elements premagnetized in one direction at different strengths is that the device required for magnetization may be simple. The correctors are preferably magnetized in their plane. To be able to place a sufficiently large number of correctors, the correctors should be sufficiently small. They may have, for example an elongate shape with a largest width of 5 mm and a largest length of 10 mm and placed in such a way that their short or long axis is directed towards the axis of the deflection unit. Their (in-plane) direction of magnetization after positioning in the ring is advantageously directed tangentially. In that case it is possible to form 2 N poles along a circular circumference by means of N correctors. The preformed elements used for the correctors may be standard-premagnetized with different values of the magnetic induction (strength). For example, a collection of preformed elements, the strongest of which is ten times as strong as the weakest, such as a collection whose strength varies in steps between roughly 1 and 10 times a given unit strength, or between roughly 1 and 20 (or 24) times a given unit strength. In a practical case the correctors from which a choice is made for positioning in the support have, for example 10 or 20 different strengths in the range between 5 and 500 µT, and more particularly in the range between 5 and 250 µT. The correctors can be placed at the desired locations in the ring in a simple manner by means of an automatic positioning machine. It is then advantageous if the ring has a plurality of positioning projections or grooves in accordance with the number of correctors to be placed. It is theoretically possible to use a ring of permanent magnetic material and to magnetize this ring at the required (large) number of locations in the desired strength and polarity. However, the required magnetization process is very complicated. The use of an annular support with a large number of (pre)magnetized correctors at fixed locations provides an extremely practical solution to the problem of correcting errors throughout the display screen. Moreover, this provides the possibility of not realising the various strengths in the same magnetic material, but of adapting the composition of the material to the required weaker magnetization for the weaker magnets. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings Fig. 1 shows diagrammatically a colour display tube with a deflection unit in a side elevation; Fig. 2 shows the deflection unit of Fig. 1, viewed from the wide end of the coil holder; Fig. 3 is a front elevation and Fig. 3A is a cross-section of an annular support with correction magnets, Fig. 4 is a perspective elevational view and Fig. 4A is a cross-section of an annular support during the process of positioning correction magnets; Fig. 5 shows a detail of a coil holder provided with a support with correction magnets; Fig. 6 is a graph showing the Q factor as a function of N; and Fig. 7 shows an alternative to the construction of Fig. 3. The colour display tube of Fig. 1 has a neck 1, a cone 2 and a display window 3 with an internal phosphor screen (display screen). Current conductors 4 emerge from the neck 1. A deflection unit 5 is arranged around the neck 1 and against the cone 2. The deflection unit has a coil holder 6 of synthetic material, for example of polystyrene/polyphenylene oxide supporting deflection coils of the saddle type. One deflection coil 7 for (field) deflection of electron beams is visible in this case. A yoke ring 9 of a soft-magnetic material, for example nickel-zinc-ferrite or manganese-zinc-ferrite is arranged around the coils with which it cooperates. The ring 9 is secured to a front flange 8 of the coil holder 6. Fig. 2 is a front elevation of the deflection unit 5 with the coil holder 6. Coils 10a, 10b of the saddle type, intended for, for example line deflection of electron beams, are supported by the coil holder 6. The coils 10a, 10b have parts which extend substantially in the longitudinal direction of the coil holder 6. These longitudinal parts are connected at their front and rear ends by means of transversal arcuate connection portions. The flange 8 provided with an edge 9 at the wide end of the coil holder 6 provides the location for said arcuate connection portions. To improve the picture quality throughout the surface of the screen, the invention provides an annular (conical) support 13 (Fig. 3, 3A) in which, in the case shown, 24 compartments 12, ... are recessed every 15° in the inner surface. Preformed, premagnitized elements 14, which have a length of approximately 6 to 7 mm, a width of approximately 5 mm and a thickness of less than about 1 mm are located in a plurality of the compartments. The required field strength (increasing in, for example 10 or 20 steps from approximately 10 µT to approximately 250 µT) and the direction of magnetization of each magnet position is computed with reference to a test pattern on a display screen for each deflection unit or for each combination of deflection unit and display tube. (A convergence measuring method is described, for example in JP-A 2 156 792 and another method is described in EP-A 135 072.) An embodiment of the support 13 was made of a synthetic material (such as nylon) having a wall thickness of 1 mm; magnetic preformed elements 14 having a thickness of 0.7 mm and comprising a mixture of 60% by volume of polyphenylene oxide/polystyrene and 40% by volume of permanent magnetic ferrite can be connected to the bottoms of the (0-7 mm deep) compartments 24 by means of glueing. The elements 14 were magnetized to produce the desired field strengths at a distance of about 8.4 mm above their centre. However, the invention is not limited thereto. For example, the ring may alternatively comprise a magnetic inert material which is different from synthetic material, such as glass or a ceramic material. The provision of glue and the positioning of the preformed elements 14 can be realised, for example on an (adapted) S(urface) M(ounting) D(evice) machine. For example, a hot-melt process is suitable for glueing. A ring 13 can be given a horizontal position in a positioning machine by means of a conveyor belt. The ring 13 can then be tilted until a processing position has been reached (Fig. 4; Fig. 4A) so that a preformed element 14 can be placed in one position by means of a standard vertical positioning tool. After positioning (and fixation) of a preformed element 14, the ring 13 is advanced one position. The position is accurately determined and fixed by making use of reference grooves 19 along the inner circumference of ring 13. The positioning routine is repeated until the ring has been filled with the associated preformed elements. There are, for example 20 types of preformed elements (different in magnetic induction). Each type of preformed element can advantageously be packed in tape and fed to the positioning machine. The ring 13 provided with at least two ears 16a, 16b is positioned at the inner side of the line deflection coils 10a, 10b (Fig. 5) at the wide end of the deflection unit 5. In a preferred embodiment the upper side of the ring 13 registers with the upper side of the deflection coils 10a, 10b. The ring 13 has a shape which fits in with the shape of the deflection coils 10a, 10b. The ears 16a, 16b are used for securing the ring 13 to the front flange 8 of coil holder 6. The ears 16a, 16b may be secured to a fixation element 15, for example by means of a screw connection, a snap connection ( via a clamping fit) or by ultrasonic welding. The ears 16a, 16b have grooves 17a, 17b of different dimensions so as to guarantee an unambiguous orientation. Fig. 6 shows in a graph Q max (Q max represents the maximum convergence error measured in a series of 100 display tubes) and Q AV (which is a measure of the average weighted convergence error at a large number of measuring points, for example 25, distributed across the screen surface) in dependence upon the number N of correctors positioned at fixed locations in an annular support and on the basis of a measuring and computing program, selected from a collection of correctors having a strength increasing (in steps) from roughly 10 to 200 µT. Fig. 6 shows with reference to measurements relating to a colour monitor tube that for normal convergence requirements a minimum of 12 corrector locations was needed. For less stringent requirements a number of 8 locations might suffice. Above 36 corrector locations further improvements were found to be marginal. In practice it is difficult to obtain uniform step sizes, which is related to the geometrical and magnetic tolerances. However, this does not detract from the principle of the invention. For example, a series of magnetized preformed elements having dimensions of 5 x 7 x 1 mm had the following values (in µT): 7.5-16.4-26.4-34.8-43.4-50.9-60.5-70.5-80.3-98.6-109.0-118.4-126.7-136.9-146.9-157.1-170.9-178.3-187.5-199.4 upon measurement by means of a Hall gauge and a Gauss meter. It is to be noted that the annular support used in the invention and having predetermined locations for accommodating field correction elements may not only be used advantageously in deflection units for monochrome or colour display tubes but also in deflection units for electron microscopes, e-beam pattern generators and the like. It is further to be noted that the collection of premagnetized preformed elements from which a selection is made may have a strength distribution linking up a minimum number of strengths with a maximum number of possibilities, for example a distribution of 1, 2, 5, 10, ..., a distribution of 1; 1.25; 2; 2.25; 2.75; 3.25; 4; 4.5; 5; 6, ..., a distribution similar to that of scale weights, etc. More particularly, a very large number of possibilities is created by stacking two or more preformed elements of the same or different strengths having parallel or antiparallel directions of magnetization. Fig. 7 shows by way of example a front view of an annular support in which locations for accommodating correctors are provided on a plurality of radiuses (in this example 7 radiuses). In this example the spacings between the locations on a distinct radius is uniform, but in an alternative embodiment these spacings might be non-uniform.

Claims: An electromagnetic deflection unit for a cathode ray tube, comprising a hollow coil holder which supports a set of coils for electron beam deflection, said coils having parts extending substantially in the longitudinal direction of the coil holder at its inner side and field correction elements at the re-entrant side of said coils, the deflection unit further comprising, arranged within the set of coils, an annular support comprising predetermined locations for accommodating field correction elements, at a plurality of said predetermined locations field correction elements being accommodated, characterized in that the field correction elements are of the permanent magnetic type. A deflection unit as claimed in claim 1, characterized in that the field correction elements produce fields having magnetic strengths in the range between 1 and 1000 µT. A deflection unit as claimed in claim 1 or 2, characterized in that the support comprises at least one set of at least 12 predetermined locations wherein said predetermined locations are regularly distributed along a circle. A deflection unit as claimed in claim 3 characterized in that the support comprises a plurality of concentric sets of predetermined locations. A deflection unit as claimed in claim 1, 2, 3 or 4, characterized in that the field correction elements comprise magnetized preformed elements which produce fields having magnetic strengths which, while taking a given tolerance into account, have a limited number of fixed values. A deflection unit as claimed in claim 1, 2, 3, 4 or 5, characterized in that the coil parts extending substantially in the longitudinal direction of the coil holder are connected at their ends by front and rear transversal connection portions, and in that the annular support is arranged at a position adjoining the front transversal connection portions of the deflection coils. A deflection unit as claimed in any of the preceding claims, characterized in that the field correction elements comprise magnetized preformed elements which are magnetized in their plane. A deflection unit as claimed in claim 7, characterized in that the directions of magnetization of the preformed elements are directed tangentially with respect to the circumference of the annular support. A colour display tube provided with a deflection unit as claimed in any one of the preceding claims.

Inventors: BOLDER ANTONIUS JACOBUS JOHANN, DE MAN RONALD JOHANNES JACOBUS, DEKKERS BERNARDUS HENDRIKUS JO, SLUYTERMAN ALBERTUS AEMILIUS S, BOLDER, ANTONIUS JACOBUS JOHANNES, DE MAN, RONALD JOHANNES JACOBUS, DEKKERS, BERNARDUS HENDRIKUS JOHANNES, SLUYTERMAN, ALBERTUS AEMILIUS SEYNO
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: H01J29
IPC codes: H01J29