DE69308269T2 - Deflection unit with a ring provided with field correction elements and cathode ray tube with this unit - Google Patents

Description

The invention relates to an electromagnetic deflection unit for a cathode ray tube, with a hollow coil body which has on the inside a set of coils for deflecting electron beams, these coils having parts which extend substantially in the longitudinal direction of the coil body, these parts being connected at their ends by front and rear cross-connecting parts, and with field correction elements on the inwardly directed side of these coils, the deflection unit further comprising, arranged within the set of coils, an annular carrier which has predetermined locations for fastening field correction elements, field correction elements being provided at a number of said predetermined locations. The invention also relates to a color picture tube with such a deflection unit.

Such a deflection unit and such a cathode ray tube equipped therewith (in particular a color picture tube) are commercially available.

By "predetermined locations" it is meant that one type of annular support with fixed locations for the correctors is used for a series of display tubes of the same type (for example, color monitor tubes with a fixed screen aspect ratio).

The ever-increasing demands placed on display systems with cathode ray tubes and deflection coils are leading to ever-improving designs. However, there will always be errors. These can be linearity errors, raster distortion errors (in monochrome picture tubes) or convergence errors (in color picture tubes). Some of these errors arise during production as a result of scatter in the manufacturing process. When assembling the tube and the coil, attempts are made in practice to reverse some of the errors made during production by adding field correction elements, usually in the form of statically or dynamically acting molded parts: so-called correctors. These can be made of (permanently) magnetic material or electrically conductive material (for example Al or Cu). The addition is made locally based on errors identified by the fitter. This leads to an improvement in quality on site. In order to generate the greatest possible field strength with the help of the smallest possible power, the coils are located on the inside of the coil body in such a way that - when the deflection unit is arranged on a picture tube - the coils are as close as possible to the electron beams in the picture tube. This design of a deflection unit means that the molded parts usually have to be glued to the inner surface of the coils. The rather uneven surface of the coils can lead to insufficient adhesion, causing molded parts to come loose. It is usual to hold the soft magnetic molded parts in place with adhesive strips and to fix them in place with glue.

The precise (manual) adjustment and fixing of the molded parts, which can have different shapes and sizes, is time-consuming and therefore expensive, so that most people only use very few, for example four or six. This generally only leads to a local improvement of the image displayed on the screen. The results of this well-known optimization process are therefore not sufficient if higher requirements are imposed.

It is now an object of the method proposed in this application to improve the quality of the image across the entire screen.

The invention further aims to provide an electromagnetic deflection unit of a construction which enables a rapid and preferably automated arrangement of the field correction elements.

It is yet another object of the invention to reverse the errors of individual deflection units.

This object is achieved in a deflection unit of the type described above according to the invention in that the field correction elements are of the permanent magnet type.

The use of correction elements of the permanent magnet type or of magnetized preformed elements is preferred and is described below as an example, but the invention is not limited to this. Differences in the strength (magnetic induction) and in the direction of the magnetization can arise, depending on the correction required for a particular combination of display tube and deflection unit. In general, corrections with magnetic fields with a field strength in the range of 1 to 1000 µT will suffice, and in most cases corrections between 5 and 500 µT will suffice. To provide these fields, correction elements are used in the form of flat elements which provide these field strengths at a certain distance from their center, this distance being associated with the distance from the electron beams.

US-A-4,535,313 describes an electromagnetic deflection unit with a conical coil body with a flange that carries coils for deflecting electron beams. The coils have segments that extend substantially in the longitudinal direction of the coil body and soft magnetic correction elements are provided in an annular carrier near these segments.

The exact magnetic force and polarity of the correction element required for each correction point can be determined in relation to the measured error pattern, followed by individual magnetization of each correction element with the desired strength (and polarity). An alternative embodiment is characterized in that the magnetized elements have a magnetic force which, taking into account a certain tolerance, has a limited number of fixed values. (For example, values which are a multiple of a certain unit force). According to a further embodiment, the elements have an elongated (in particular rectangular) shape and a flat magnetization direction, such as parallel to the short or long axis. This offers the possibility of magnetizing all elements in the same direction, while an arrangement in a first position or in a position rotated by 180º provides a choice of two mutually opposite polarities. The power and polarity of the correction elements to be arranged in the ring can be determined by means of a computer program in relation to the known effects of a reference correction element and the measured error pattern (for example at 25 points on the screen). Each correction element has its own influence on the convergence pattern.

The correction elements can be applied quickly and possibly automatically using an annular correction element carrier having, for example, a number of compartments at fixed locations along a circular circumference for accommodating correction elements. The greater the number of correction elements, the greater the improvement achieved. In particular, it has been found that the Q-factor (ie a measure of the average weighted convergence error at a plurality (for example 25) measuring points across the screen) can be reduced by at least 10 to 20% and can even be reduced by at least 50%.

The dimensions of the correction elements are chosen in such a way that the required (possibly large) number can be mounted in an annular support with specific dimensions (determined by the deflection unit with which it must work together).

Also important is the arrangement of the annular support with the correction elements at a location within the horizontal deflection coil system, in particular at a location between the center thereof and the front, preferably near the front connecting parts which connect the longitudinally extending parts of the saddle-type horizontal deflection coils.

This means that the convergence of the electron beams is influenced at a time when the electron beams are deflected, which is very effective.

The performance of each deflection unit can be measured on a standard picture tube and fitted with a ring containing correction elements of the strength required to correct scattering errors. With a suitable measuring method, for example, the convergence is measured at 25 points on the screen.

An alternative form is to measure each deflection unit on the picture tube with which it is to form a combination, to assemble one with correction elements on the basis of the measurement data, to remove the deflection unit from the picture tube in order to arrange the ring and to put the deflection unit back in place.

It has been found that adapting a deflection unit with a ring with correction elements according to the invention to a picture tube is easier and therefore faster than adapting deflection units without such a provision to a picture tube. This is a consequence of the fact that the error pattern is more regular. (During the adjustment, an attempt is made to correct errors in the combination as far as possible by shifting and/or tilting the deflection unit.)

The ring-shaped carrier can be arranged on the (glass) casing of the picture tube, i.e. separately from the deflection unit. In one embodiment, however, the ring-shaped carrier is attached to the coil body. This makes handling the deflection unit easier. The attachment can be implemented in various ways.

The annular support may have a very thin wall, of the order of 1 mm, while compartments with a bottom thickness of between 0.1 and 0.5 mm may be provided therein. The correction elements may be in the form of very thin flat discs (e.g. comprising a permanently magnetic ferrite material) provided in the compartments in the annular wall (e.g. glued or clamped) and not or hardly protruding from the wall. Since the annular support takes the place of separately provided molded parts of a similar thickness, the use of the annular support causes little or no increase in the distance of the inner deflection coils from a color picture tube located within the deflection unit. Depending on the correction required, the support may be provided with correction elements at only one location, with correction elements at a number of locations, or with correction elements at all available locations.

An advantage of using preformed parts that have been previously magnetized in one direction with different strengths is that the arrangement required for magnetization can be simple. Preferably, the correction elements are magnetized in their plane. In order to be able to attach a sufficiently large number of correction elements, these should be small enough. For example, they can have an elongated shape with a maximum width of 5 mm and a maximum length of 10 mm and be arranged in such a way that the short or long axis points to the axis of the deflection unit. Their magnetization direction in their own plane after arrangement in the ring is advantageously directed tangentially. In this case, it is possible to form 2 N poles on a circle circumference with N correction elements. The preformed elements used for the correction elements can be premagnetized with different values of magnetic induction (strength) in accordance with standards. So For example, a series of preformed elements whose thickness varies in stages between about 1 and 10 times a certain unit thickness, or between about 1 and 20 (or 24) times a certain unit thickness. In a practical case, the corrective elements selected for arrangement in the carrier have, for example, 10 or 20 different thicknesses in the range from 5 to 500 µT, and in particular in the range from 5 to 250 µT, their thickness differences being approximately a multiple of the unit difference. These can be arranged in a simple manner at the required locations in the ring by means of an automatic assembly machine. It is advantageous if the ring is provided with a number of positioning cams or slots corresponding to the number of corrective elements to be arranged.

Theoretically, it is possible to use a ring made of permanent magnetic material and magnetize it at the required (large) number of locations with the desired strength and polarity. However, a suitable magnetization process is very complicated. The use of a ring-shaped carrier with a large number of (pre)magnetized correction elements at fixed locations, each with a magnetic strength that is approximately a multiple of a unit strength, therefore offers an extremely practical solution to the problem of correcting errors across the entire screen. In addition, this opens up the possibility of not realizing the respective strengths in the same magnetic material, but of adapting the composition of the material to the required weaker magnetization for the weaker magnets.

Embodiments of the invention are shown in the drawing and are described in more detail below. They show:

Fig. 1 is a schematic representation of a color picture tube with a deflection unit in side view,

Fig. 2 the deflection unit according to Fig. 1 seen from the wide end of the coil body,

Fig. 3 is a front view and Fig. 3A is a section through a ring-shaped carrier with correction magnets,

Fig. 4 is a diagrammatic view and Fig. 4a is a section through a ring-shaped carrier when attaching correction magnets,

Fig. 5 a detail of a coil body with a carrier with correction magnets,

Fig. 6 is a graph showing the Q factor as a function of N ; and

Fig. 7 shows an alternative form of a construction according to Fig. 3.

The color picture tube according to Fig. 1 has a neck 1, a cone 2, a picture window 3 with an inner phosphor screen (display screen). Current conductors 4 emerge from the neck 1. A deflection unit 5 is provided around the neck 1 and on the cone 2. The deflection unit has a coil body 6 made of plastic, for example polystyrene/polyphenylene oxide, which carries the saddle-shaped deflection coils. A deflection coil 7 for (vertical) deflection of electron beams is visible in this case. A yoke ring 9 made of soft magnetic material, for example nickel-zinc ferrite or manganese-zinc ferrite, is provided around the coils and works together with them. The ring 9 is attached to the front lens 8 of the coil body 6.

Fig. 2 shows a front view of the deflection unit 5 with the coil body 6. The saddle-shaped coils 10a, 10b for, for example, horizontal deflection of electron beams are carried by the coil body 6. The coils 10a, 10b have parts that extend essentially in the longitudinal direction of the coil body 9. These longitudinal parts are connected to one another at the front and rear ends by arc-shaped cross-connecting parts. The flange 8 provided with an edge 9 at the wide end of the coil body 6 offers space for the aforementioned arc-shaped connecting parts.

In order to improve the image quality over the entire screen surface, the invention provides an annular (conical) support 13 (Fig. 3, 3A) in which, in the case in question, 24 spaces 12, ... are provided at a distance of 15º each, in one or more of which one or more premagnetized moldings 14 are provided. The moldings 14, which in this case have a length of about 7 mm, a width of about 5 mm and a thickness of about 1 mm, are located on the inside. The required strength (increasing in, for example, 20 almost equal steps from 10 µT to about 200 µT) and the direction of magnetization Each magnet position is calculated for each deflection unit or each deflection unit-picture tube combination using a test pattern on the screen. (One convergence measurement method is described, for example, in JP-A 2 156 792 and another in EP-A 135 072). In one embodiment, the carrier 13 was made of plastic (such as nylon) with a wall thickness of 1 mm; magnetic molded parts 14 made of a mixture of 60 vol. % polyphenylene oxide/polystyrene and 40 vol. % permanent magnet ferrite can be bonded to the floors of the chambers 24 (0 - 7 mm deep) using an adhesive process. The elements 14 were magnetized at a distance of about 8.4 mm above their center to generate the desired field strengths.

However, the invention is not limited to this. For example, the ring can alternatively be made of magnetically inert materials other than plastic, such as glass or ceramic. The application of adhesive and the attachment of the molded parts 14 can be carried out, for example, using an (adapted) surface mounting machine (SMD assembly). A "hot melt" process can be considered as an adhesive process, for example.

A ring 13 can be brought into an assembly machine in a horizontal position using a conveyor belt. The ring 13 can then be tilted into the processing position (Fig. 4; Fig. 4A), whereby a molded part 14 can be assembled in just one place using a standardized vertical assembly tool. After assembling (and fixing) a molded part 14, the ring 13 is displaced by one position. This position is precisely determined and fixed using reference slots 19 on the inner circumference of the ring. The assembly routine is repeated until the ring is filled with assigned molded parts.

For example, there are 20 types of molded parts (various magnetic induction). These can be conveniently packaged into a strip and fed to the assembly machine.

The ring 13, which is provided with at least two ears 16a, 16b, is provided on the wide side of the deflection unit 5 on the inside of the horizontal deflection coils 10a, 10b (Fig. 5). In a preferred embodiment, the top of the ring 13 is on the same level as the top of the deflection coils 10a, 10b. The ring 13 has a shape that follows the shape of the deflection coils 10a, 10b. The Ears 16a, 16b serve to attach the ring 13 to the front lens 8 of the coil body 6. The ears 16a, 16b can be attached to a fastening element 15, for example, by a screw fastening, by a snap connection (by means of a clamp fit) or by an ultrasonic welding process. The ears 14a, 14b have slots 17a, 17b that differ in their dimensions so that clear orientation is guaranteed.

Fig. 6 shows in a graph Qmax (Qmax represents the maximum orientation error measured in a row of 100 picture tubes) and QAV (this is the measure of the average weighted convergence error in a large number of, for example, 25 measuring points distributed over the screen surface) as a function of the number N of correction elements provided at fixed locations in a ring-shaped carrier, based on a measuring and calculation program, selected from a collection of correction elements with (stepwise) increasing strength from about 10 to 200 µT.

Fig. 6 shows, with reference to measurements on a color monitor tube, that for normal convergence requirements a minimum of 12 correction points are required. For less stringent requirements a number of 8 points is sufficient. More than 36 correction points have shown only marginal improvements.

In practice, the sizes of the steps are not uniform due to geometric and magnetic tolerances. However, this does not affect the principle of the invention. For example, a series of magnetized molded parts with dimensions of 5x7x1 mm showed 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 when measured using a Hall probe and a Gaussian measuring instrument.

It should be noted that the ring according to the invention with predetermined locations for arranging field correction elements can be used advantageously in electron microscopes, "e-beam generators" and the like, in addition to monochrome and color picture tubes.

It should also be noted that the collection of pre-magnetized molded parts from which to choose may have a distribution of strengths that combines a minimum number of strengths with a maximum number of possibilities, for example a distribution of 1, 2, 5, 10 ... . In particular, by having two or more molded parts When they are stacked on top of each other with parallel or anti-parallel magnetization directions, a multitude of possibilities are created.

Fig. 7 shows, as an example, a front view of an annular carrier, where locations for receiving correction elements are provided on a plurality of radii (in this example 7 radii). In this example, the spaces between locations on a certain radius are equal to each other, but in another embodiment these spaces may be non-uniform.

Claims (9)

1. Electromagnetic deflection unit for a cathode ray tube, comprising a hollow coil body having on the inside a set of coils for deflecting electron beams, said coils having parts extending substantially in the longitudinal direction on the inside of the coil body, and vertical correction elements on the downstream side of said coils, the deflection unit further comprising, arranged in the set of coils, an annular carrier containing predetermined locations for arranging vertical correction elements, vertical correction elements being provided at a number of said predetermined locations, characterized in that the vertical correction elements are of the permanent magnet type. 2. Deflection unit according to claim 1, characterized in that the vertical correction elements generate fields with a magnetic strength in the range between 1 and 1000 µT. 3. Deflection unit according to claim 1 or 2, characterized in that the carrier has at least one set of at least 12 predetermined locations, said predetermined locations being distributed at regular intervals over a circle. 4. Deflection unit according to claim 3, characterized in that the carrier has a number of concentric sets of predetermined locations. 5. Deflection unit according to claim 1, 2, 3 or 4, characterized in that the vertical correction elements comprise magnetized preformed elements which generate fields with a magnetic strength which, taking into account a certain tolerance, has a limited number of fixed values. 6. Deflection unit according to claim 1, 2, 3, 4 or 5, characterized in that the coil parts, which extend substantially in the longitudinal direction of the coil body, are connected to one another at their ends by front and rear cross-connecting parts, and that the annular carrier is provided at a location which is the front cross-connection parts of the deflection coils. 7. Deflection unit according to one of the preceding claims, characterized in that the vertical correction elements comprise pre-magnetized preformed elements which are magnetized in their plane. 8. Deflection unit according to claim 7, characterized in that the magnetization direction of the preformed elements extends tangentially to the circumference of the annular carrier. 9. Color display tube with a deflection unit according to one of the preceding claims.