MULTIPOLAR ROTOR

Abstract: Method and devices for producing a magnetic object, to be moulded in a moulding device from a mixture of grains of magnetic material and hardening binding agent, said object having pole areas of small dimensions, the mixture being subjected in a moulding cavity of a moulding body of the moulding device to temperature changes, gravity, mechanic forces or magnetic forces, or combinations of those. The invention comprising the reduction of a strong, perma­ nent magnet to fully magnetized anisotropic permantly magnetic ma­ terial, the reduction of the fragments of fully magnetized aniso­ tropic permanently magnetic material to grains, until all grains are smaller than the width of a pole area, mixing those grains with the hardening binding agent, inserting the mixture into the moulding device, and ensuring that the mixture hardens in the moulding device, providing the permanently magnetized object as the final product.

Description: The present invention relates to a method of manufacturing a multipolar magnet having a plurality of poles of predetermined size arranged at a surface thereof, the method comprising the steps of mixing magnetic particles with a binding agent, inserting the mixture in a mould, and hardening said mixture in the mould in the presence of a multipolar static magnetic field generated by magnetic means. Furthermore, the invention relates to a multipolar magnet having a plurality of poles of predetermined size arranged at a surface thereof, said magnet comprising a hardened mixture of magnetic grains and a binding agent and having at least one anisotropic direction. Such a method and magnet are known from Patents Abstracts of Japan, vol. 11, no. 240 (E-529) (2687), based on JP-A-62-52913. This document discloses a method of manufacturing a multipolar anisotropic cylindrical magnet in which a kneaded matter, composed of magnetic powder and a resin, is injected into a cylindrical cavity of a mould, in which cavity a magnetic body is moulded. Around the cylindrical cavity a multipolar static magnetic field is formed by means of an arrangement of permanent magnets. Due to this magnetic field an anisotropic direction, hence an axis of easy magnetization, is induced in the magnetic body. After that, the obtained composite magnet formation body is processed into a predetermined size and is magnetized in the same direction as the anistropic direction in order to obtain the multipolar magnet. As a result of the final step of magnetizing the moulded composite body, in order to obtain a multipolar magnet having alternately magnetized poles, the known method is not suitable for the manufacture of small multipolar objects, as on the one hand high magnetic fields, usually generated by means of a magnetic charging yoke, are required to force the magnetic dipoles in directions in accordance with the desired multipolar pattern, and on the other hand a high spatial resolution of the alternating magnetic field thus generated is required corresponding to the small dimensions of the desired alternately magnetized poles. Therefore, the multipolar magnet thus obtained cannot have a small dimension, for instance a diameter of less than 4 mm, while at the same time a large number (for example sixty) of well-defined strong poles is provided. This, however, is desirable when the multipolar magnet is used in small-sized applications, as for instance the rotor of a stepper motor in a small timepiece. The object of the present invention is to remove these drawbacks. According to the present invention a method of manufacturing a multipolar magnet is provided, as mentioned in the preamble, characterized in that the mixing is carried out by using magnetic particles comprising fully magnetized anisotropic permanent-magnet grains having a size of the same order of magnitude as, but smaller than the short dimension of the poles, said grains being obtained by reducing a strong permanent magnet to grains of predetermined size, wherein, during the hardening of the mixture, said grains are positioned along the surface by the multipolar, static magnetic field, so as to establish the poles of the multipolar magnet. The multipolar magnet according to the present invention is characterized in that said magnetic grains comprise fully magnetized anisotropic permanent-magnet grains in a graded composition with respect to their size, including large grains of a size of the same order of magnitude as, but smaller than a short dimension of the poles, wherein said large grains are positioned along the surface, so as to define the poles of the multipolar magnet. By using grains that are fully magnetized before they are mixed with the binding agent, the magnetizing of the magnet after the moulding process may be omitted. In order to avoid the formation of an inextricable conglomerate of grains due to their magnetic interaction, the size of the grains is of the same order of magnitude as the short dimension of the poles. During the moulding process the fully magnetized grains merely have to be properly orientated which can easily be effected by a weak magnetic field generated by the magnetic means, as forces and torques exerted on the grains during the interaction with the magnetic field can be significant because of the net magnetization being already present in the grains. Thus, the orientation of the grains establishes and defines the poles at the surface of the magnet, wherein smaller particles may fill up the space between the grains, and no subsequent magnetic processing of the magnet is required. Preferably, the size of the grains is 150 µm for the short dimension of the poles of 200 µm. The number of poles may equal sixty and the diameter of the multipolar magnet may equal 4 mm or less, thus providing a magnet that is very suitable to be used as the rotor of a stepper motor with a stepping angle of 6 degrees in a time-piece of small dimensions, the multipolar magnet having the shape of a cylindrical sleeve having a recess. The cylindrical sleeve may be provided, in its recess, with a plate, ring or a collection of ring segments made of soft iron, so as to establish a well-conducting internal flux return path for the poles of the magnet. Further details, characteristics and properties will be elucidated in the following description. Several figures will be referred to, of which figure 1 represents a section along the axis of a schematically drawn moulding device, figures 2A, 2B, 2C represent a similar section of a part of the moulding device, in which a presser means has been inserted into the moulding device, figure 3 also represents a longitudinal section along the axis of the schematically drawn moulding device, in which a composed presser means has been inserted into the moulding device, figure 4 shows a cross-sectional view along the line IV-IV in figure 1, figure 5 shows a cross-sectional view along the line V-V in figure 3, figure 6 represents a view similar to figure 5, in which the mandrel comprises magnetic means, figure 7 gives a view similar to figures 5 and 6, in which mandrel and moulding device are connected by ribs, figure 8 represents a schematic view of the positioning of the fragments to be ground, consisting of fully magnetized anisotropic permanent-magnet material, figure 9 schematically represents the grinding device, and figures 10A, 10B and 10C schematically represent a top view with enlargement and side view, respectively, of a permanently magnetized object as the final product, obtained with the moulding device. The merit of the invention can particularly be elucidated by means of figures 1, 4, 8, 9 and 10. Figure 8 shows fragments (60) of fully magnetized anisotropic permanent-magnet material, which have been obtained by breaking a strong magnet of sintered permanent-magnet material such as SmCo₅ or Sm₂Co₁₇ or other desired strong, permanent-magnet material into small fragments. This material has to be reduced to grains, e.g. in a grinding device as described hereafter, and represents the starting material then. In order to have the grains take up a fixed position in the final product they are combined to a mixture with a hardening binding agent. From this mixture, an object (80) of small dimensions will have to be moulded as can be seen in figures 10A and 10C. This object could e.g. be the rotor element to be mounted in a stepper motor with stator poles of a timepiece, which rotor has a diameter of 4 mm and has along its periphery 60 pole areas (90, 91) arranged therein of alternatingly north poles N and south poles S. In a moulding device (10) for the production of such a rotor, the invention shows a first moulding body (11), a second moulding body (12), and a passage member (13) which can be connected between the two bodies. Both moulding bodies (11, 12) are identical in structure, however, they do not have the same size. They incorporate, inserted near their inner surfaces, magnetic means (30, 31) as indicated in figure 4, in which magnetic poles, referred to as N and Z for north and south, respectively, are situated at the inner circumference (32) of the moulding bodies. The purpose of these magnetic means is to exert a magnetical influence on the mixture of the starting material and the hardening binding agent, and in particular on the portion near the inner circumference (32), in order to establish pole areas, particularly linking up at the N, Z poles, to which within the mixture a garland (92) of magnetic flux lines corresponds. This magnetic exertion begins with the introduction of the mixture into the second moulding body (12). During carefully feeding the mixture through the moulding device to the first moulding body (11) in the direction of the arrow indicated by M, the pole patterns within the mixture will be maintained. When a part of the mixture has arrived in the first moulding body (11) it will be able to harden there, or it will have hardened substantially or completely, enabling the first moulding body (11) to be removed and replaced by a subsequent similar moulding body (11'), the filled first moulding body thus providing a permanently magnetized object as a final product. The magnetic means (30, 31) in the two moulding bodies (11, 12) may be slices or discs of a desired permanently magnetic material. Strong magnets with high remanence B r are preferred. For this purpose certain iron compounds, SmCo alloys such as SmCo₅, and Sm₂Co₁₇, as well as B-doped, Nd-Fe alloys are known. The passage member (13) will preferably taper gradually from the second moulding body (12) to the first moulding body (11). E.g. the inner circumference can be a truncated cone, however, other shapes of the inner circumference are also possible. Preferably each cross-section of the passage member will have to be identical in shape with that of a moulding body in order to disturb as little as possible the pole pattern formed within the mixture when passing it through the moulding device. For the two moulding bodies, an inner circumference that is shaped as a regular polygon is also conceivable. In that case each side of the polygon will have a magnetic N- or S-pole. Accordingly, the passage member will be a truncated pyramid, its cross-section perpendicular to the axis being a regular polygon which is identical in shape with the cross-section of the two moulding bodies. Provided there is a gradual transition, it is possible that the successive cross-sections of the passage member (13) start as circles and end as regular polygons, or the other way around, to which the two moulding bodies (11, 12) have to fit accordingly. In order to maintain the pole pattern in the mixture in the best possible way it is preferred to also apply magnetic means as found in the two moulding bodies in at least a part of the passage member (13) near the surfaces at the inner circumference. Slices or discs of a desired permanently magnetic material extending accordingly from the second moulding body (12) in the direction of the first moulding body (11) will prevent a possible disturbance of the pole patterns in the mixture. On the other hand, with the use of a suitable binding agent in the mixture, a passage member of iron or other soft-magnetic material may offer a sufficiently conducting path to the magnetic flux lines from the poles in the mixture. Suitable size ratios of the moulding body (11, 12) are e.g. 10 mm and 4mm for the respective inner diameters, with a length of 30 mm for the passage member (13), i.e. the height of the truncated cone. However, other dimensions are also possible and even desirable if the hardening of the binding agent requires such. It will be clear to an expert that the reduction factor 2.5 can be easily deviated from. In figure 10B the dotted part (81) in figure 10A of the object (80) to be produced is shown enlarged. The drawing represents a possible structure of pole areas (90, 91) composed of grains of fully magnetized anisotropic permanent-magnet material for an N-pole (90) and a Z-pole (91). On the dimensions of the grains, the following can be remarked. In order to make the poles in the pole areas (90, 91) of the final product (80) as strong as possible, it is necessary that the mixture in the pole areas comprises an as large as possible fraction of starting material. In other words: the filling factor, to be determined as the ratio of the volume of starting material per volume unit of mixture, should be as close to 1 as possible. Such a mixture should comprise grains of the maximally admissable size, viz. the width of a pole in the final product on the one hand, and a graded composition of smaller grains in order to fill up the space between the larger grains on the other hand. It should be remarked that the small grains are preferably not so small that they can form an inextricable conglomerate, which has a highly reduced magnetic moment as a whole, as a consequence of differences in orientation of the separate parts. Such a mixture will provide a minimal surface to be enveloped by the binding agent and will thus result in the largest possible filling factor. From the above it is easily deducible that the maximum pole width at the surface of an above-described rotor (80) with a diameter of 4 mm and with 60 poles is about 0.2 mm (= 200 µm). For the grains to be positioned correctly and somewhat interspaced, i.e. not within the interpolar areas, they should not be larger than about 150 µm. Similar calculations are possible for other rotor dimensions. With the moulding device (10) as discussed by means of figures 1 and 4, having magnetic means (30, 31) on the inner circumference (32) of said moulding device, the pole patterns can be established in the desired structure as indicated in figure 10B. It will be understood that when the mixture is introduced into the second, larger moulding body (12) the grains can be positioned correctly. Particularly in the second, larger moulding body (12) the grains can be positioned in the right direction, for in said moulding body the grains are the most versatile on the one hand, since the binding agent is at its most fluid there, and because the grains will have enough space there to move on the other hand. Once they have been arranged in patterns, the pole patterns will be maintained in the passage member (13) up to the first moulding body, where the final product is delivered as described above. Before the mixture can be composed, the fragments (60) will have to be reduced to the above-described grains of desired dimensions. In this respect the following should be noted. Figure 8 shows a collection of fragments (60) of the fully magnetized anisotropic permanent-magnet material that is to be processed. These fragments have been obtained by the fragmentation of strong, permanent magnets of desired magnetic material into small fragments. These fragments (60) have to be reduced to granulated material or granulate of desired dimension. In order to prevent the fragments (60) and, after grinding, the grains from lumping into a head-to-tail arrangement, the present invention provides a solution by means of a grinding device, as schematically shown in figure 9. The fragments (60) are introduced between two grinder bodies (70, 71) with facing magnetic surfaces (72, 73) of matching polarity. Since the fragments (60) will be directed accordingly by this arrangement a more uniform processing of all the fragments is possible and the grains are more easily separated from one another after grinding. The grinder bodies (70, 71) can be permanent magnets, or electromagnets, or a combination of those. In order to conduct the flux lines these grinder bodies (70, 71) can partly consist of iron or another magnetic material, e.g. in a part (74) as indicated in figure 9. The grinder bodies (70, 71) can be rotated about a joint axis (A), and they can be pressed, adjustably or not, as indicated in the direction of the arrows (f). The entire grinding device can be combined with a yoke of suitable magnetic material, again intended to conduct the flux lines, and a bottom portion (75) can be integrated with the yoke. The passage of the mixture through the moulding device (10) in the direction of the arrow, indicated by M, can be established in several ways. The hardening rate of the binding agent, the length of the moulding device (10) and its positioning (horizontally, inclined, vertically, with removable moulding body (11) above or below) will also determine the passage of the mixture. Thus even the elimination of gravity can be taken into account. The passage through will particularly be established by presser means (20, 21, 22, 23) as indicated in figures 2A, 2B, 2C and 3. In these figures, the displacement of these means has been indicated by arrows (a, b, c, d and e). It will be understood that the presser means preferably consists of non-magnetic material so as not to disturb the pole patterns. It can also be remarked that the filling material between the magnetic means (30, 31) is non-magnetic, e.g. a synthetic material or a metal, so that the flux line pattern at the inner circumference (32) of the moulding device will not be disturbed either. Possibly the two moulding bodies (11, 12), and, if necessary, the passage member (13), can be enveloped by a sleeve of magnetically conductive material in order to conduct the flux lines. In figure 2A the presser means (20) is a cylindrical block that closely fits into the supply opening of the second moulding body (12). When a large quantity of the present mixture is introduced into the moulding device (10), the mixture can be pressed to the first moulding body (11) by careful pressing, with which the pole pattern will have to be maintained, and during which in the meantime the mixture can be replenished, or the temperature can be increased or decreased for a part of the moulding device. It is clear that the block (20) can only be displaced up to the passage member (13). Figure 2B shows an alternative way to leave an interspace (33) between the presser means (21), also being a cylindrical block, and the moulding device (10). Although the length of the block (21) is the same as that in figure 2A, it may vary, dependent on the requirements at the used position of the moulding device (10), the binding agent used, and the chosen passage length. It will be clear that the form and the cross-sectional dimensions of the interspace (33) are important to the maintenance of the pole pattern in the mixture. Preferably the respective circumferences of the block (21) and the inner circumference (32) of the second moulding body (12) will be concentric with the axis of the moulding device (10). Figure 2C shows a presser means in the form of a mandrel (22), also having an interspace (33) between the mandrel and the moulding device (10) as indicated above, in which the interspace (33) extends over at least a part of the passage member (13). In a favourable way the mandrel (22) can extend up to the first moulding body (11), contrary to the way it has been drawn in figure 2. The mandrel (22) may even end in a tip, at the point where the first moulding body (11) begins, with suitable diameter, chosen to that purpose, of its cylindrical part and with evenly tapering interspace (33),as indicated above. Figure 3 shows a preferred embodiment of a presser means according to the invention. The presser means is composed of the above-described mandrel (22) and a cylindrical presser sleeve (23) to be displaced over the mandrel (22) into the second moulding body (12) in a close-fitting arrangement. After insertion of the mixture and, subsequently, of the mandrel, as indicated in the situation of the figure, the mixture can be evenly pressed with the sleeve (23). Replenishment of the mixture, removal of the first moulding body (11) and possible heating or cooling can be performed as indicated above. It will be clear that the sleeve (23) can be pressed up to the passage member (13). Figure 5 shows a cross-sectional view along the line V-V in figure 3. Interpolar areas 34, situated between alternating N- and Z-poles, are also schematically indicated. Figure 6 shows a view similar to that of figure 5, but here magnetic means (40, 41) have also been incorporated in the mandrel (22). These magnetic means have alternating N- and Z-poles at the surface of the mandrel. When inserting the mandrel, the N- and Z-poles at the inner circumference (32) of the moulding device (10), and the N- and Z-poles at the surface of the mandrel (22) will have to be aligned in the manner as drawn in the figure. This may be established e.g. by giving the mandrel (22) a fixed position with respect to the moulding device (10). The thus formed magnetic domains in the mixture will have the shape of bar magnets according to this cross-section. Figure 7 represents the case in which the interpolar areas (34, 44) of the moulding device (10) and the mandrel (22), respectively, are interconnected by ribs (50). These ribs can also only extend partially from the moulding device (10) to the mandrel (22), or vice versa. In accordance with the three above-mentioned cases the presser sleeve (23) will comprise a cylindrical comb-like means which fits into respective channels (51) as indicated in figure 7, or in the other cases, a sleeve wall provided with relief, fitting into grooves which will be formed between the protruding ribs (50). It has to be remarked that the ribs (50) may possibly not extend quite up to the first moulding body (11). On the one hand this is induced by lack of room, on the other hand the "bar magnets" as indicated above would get so close to one another that further extending ribs would narrow down the pole areas of these bar magnets and thus hamper their effectiveness. In order to have the pressing of the mixture performed as gradually and evenly as possible, the inner circumference (32) of the moulding device (10) and the surfaces of the fixedly positioned mandrel (22) in figures 5, 6 or 7, and of the ribs (50) in figure 7, can be provided with a well-gliding coating, e.g. of teflon. The ribs (50) may also be entirely made of teflon. In figures 1 and 3 it has not been indicated that the first moulding body (11) to be filled may comprise a bottom portion, with which also a shaft, extending from the bottom along the axis in the moulding device (10), e.g. over the entire length of the moulding body (11), can be provided. Such a recess could function as the place to secure a shaft. In order to further increase the filling factor, the method according to the invention for producing permanently magnetized objects of the type as described above may also comprise the mixing, in a suitable manner, of binding agent and starting material, and feeding this mixture to the moulding device on the one hand, and sucking the mixture by means of vacuum into the first moulding body (11) on the other hand. The first action is performed e.g. by feeding the starting material through a thin layer of binding agent by channels or ducts, and particularly by drawing the starting material through it by means of magnets. In case of a moulding device (10) according to figures 5, 6 and 7 the channels are preferably injection moulding channels. Along a supply end thereof, magnets can be periodicially passed. Of course it is important that during this mixing, the layer of binding agent around a grain becomes as thin as possible. The second action, viz. sucking by means of vacuum, will ensure that possible air or gas bubbles are sucked off. The density of the starting material can be further improved by this method. An object (80), obtained as final product with the above-described devices and methods, may have a shape as drawn in figures 10A, 10C, showing a top and side view, respectively, of such an object. The N- and Z-poles (90, 91) applied therein alternate and in this way may provide a multipolar rotor for a stepper motor in a timepiece. The possible recess, extending along the axis, destined for later securing of the rotor in a timepiece, has not been drawn. If the dimensions of the shaft give rise to such action, it can be made of soft iron, so that it can serve as a magnetic conductor in the magnetic circuit of stator and rotor. It then forms a well-conducting internal return path for the permanent-magnetic poles of the rotor, and improves the external return path for the electromagnetically powered stator poles. The described magnetic function can also be performed by a plate, ring or collection of ring segments made of soft iron and inserted in a recess in the rotor body (80). Multipolar rotors with diameters smaller than 4 mm can be produced by means of the above-described methods and devices. If such rotors are applied in stepper motors with a small stepping angle (e.g. 6°) for timepieces, this could result in a considerable saving of space in the timepiece housing. Perhaps unnecessarily it is pointed out that the above-described method and devices can also be applied for producing other objects having pole areas arranged at the surface. It will be clear to any expert that changes and alterations can be made in a suitable manner to the present methods and devices. One could e.g. think of specially chosen atmospheric conditions. The object (80) could also incorporate an iron core or iron ring for conducting the flux lines. It goes without saying that such changes are not beyond the scope of the present invention, as determined in the enclosed claims.

Claims: Method of manufacturing a multipolar magnet (80) having a plurality of poles (90, 91) of predetermined size arranged at a surface thereof, the method comprising the steps of mixing magnetic particles with a binding agent, inserting the mixture in a mould (10), and hardening said mixture in the mould (10) in the presence of a multipolar static magnetic field generated by magnetic means (30, 31) characterized in that the mixing is carried out using magnetic particles comprising fully magnetized anisotropic permanent-magnet grains (60) having a size of the same order of magnitude as, but smaller than a short dimension of the poles (90, 91), said grains (60) being obtained by reducing a strong permanent magnet to grains (60) of said predetermined size, wherein, during the hardening of the mixture, said grains are positioned along the surface by the multipolar static magnetic field so as to establish the poles (90, 91) of the multipolar magnet (80). Method according to claim 1, characterized in that the size of the grains (60) is about 150 µm for the short dimension of the poles (90, 91) of 200 µm. Method according to claim 1 or 2, characterized in that the number of poles (90, 91) amounts to sixty. Method according to any of the claims 1-3, characterized in that the shape of the multipolar magnet (80) is that of a cylindrical sleeve having a recess and an outer cylindrical surface, wherein the plurality of alternating north poles (90) and south poles (91) are located near the outer cylindrical surface. Method according to claim 4, characterized in that the recess of the cylindrical sleeve is provided with a plate, a ring or a collection of ring segments made of soft iron. Method according to any of the claims 1-5, characterized in that he multipolar magnet (80) has a diameter which is smaller than 4 mm. Method according to any of the claims 1-6, characterized in that said grains (60) are made of a SmCo-alloy. Multipolar magnet (80) having a plurality of poles (90, 91) of predetermined size arranged at a surface thereof, said magnet (80) comprising a hardened mixture of magnetic grains and a binding agent and having at least one anisotropic direction, characterized in that said magnetic grains comprise fully magnetized anisotropic permanent-magnet grains (60) in a graded composition with respect to their size, including large grains of a size of the same order of magnitude as, but smaller than a short dimension of the poles (90, 91), wherein said large grains are positioned along the surface, so as to define the poles (90, 91) of the multipolar magnet (80). Multipolar magnet according to claim 8, characterized in that the size of the grains (60) is about 150 µm for the short dimension of the poles (90, 91) of 200 µm. Multipolar magnet according to claim 8 or 9, characterized in that the number of poles (90, 91) amounts to sixty. Multipolar magnet according to any of the claims 8-10, characterized in that the shape of the multipolar magnet (80) is that of a cylindrical sleeve having a recess and an outer cylindrical surface, wherein the plurality of alternating north poles (90) and south poles (91) are located near the outer cylindrical surface. Multipolar magnet according to claim 11, characterized in that the recess of the cylindrical sleeve is provided with a plate, a ring or a collection of ring segments made of soft iron. Multipolar magnet according to any of the claims 8-12, characterized in that the multipolar magnet (80) has a diameter which is smaller than 4 mm. Multipolar magnet according to any of the claims 8-13, characterized in that said grains (60) are made of a SmCo-alloy.

Inventors: KNAPEN, PETRUS MATHEUS JOSEPHUS
Applicants: KINETRON B.V.
Application year: 1988
Main classification: H01F41
IPC codes: H01F7, H02K15, H01F41