Method of making a radial anisotropic ring magnet

Abstract: A magnet is formed as a ring magnet (10) and magnetised by radial application of a magnetic field while the intensity of the magnetic field is changed periodically along the circumference of the ring magnet (10) to give a circumferentially sinusoidal wave form distribution of magnetic flux density to the ring magnet (10) in the magnetised state of the magnet.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of producing a ring magnet having a circumferentially formed sinusoidal waveform distribution of magnetic flux density. An AC servomotor which can generate constant torque regardless of the rotational angle of a rotor has been required. To satisfy this requirement, the theoretical requirement is for a rotor-side magnet having a sinusoidal waveform distribution of magnetic flux density along the circumference thereof. Therefore, as a conventional method, circular blocks (eccentric segments) 100 each having its thickness changed along the circumference thereof as shown in Fig. 4 are stuck to one another in the rotor side to form a ring as a whole (in this example, four circular blocks 100 are circumferentially stuck to one another) so that sinusoidal or substantially sinusoidal waveform distribution of magnetic flux density is provided correspondingly to the change of the thickness of the circular blocks 100. In the case where a plurality of circular blocks 100 are stuck to one another as described above to form a ring magnet, however, the number of the circular blocks 100 must be made to correspond to the number of magnetic poles. For example, four circular blocks must be stuck to one another when the number of magnetic poles is four, and eight circular blocks must be stuck to one another when the number of magnetic poles is eight. Much labour as well as a lot of steps are required for sticking the circular blocks 100 to another. Further, in the case where the circular blocks 100 are stuck to one another as described above, there is a risk that the circular blocks may be separated when the motor is in use. In addition, a problem may arise on positional accuracy in sticking of the circular blocks 100. That is, it is difficult to eliminate the shifting of the sticking positions of the circular blocks 100 perfectly. This causes cogging in the motor, that is, rotational unevenness. Japanese Patent Publication No. 60-124812 discloses a method of producing a radial anisotropic permanent magnet in one piece by injection moulding in a radial magnetic field to form a ring magnet having a circumferentially sinusoidally varying radial thickness. European Patent Publication No. 0369462 discloses a method of producing a polar anisotropic permanent magnet by compressing a magnetic powder in a cavity under a polar magnetic field. Japanese Patent Publication No. 55-165607 discloses a method of producing a radial anisotropic permanent magnet by plastic working in a radial magnetic field having a circumferentially alternating polarity to form a ring magnet having a constant radial thickness and a magnetic flux density circumferentially varying in polarity. SUMMARY OF THE INVENTION The present invention is provided to solve the aforementioned problems. Accordingly, the present invention provides a method of producing a radial anisotropic ring magnet comprising a continuous ring having a constant radial thickness and a circumferentially sinusoidal waveform distribution of magnetic flux density in the magnetised state, characterised by forming the ring with constant radial thickness prior to magnetisation. The intensity of the magnetic field along the circumference of the ring may be varied to produce a circumferentially sinusoidal waveform distribution of magnetic flux. The variation in intensity of the magnetic field may be provided by the use of two yokes, the first yoke being placed within the ring and the second yoke being placed around the ring. More particularly, the present invention provides a method of producing a radial anisotropic ring magnet, further comprising the steps of: forming a ring-shaped preformed material having its wall thickness changed circumferentially and periodically; applying plastic working to said ring shaped preformed material to make a continuous ring having constant radial thickness so that the degree of said plastic working changes circumferentially; and In recent years, rare earth metal magnets having magnetic anisotropy have been noticed and considered to be applied to various fields. In application of this type rare earth metal magnet to an AC servomotor, magnetization is performed by radially applying a magnetic field to the rare earth metal magnet. According to the present invention, a circumferentially sinusoidal waveform distribution of magnetic flux density is given to a magnet in the magnetized state thereof by preliminarily forming the magnet as a ring magnet with constant radial thickness and subsequently magnetizing the ring magnet in a radial magnetic field while changing the intensity of the magnetic field irregularly along the circumference of the ring magnet, that is, periodically strongly or weakly along the circumference of the ring magnet. The magnet thus produced has a single form, that is, an integral type ring form. Accordingly, not only it is easy to mount the magnet onto a shaft of the rotor, but the mounting can be performed by a smaller number of steps. Unlike the case of sticking circular blocks to one another, there is no risk such as separation of the stuck magnet in use of the motor. Further, according to the present invention, it is confirmed that a good sinusoidal waveform distribution of magnetic flux density can be given to a magnet. There arises an effect in that unevenness of rotational torque in the motor can be suppressed to thereby make it possible to apply the magnet to the field of robots which requires high-accurate control. The magnet produced according to the present invention has also a single form, that is, an integral type ring form. Accordingly, in application of the magnet to an AC servomotor, not only the magnet can be mounted on the rotor by a small number of steps but there is no risk such as separation of the magnet in use. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a view for explaining a method according to a first embodiment of the present invention, Fig. 2 is a view for explaining a method according to a second embodiment of the present invention, Figs. 4A and 4B are views for explaining the background of the invention. DETAILED DESCRIPTION OF THE INVENTION Preferred embodiments 1 and 2 of the present invention will be described below in detail to make the features of the invention more clear. Embodiment 1 Rare earth metal magnet material powder composed of Nd, Fe, Co and B was subjected to hot pressing and then subjected to backward extrusion to thereby prepare a ring magnet 10 having an outer diameter of 40 mm, an inner diameter of 36mm and a length of 20 mm (see Fig. 1). Then, the ring magnet was disposed between an inner magnetizing yoke 12 and an outer magnetizing yoke 14 as shown in the drawing and then magnetized by application of the magnetic field formed radially by passing an electric current through coils 16. Here, the inner circumferential side of the outer magnetizing yoke 14 was constituted by four circular surfaces 18 so that not only gaps 20 were formed between the inner circumferential side of the outer yoke 14 and the outer circumferential side of the ring magnet 10 but the depth of the gaps 20 changed circumferentially periodically. In the case where radial magnetization of the ring magnet 10 is performed by using such magnetizing yokes 12 and 14, the magnet 10 is magnetized intensively at positions where the inner yoke 12 and the outer yoke 14 are nearest to each other, that is, at positions where the gaps 20 are shallowest, but magnetized relatively weakly at positions where the inner yoke 12 and the outer yoke 14 are farthest from each other, that is, at positions where the gaps 20 are deepest. As a result, the magnetic flux density of the magnet thus prepared has a predetermined distribution along the circumference. Accordingly, a circumferentially sinusoidal distribution of magnetic flux density can be given to the ring magnet 10 by controlling the depth and pattern of the gaps 20. Embodiment 2 Rare earth metal magnet material powder composed of Nd, Fe, Co and B was subjected to hot pressing to thereby prepare a ring preformed material 22 (outer diameter: 40 mm) as shown in Fig. 2. The inner circumferential side of the ring preformed material 22 was constituted by four circular surfaces 24 (circular arcs with a radius of 53 mm) so that the wall thickness thereof changed circumferentially periodically. Then, the preformed material 22 was subjected to backward extrusion to thereby prepare a radial anisotropic ring magnet having the same structure as Embodiment 1. Here, thick wall portions 26 of the preformed material 22 are higher in the degree of working, while thin wall portions 28 thereof are lower in the degree of working. As a result, the thick wall portions are higher in the degree of orientation (in magnetic domain) of particles, and the thin wall portions are relatively lower. That is, the degree of orientation changes periodically along the circumference of the ring magnet so that it becomes higher or lower. Then, the ring magnet is set in a magnetizing yoke so that the portions higher in the degree of working come to the center of poles, and then the ring magnet is magnetized by application of a magnetic field thereto. Thus, a magnet having a sinusoidal waveform distribution of magnetic flux density is produced. [Comparative Test] Comparative test was applied to the magnets produced in the Embodiments 1 to 3 and a magnet having a structure in which blocks were stuck to one another to form a ring. Results of the comparative test were as shown in Table 1. Number of Times for Sticking to Shaft Cogging Distortion ratio of Flux Density Waveform to Sine Wave Eccentric Segment 100% 100% 100% Example 1 10% 10% 20% Example 2 10% 20% 100% Example 3 10% 15% 80% In the table, numerical values are expressed as values (%) relative to the values (100 %) of the magnet constituted by blocks shown in Fig. 4. It is apparent from the results that the present invention provides a ring magnet having a distribution of magnetic flux density not inferior to the conventional magnet and molded into one body to obtain less irregularity of rotational torque, and that the ring magnet is simplified in the number of steps for mounting the rotor onto the shaft.

Claims: A method of producing a radial anisotropic ring magnet comprising a continuous ring (10) having a constant radial thickness and a circumferentially sinusoidal waveform distribution of magnetic flux density in the magnetised state, characterised by forming the ring with constant radial thickness prior to magnetisation. A method of producing a radial anisotropic ring magnet according to claim 1, wherein on magnetisation of the ring (10), the intensity of the magnetic field along the circumference of the ring is varied to produce a circumferentially sinusoidal waveform distribution of magnetic flux density in the ring. A method of producing a radial anisotropic ring magnet according to claim 2, wherein the variation in intensity of the magnetic field is provided by use of two yokes, the first yoke being placed within the ring and the second yoke being placed around the ring. A method of producing a radial anisotropic ring magnet as claimed in claim 1, comprising the steps of: forming a ring-shaped preformed material (22) having its wall thickness changed circumferentially and periodically; applying plastic working to said ring shaped preformed material to make a continuous ring having constant radial thickness so that the degree of said plastic working changes circumferentially; and magnetising the continuous ring by radially applying a magnetic field to give a predetermined magnetic characteristic corresponding to the degree of plastic working applied to said preformed material.

Inventors: KASAI YASUAKI, YAMADA HIYOSHI, YOSHIKAWA NORIO, KASAI, YASUAKI, YAMADA, HIYOSHI, YOSHIKAWA, NORIO
Applicants: DAIDO STEEL CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1992
Main classification: H01F7
IPC codes: H01F1, H02K15, H01F7, H01F41, H01F13, H02K1