Method for manufacturing a magnetic encoder

Abstract

To provide a magnetic encoder capable of ensuring moldability, being less likely to deform after thermal hardening, achieving high thickness accuracy, and being reduced in a thickness.SOLUTION: A ring-shaped magnetic encoder is obtained by mixing rare earth magnet powder and a resin and compressing them to form a non-heated body, thereafter thermally hardening a resin contained in the non-heated body to form a thermal hardened body, and includes a magnetic track in which magnetic poles circumferentially different to each other are magnetized at a predetermined magnetic pole pitch. When a thickness of the non-heated body is denoted by T(mm), and a maximum grain size of the rare earth magnet powder is denoted by G(μm), a ratio R is T / G. The ratio R is 6.67 or more, and the grain size of the rare earth magnet powder is 45 μm to 150 μm.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a magnetic encoder.

Background Art

[0002] Conventionally, a magnetic encoder used for detecting the rotational position of a device is known. As this magnetic encoder, there is provided a magnetic encoder having two magnetic tracks provided in concentric ring shapes and having different numbers of magnetic poles from each other, and a magnetic sensor for detecting the magnetic fields of these respective magnetic tracks, and based on the phase difference of the magnetic field signals detected by the magnetic sensor, a rotation detection device configured to calculate the absolute angle of the magnetic encoder has been proposed (see, for example, Patent Document 1).

[0003] The magnetic encoder in the rotation detection device of Patent Document 1 is configured, for example, by vulcanizing and adhering an elastic member mixed with magnetic powder to a magnetic core made of a magnetic material, and forming this elastic member as a rubber magnet in which magnetic poles are alternately formed in the circumferential direction to form magnetic tracks. Further, as another configuration example of the magnetic encoder, it is described that a resin molded body obtained by molding a resin mixed with magnetic powder may be provided on a magnetic core made of a magnetic material, and this resin molded body may be used as a resin magnet in which magnetic poles are alternately formed in the circumferential direction to form magnetic tracks. In such rubber magnets and resin magnets, usually, ferrite-based magnet powder is used as the magnetic powder.

[0004] Considering the improvement of the magnetic characteristics of the magnetic encoder, it is preferable that the blending amount of the magnetic material contained in the magnetic encoder is large, and it is also preferable that the magnetic characteristics of the magnet powder are high. For this reason, the magnetic characteristics are improved when rare earth magnet powder is used rather than ferrite-based magnetic powder. For this reason, it is conceivable to configure the magnetic encoder, which is a magnetizable object, with a rare earth bonded magnet.

[0005] When magnetizing multiple magnetic poles (N poles, S poles) that form the magnetic track of a magnetic encoder, a coil-energized magnetization device (the so-called pulse magnetization method) is generally used. In this coil-energized magnetization device, for example, a pulse current is passed through a field section having a coil wound around a magnetization yoke, and the resulting magnetic field magnetizes the object to be magnetized. However, as described in the "Problems to be Solved by the Invention" section of Patent Document 2, for example, if the magnetic pole width of the magnetic sensor that detects the magnetic track of the magnetic encoder is limited to 1.28 mm, and the magnetic encoder, which is the object to be magnetized, is made of rare-earth bonded magnets, it is difficult to saturate magnetize and form multiple magnetic poles with such a narrow magnetic pole pitch as 1.28 mm using the pulse magnetization method described above.

[0006] For this reason, a method has been proposed that allows for suitable magnetization even when multi-pole magnetization is performed by continuously applying a magnetizing magnetic field to the object to be magnetized using a permanent magnet, which is the field source, while heating the object to be magnetized above the Curie point of the magnet and cooling it to below the Curie point (see, for example, Patent Document 3).

[0007] Patent Document 3 describes a magnetization apparatus that provides an anisotropic rare-earth iron bulk magnet obtained by hot-plastic deformation of magnet powder at a predetermined temperature to impart anisotropy; an isotropic rare-earth iron bulk magnet obtained by sintering magnet powder at a predetermined temperature using a discharge plasma sintering (SPS) apparatus; an isotropic rare-earth iron bonded magnet obtained by mixing isotropic magnet powder and epoxy resin, compressing and curing at a predetermined temperature; and an isotropic rare-earth iron bonded magnet obtained by using isotropic magnet powder, mixing and compressing it with epoxy resin, and then curing the epoxy resin at a predetermined temperature. It is stated that the apparatus can suitably magnetize even when multi-pole magnetization is performed with a narrow pole pitch such as 1.28 mm. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2008-267867 [Patent Document 2] Japanese Patent Publication No. 2017-32327 [Patent Document 3] Japanese Patent Publication No. 2021-093521 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Rare earth bonded magnets are obtained by mixing and compressing rare earth magnet powder and resin to produce an unheated green body, then heat-curing the resin contained in the green body to produce a thermoset cured body, and finally magnetizing the cured body at a predetermined pole pitch.

[0010] However, because the magnetic powder contains a mixture of magnetic powders with various particle sizes, the moldability when the green body is thinned and the variation in density of the green body make it prone to deformation after heat curing. Furthermore, only the magnetic powder contributes to the magnetic properties, and the even distribution of the magnetic powder is important for high-precision magnetic encoders. However, while Patent Document 3 describes that isotropic rare-earth iron bonded magnets can be suitably magnetized even when multi-pole magnetization is performed with a narrow magnetic pole pitch, it does not describe the moldability when the green body is thinned or deformation after heat curing, leaving room for further investigation.

[0011] The present invention has been made in view of the above, and aims to provide a magnetic encoder using rare earth bonded magnets that improves moldability when the green body is made thinner, is less prone to deformation after heat curing of the green body, and can be suitably magnetized even when multi-pole magnetization is performed with a narrow magnetic pole pitch. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems and achieve the objective, a magnetic encoder according to one aspect of the present invention is obtained by mixing and compressing rare earth magnet powder and resin to produce an unheated body, and then thermosetting the resin contained in the unheated body to produce a thermoset body, and having a ring-shaped magnetic encoder having magnetic tracks in which different magnetic poles in the circumferential direction are magnetized at a predetermined magnetic pole pitch, When the ratio R when the thickness of the unheated body is T (mm) and the maximum particle size of the rare earth magnet powder is G (μm) is defined as T / G, the ratio R is 6.67 or more, and the particle size of the rare earth magnet powder is 45 μm to 150 μm.

Advantages of the Invention

[0013] According to an aspect of the present invention, there is an effect that a magnetic encoder can be provided which ensures moldability, hardly causes deformation after thermosetting, has high thickness accuracy, and can be thinned. Therefore, the magnetic encoder according to an aspect of the present invention is suitable for joint portions of various robots, angle detection of motors, rotation detection of vehicle-use bearings, and the like.

Brief Description of the Drawings

[0014] [Figure 1] FIG. 1 is a schematic perspective view of a magnetic encoder according to an embodiment. [Figure 2] FIG. 2 is a schematic partial plan view of a magnetic encoder according to an embodiment.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, a magnetic encoder according to an embodiment of the present invention will be described in detail based on the drawings. Note that the present invention is not limited by this embodiment.

[0016] [Magnetic Encoder According to an Embodiment] A magnetic encoder according to an embodiment will be described. From the viewpoint of improving the magnetic characteristics of the magnetic encoder, it is preferable to use an isotropic rare earth bonded magnet for the magnetic encoder. When a rare earth bonded magnet is used, it can be suitably magnetized even when multi-pole magnetization is performed with a narrow pole pitch, and cost reduction can be expected.

[0017] The rare earth bonded magnet is obtained by mixing rare earth magnet powder and resin, compressing them to produce a green body which is an unheated body, then thermally curing the resin contained in the green body to produce a cured body which is a thermoset body, and magnetizing the cured body at a predetermined magnetic pole pitch. The rare earth bonded magnet is composed of rare earth magnet powder, a resin as a binder, and voids, and the volume ratios of each are approximately about 80% for rare earth magnet powder, about 10% for resin, and about 10% for voids (pores).

[0018] Since the rare earth magnet powder contains rare earth magnet powders with various particle sizes mixed therein, when thinning the green body, the formability during molding and the variation in the density of the green body tend to cause deformation after thermal curing. Also, only the rare earth magnet powder contributes to the magnetic force characteristics, and it is important for the high-precision of the magnetic encoder that the rare earth magnet powder is distributed evenly without bias. Note that the particle size of the rare earth magnet powder hardly changes in the unheated body, the thermoset body, and the magnetic encoder after magnetization.

[0019] FIG. 1 is a schematic perspective view schematically showing a magnetic encoder according to an embodiment, and FIG. 2 is a schematic partial plan view schematically showing the magnetic encoder according to the embodiment. As shown in these FIGS. 1 and 2, the magnetic encoder 1 using a rare earth bonded magnet has two magnetic tracks, a main track 2 and a sub-track 3, and the main track 2 and the sub-track 3 are arranged on the outer and inner circumferences concentrically with a non-magnetized region 4 interposed therebetween.

[0020] As shown in FIG. 2, the main track 2 and the sub-track 3 are magnetized such that N poles and S poles are alternately adjacent to each other. The main track 2 and the sub-track 3 are magnetized with a plurality of magnetic pole pairs (N poles and S poles) at equal pitch in the circumferential direction, and the number of magnetic poles of the main track 2 and the sub-track 3 are made different from each other. A sensor module 5 provided with magnetic sensors 6A, 6B for detecting the magnetic fields of the main track 2 and the sub-track 3 respectively is arranged on a part of the magnetic poles of the main track 2 and the sub-track 3 as shown in FIG. 2.

[0021] When the magnetic encoder 1 is rotated around a concentric center, a phase difference occurs between magnetic sensors 6A and 6B because the number of magnetic poles on the main track 2 and the sub-track 3 are different. By detecting this phase difference and calculating the absolute angle of the magnetic encoder 1, the rotational position of the device on which the magnetic encoder 1 is installed can be detected.

[0022] [Manufacturing of a magnetic encoder according to an embodiment] The manufacturing of the magnetic encoder 1 utilizes rare-earth bonded magnets because they allow for suitable magnetization even with a narrow pole pitch and multi-pole magnetization, while also offering the potential for cost reduction. Rare-earth bonded magnets are manufactured by, for example, ultra-rapid cooling of rare-earth magnet powder. Specifically, a rare-earth alloy is melted by high-frequency induction heating under reduced pressure or in an argon atmosphere. Next, the molten alloy is sprayed onto a copper rotating roll and ultra-rapidly cooled (rapidly cooled) to produce ribbon-shaped thin strips. These thin strips are then broken into pieces of, for example, several mm to several tens of mm in length, and then crushed using a pulverizer or similar device to obtain powder.

[0023] Next, the crushed powder is classified using a sieve with a predetermined mesh size, and then heat-treated. Furthermore, the rare earth magnet powder and a thermosetting resin (epoxy resin) binder are mixed in a predetermined ratio to produce a compound. The predetermined ratio is, for example, 97-98% by weight of rare earth magnet powder to 2-3% by weight of thermosetting resin. A small amount of lubricant (for example, calcium stearate) may also be added to the compound. Next, the compound is filled into a mold and compressed under a predetermined pressure to produce a ring-shaped green body. The green body removed from the mold is placed in an oven and cured (heat-cured) at a predetermined temperature, for example, about 150°C for a predetermined time to produce a cured body.

[0024] After preparing the cured body, a rust-preventive measure is applied to the surface of the cured body to prevent oxidation. Known methods such as electrodeposition coating and spray coating are used as rust-preventive measures. After applying the rust-preventive measure to the surface of the cured body, a vernier-type magnetic encoder is constructed, for example, as shown in Figure 2, in which two magnetic tracks are formed concentrically on one axial end face of the cured body. Specifically, the main track 2 on the outer circumference is magnetized with a predetermined magnetic pole pitch, and the sub-track 3 on the inner circumference is magnetized with a predetermined magnetic pole pitch. Here, the magnetic pole pitch is the distance in the circumferential direction between adjacent N poles and S poles on each magnetic track.

[0025] When the main track 2 is magnetized with n pole pairs, the sub-track 3 is magnetized with (n-1) pole pairs. The pole pitch of the magnetic track formed on the magnetic encoder 1 is formed to be the same width as the pole width of the magnetic sensors 6A and 6B when the pole width of the magnetic sensors 6A and 6B that detect the magnetic track of the magnetic encoder 1 is limited. For example, if the pole width of magnetic sensor 6A is limited to 1.28 mm (or 1.5 mm), the main track 2 of the magnetic encoder 1 will be magnetized to 1.28 mm (or 1.5 mm), as shown in Figure 1.

[0026] Magnetization is what is commonly known as magnetization, and the magnetization method is, for example, as disclosed in Patent Document 3, a method in which the object to be magnetized is heated to a temperature above the Curie point of its magnetic powder, and while it is cooling to below the Curie point, a magnetizing magnetic field is continuously applied to the object using a permanent magnet, which is a field source. This allows for suitable magnetization even when multi-pole magnetization is performed with a narrow magnetic pole pitch. Thus, a magnetic encoder using rare-earth bonded magnets is manufactured. [Examples]

[0027] The above embodiments will be described in more detail below based on the examples. However, the above embodiments are not limited in any way by the following examples and comparative examples.

[0028] [Examples 1-2 and Comparative Examples 1-3] Samples 1 to 5 were prepared as rare earth magnet powders, corresponding to Examples 1 to 2 and Comparative Examples 1 to 3. For all five samples, Nd-Fe-B magnet powder (manufactured by Magnequench, model number MQP-10-11), a magnetically isotropic rare earth iron-based magnet powder produced by ultra-rapid quenching, was used as the rare earth magnet powder.

[0029] Rare earth iron-based magnetic powders include R-Fe-B magnets (where R is a rare earth element such as Ce, Pr, Nd, Gd, Tb, Dy, Ho, including Y) or R-Fe(Co)-B magnets (where R has the meaning described above) in which part of Fe is replaced with Co in the aforementioned magnets, and R-Fe-BM magnets or R-Fe(Co)-BM magnets (where R has the meaning described above, and M of one or more of Si, Al, Nb, Zr, Hf, Mo, Ga, P, C), and R2Fe, which has an alloy composition consisting of unavoidable impurities. 14 B, R2Fe(Co) 14 B nanocrystalline structure, or αFe and R2Fe 14 B, R2Fe(Co) 14 A magnetically isotropic rare-earth iron-based quenched magnetic powder containing a nanocomposite structure with B (where R represents the aforementioned meaning) is preferred.

[0030] Alternatively, the rare earth-iron magnetic powder according to the embodiment is an alloy composition consisting of an Sm-Fe-N magnet, an Sm-Fe-M'-N magnet (where M' represents one or more of Hf, Zr, Si, Nb, Ti, Ga, Al, Ta, and C) using one or more combinations of Hf, Zr, Si, Nb, Ti, Ga, Al, Ta, and C, and Sm2Fe 17 N x (x≈3) nanocrystalline structure, or αFe and Sm2Fe 17 N xIt is also acceptable to use magnetically isotropic rare-earth iron-based quenched magnetic powders containing a nanocomposite structure with (x≒3). Furthermore, the aforementioned R-Fe-B magnetic powders and Sm-Fe-N magnetic powders may be mixed, and it is also acceptable for one or both of them to be magnetically anisotropic magnetic powders.

[0031] Next, a compound was prepared by mixing the rare earth magnet powder of each sample with a thermosetting resin (epoxy resin: Pernock, model number XW2310) as a binder at a predetermined mixing ratio (rare earth magnet powder: 97.5% by weight, thermosetting resin: 2.5% by weight). The mixing ratio of rare earth magnet powder to binder was the same for all samples 1 to 5. A lubricant (e.g., calcium stearate) may also be added to this compound. The compound was then filled into a mold and compressed under a predetermined pressure to produce green bodies (samples 1 to 5). As shown in Figure 1, the shape of the green bodies is ring-shaped with an outer diameter B: Φ56 mm, an inner diameter A: Φ41 mm, and a thickness T: 1.0 mm.

[0032] To saturate magnetize a sample, it is preferable that the sample thickness T is at least half the magnetic pole pitch. When the thickness T is at least half the magnetic pole pitch, the semicircles of the magnetic field lines are contained within the sample, thus suppressing a decrease in magnetic force. For example, if the magnetic pole width of the magnetic sensor 6A that detects the magnetic track of a magnetic encoder is limited to 1.28 mm, as shown in Figure 2, the magnetic pole pitch of the magnetic track (main track 2) formed on the magnetic encoder is formed to be 1.28 mm.

[0033] Therefore, the sample thickness T is not particularly limited as long as it is 0.64 mm or more, which is half the magnetic pole pitch of the magnetic track (1.28 mm). However, if the sample thickness T is made unnecessarily large, the magnetization depth during magnetic track formation will not be sufficient and will not contribute to the magnetic force, resulting in wasted thickness and increased costs. On the other hand, if the sample thickness T is set to about 0.7 mm, it is more than half the magnetic pole pitch, but for example, handling is poor, and there is a high possibility that the green body will be damaged when ejecting the green body that has been compression molded in the mold. Also, it becomes more prone to damage during manual handling, so from the viewpoint of preventing damage, the sample thickness T was set to 1.0 mm.

[0034] Here, the handling characteristics of each sample of the green body were evaluated (○: good, no chipping; △: average, some chipping; ×: unacceptable, many chips). Handling characteristics refer to the ease of handling when removing the ring-shaped green body, which was made by filling a mold with a compound prepared by mixing rare earth magnet powder and a binder and then compression molding it, from the mold. The condition of cracks and chips that occurred on the green body when removing it from the mold with an ejector pin provided in the mold was visually observed and evaluated as handling characteristics. The results of the handling characteristics evaluation, along with other evaluation results, are summarized in Table 1.

[0035] [Table 1]

[0036] Next, the green material removed from the mold was placed in an oven and cured at a temperature of approximately 150°C for a predetermined time to produce a cured body. The thickness of the cured body removed from the oven was measured at eight diagonal positions using a micrometer to evaluate the thickness variation (maximum value - minimum value) and thickness accuracy, and the evaluation results are shown in Table 1. In Table 1, samples with a thickness variation value greater than 0.08 mm were marked as unacceptable and their thickness accuracy was indicated by ×, samples with a variation value between 0.08 and 0.06 mm were marked as average and their thickness accuracy was indicated by △, and samples with a variation value less than 0.06 mm were marked as good and their thickness accuracy was indicated by ○.

[0037] Table 1 shows that, regarding the evaluation of handling properties, samples 1-3 exhibited cracks and chips on their outer edges when ejecting the samples (green bodies) from the mold using ejector pins. Samples 1 and 2, in particular, exhibited significant chipping, making them unsuitable. Such chipping can lead to problems in subsequent processes, such as the inability to form tracks if the chipping extends to the track formation area during magnetization. Furthermore, because Nd-Fe-B type magnet powder is used as the rare-earth magnet powder, it is prone to rusting. Therefore, it is necessary to coat the sample surface with an anti-corrosion film to prevent oxidation, but the areas where chipping occurred will not have a uniform anti-corrosion film, and rust may develop in these areas. In contrast, samples 4 and 5 did not exhibit chipping when ejecting the samples (green bodies) from the mold, demonstrating good handling properties.

[0038] Furthermore, when evaluating the thickness accuracy of the cured bodies produced by curing the green material (maximum - minimum value), samples 1 and 2 showed variations greater than 0.06 mm, which was unacceptable, while samples 3 to 5 showed variations of 0.06 mm or less, which was good. In particular, samples 4 and 5 showed good thickness accuracy with a variation of 0.03 mm. When the thickness variation becomes large, if the sample is used as a magnetic encoder, variations will occur in the magnitude of the magnetic flux from the track detected by the magnetic sensor that detects the magnetic track, which can lead to a decrease in the angular accuracy of the magnetic encoder.

[0039] Table 1 shows the evaluation results for handling and thickness accuracy, indicating that both samples 4 and 5 show good results. Referring to the maximum particle size in the particle size distribution of the rare earth magnet powder, it can be seen that the maximum particle size in samples 4 and 5 is smaller than that in samples 1-3.

[0040] Here, when the ratio R of the thickness T (mm) of the green body (samples 1-5) to the maximum particle size G (μm) of the rare earth magnet powder in each sample 1-5 is defined as T / G, as shown in Table 1, sample 1 has a ratio of approximately 2.82, sample 2 has a ratio of approximately 3.92, sample 3 has a ratio of approximately 4.44, and samples 4-5 have a ratio of approximately 6.67. From this, considering the evaluation results of handling and thickness accuracy in Table 1, the thickness T of the sample is preferably approximately 4.5 times or more the maximum particle size of the rare earth magnet powder, and more preferably approximately 6.7 times or more the maximum particle size of the rare earth magnet powder.

[0041] Furthermore, based on the evaluation results for handling and thickness accuracy shown in Table 1, the optimal particle size for the rare earth magnet powder is 45 μm to 225 μm, preferably 45 μm to 150 μm. Note that if the particle size of the rare earth magnet powder is less than 45 μm, it is prone to jamming in the clearance (gap) between the molds, potentially making molding difficult. Also, if the particle size of the rare earth magnet powder exceeds 150 μm, it can cause cracks and chips in the green material, which is undesirable.

[0042] In the above-described embodiment, an axial magnetic encoder having a vernier type in which two magnetic tracks are formed concentrically on a ring-shaped rare-earth bonded magnet was explained, but the same can be applied to a magnetic encoder with one magnetic track. Furthermore, the same can be applied to a radial type magnetic encoder in which the magnetic track is formed on the outer surface of a cylindrical rare-earth bonded magnet. [Explanation of Symbols]

[0043] 1 Magnetic encoder, 2 Main track, 3 Sub-track, 4 Unmagnetized area, 5 Sensor module, 6A, 6B Magnetic sensors, A Inner diameter, B Outer diameter

Claims

1. A step of mixing rare earth magnet powder and resin and compressing to produce an unheated body, A step of producing a thermoset body by heat-curing the resin contained in the unheated body, The process includes manufacturing a ring-shaped magnetic encoder having a magnetic track in which different magnetic poles in the circumferential direction are magnetized at a predetermined magnetic pole pitch, A method for manufacturing a magnetic encoder, wherein, in the step of producing the unheated body, when the thickness of the unheated body is T (mm) and the maximum particle size of the rare earth magnet powder is G (μm), the ratio R is T / G, the ratio R is 6.67 or more, and the particle size of the rare earth magnet powder is 45 μm to 150 μm.

2. The method for manufacturing a magnetic encoder according to claim 1, wherein the particle size of the rare earth magnet powder is 75 μm to 150 μm.

3. The method for manufacturing a magnetic encoder according to claim 1 or 2, wherein the magnetic track has a main track and a sub-track arranged concentrically, and the main track is located on the outer circumference side of the sub-track with an unmagnetized region in between.

4. The method for manufacturing a magnetic encoder according to claim 1 or 2, wherein the rare earth magnet powder is a rare earth iron-based (Nd-Fe-B-based) magnet powder.

5. The method for manufacturing a magnetic encoder according to claim 3, wherein the rare earth magnet powder is a rare earth iron-based (Nd-Fe-B) magnet powder.

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