JP7835652B2 - Method for manufacturing a magnetic encoder - Google Patents

Description

This invention relates to a magnetic encoder.

Conventionally, magnetic encoders used for detecting the rotational position of equipment are known. One proposed rotation detection device comprises a magnetic encoder arranged in a concentric ring shape, having two magnetic tracks with different numbers of magnetic poles, and magnetic sensors that detect the magnetic fields of each of these magnetic tracks. The device is configured to calculate the absolute angle of the magnetic encoder based on the phase difference of the magnetic field signals detected by the magnetic sensors (see, for example, Patent Document 1).

The magnetic encoder in the rotation detection device described in Patent Document 1 is constructed as a rubber magnet, for example, by vulcanizing and bonding an elastic member mixed with magnetic powder to a magnetic core, and then forming magnetic tracks by alternately forming magnetic poles in the circumferential direction on this elastic member. Another example of a magnetic encoder configuration is described as a resin magnet, where a resin molded body, made from a resin mixed with magnetic powder, is provided on a magnetic core, and this resin molded body is then formed to create magnetic tracks by alternately forming magnetic poles in the circumferential direction. In such rubber magnets and resin magnets, ferrite-based magnetic powder is typically used as the magnetic powder.

To improve the magnetic properties of a magnetic encoder, it is preferable to have a higher proportion of magnetic material in the encoder, and also to have high magnetic properties in the magnet powder. Therefore, using rare-earth magnet powder results in improved magnetic properties compared to ferrite-based magnetic powder. For this reason, it is conceivable to construct the magnetic encoder, the object to be magnetized, using rare-earth bonded magnets.

When magnetizing multiple magnetic poles (N poles and 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. This coil-energized magnetization device, for example, uses a field section having a coil wound around a magnetization yoke to apply a pulse current, thereby magnetizing the object to be magnetized using the resulting magnetic field. However, as described in the "Problems to be Solved by the Invention" section of Patent Document 2, 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 is constructed using rare-earth bonded magnets, it is difficult to saturate magnetize and form multiple magnetic poles with such a narrow pole pitch as 1.28 mm using the pulse magnetization method described above.

Therefore, a method has been proposed to suitably magnetize even multi-pole magnetization by continuously applying a magnetizing magnetic field to the object to be magnetized using a permanent magnet acting as a field source while heating the object to be magnetized above the Curie point of the magnet and then cooling it to below the Curie point (see, for example, Patent Document 3).

Patent Document 3 describes a magnetization apparatus that includes 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 and compressing isotropic magnet powder and epoxy resin and curing it 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 states that this apparatus can suitably magnetize multiple poles even with a narrow pole pitch such as 1.28 mm.

Japanese Patent Publication No. 2008-267867 Japanese Patent Publication No. 2017-32327 Japanese Patent Publication No. 2021-093521

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

However, because the magnetic powder contains a mixture of magnetic powders with varying particle sizes, the moldability when the green body is thinned and the deformation after heat curing due to variations in the density of the green body become more likely. Furthermore, since only the magnetic powder contributes to the magnetic properties, even distribution of the magnetic powder is crucial for high-precision magnetic encoders. While Patent Document 3 describes that isotropic rare-earth iron bonded magnets can be suitably magnetized even with a narrow pole pitch and multi-pole magnetization, it does not mention the moldability when the green body is thinned or the deformation after heat curing, leaving room for further investigation.

The present invention has been made in view of the above, and aims to provide a magnetic encoder using rare-earth bonded magnets that offers improved 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.

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 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, and the ratio R is 6.67 or greater, and the particle size of the rare earth magnet powder is between 45 μm and 150 μm.

According to an embodiment of the present invention, it is possible to provide a magnetic encoder that ensures moldability, is less prone to deformation after heat curing, has high thickness accuracy, and can be made thinner. Therefore, the magnetic encoder according to an embodiment of the present invention is suitable for applications such as angle detection in the joints and motors of various robots, and rotation detection in bearings for vehicle applications.

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

A magnetic encoder according to an embodiment of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to this embodiment.

[Magnetic encoder according to an embodiment]
A magnetic encoder according to an embodiment will now be described. From the viewpoint of improving the magnetic properties of the magnetic encoder, it is preferable to use isotropic rare-earth bonded magnets in the magnetic encoder. When rare-earth bonded magnets are used, magnetization can be performed effectively even with a narrow pole pitch and multi-pole magnetization, and cost reduction can be expected.

Rare-earth bonded magnets are obtained by mixing and compressing rare-earth magnet powder and resin to create an unheated green body, then thermosetting the resin in the green body to create a cured body, and finally magnetizing the cured body at a predetermined pole pitch. Rare-earth bonded magnets are composed of rare-earth magnet powder, a binder resin, and voids. The volume ratios are approximately 80% rare-earth magnet powder, 10% resin, and 10% voids (pores).

The presence of rare-earth magnet powder with varying particle sizes can affect the moldability when thinning the green material, and variations in density can easily lead to deformation after heat curing. Furthermore, since only the rare-earth magnet powder contributes to the magnetic properties, an even distribution of the rare-earth magnet powder is crucial for high-precision magnetic encoders. The particle size of the rare-earth magnet powder remains largely unchanged in the unheated, heat-cured, and magnetized magnetic encoders.

Figure 1 is a schematic perspective view of a magnetic encoder according to an embodiment, and Figure 2 is a schematic partial plan view of a magnetic encoder according to an embodiment. As shown in Figures 1 and 2, the magnetic encoder 1 using rare-earth bonded magnets has two magnetic tracks: a main track 2 and a sub-track 3. The main track 2 and the sub-track 3 are arranged concentrically on the outer and inner circumferences, respectively, with an unmagnetized region 4 in between.

As shown in Figure 2, the main track 2 and the sub-track 3 are magnetized such that north and south poles are arranged alternately adjacent to each other. The main track 2 and sub-track 3 are magnetized with multiple magnetic pole pairs (north and south poles) at equal pitches in the circumferential direction, and the number of magnetic poles on the main track 2 and sub-track 3 differs from that of the sub-track 3. The sensor module 5, equipped with magnetic sensors 6A and 6B for detecting the magnetic fields of the main track 2 and sub-track 3, respectively, is positioned on some of the magnetic poles of the main track 2 and sub-track 3, as shown in Figure 2.

When the magnetic encoder 1 is rotated around a concentric center, a phase shift 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 shift 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.

[Manufacturing of a magnetic encoder according to an embodiment]
The magnetic encoder 1 is manufactured using 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 in a pulverizer or the like to obtain powder.

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 (e.g., 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, around 150°C, for a predetermined time to produce a cured body.

After fabricating 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 for rust prevention. 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, with two magnetic tracks concentrically formed on one axial end face of the cured body. Specifically, the main track 2 on the outer circumference is magnetized with a predetermined pole pitch, and the sub-track 3 on the inner circumference is magnetized with a predetermined pole pitch. Here, the pole pitch is the circumferential distance between adjacent north and south poles on each magnetic track.

When the main track 2 is magnetized with n pole pairs, the sub-track 3 is magnetized with (n-1) pole pairs. The magnetic pole pitch is determined by the pole width of the magnetic sensors 6A and 6B that detect the magnetic track of the magnetic encoder 1. If the pole width of the magnetic sensors 6A and 6B is limited, the magnetic pole pitch of the magnetic track formed on the magnetic encoder 1 will be the same width as the pole width of the magnetic sensors 6A and 6B. 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.

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 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 the field source. This allows for suitable magnetization even with a narrow pole pitch and multi-pole magnetization. Thus, a magnetic encoder using rare-earth bonded magnets is fabricated.

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.

[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 type magnet powder (manufactured by Magnequench, model number MQP-10-11), which is a magnetically isotropic rare earth iron-based magnet powder produced by an ultra-rapid quenching method, was used as the rare earth magnet powder.

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 represents the meaning described above) in which a portion of Fe in the above magnet is replaced with Co, and further, R-Fe-B-M magnets or R-Fe(Co)-B-M magnets (where R represents the meaning described above, and M represents a combination of one or more of Si, Al, Nb, Zr, Hf, Mo, Ga, P, C) having an alloy composition consisting of unavoidable impurities _{, R2Fe14B, R2Fe ( Co} ) _14B nanocrystalline structure, or αFe _{and R2Fe14} A magnetically isotropic rare-earth iron-based quenched magnetic powder containing a nanocomposite structure of B, R ₂ Fe(Co) ₁₄ B (where R represents the aforementioned meaning) is preferred.

Alternatively, the rare earth-iron-based magnetic powder according to the embodiment may include a magnetically isotropic rare earth-iron-based quenched magnetic powder having 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 in combination of one or more of Hf, Zr, Si, Nb, Ti, Ga, Al, Ta, and C), and an alloy composition consisting of unavoidable impurities, such as an Sm ₂ Fe ₁₇ N _x (x ≈ 3) nanocrystalline structure or a nanocomposite of αFe and Sm ₂ Fe ₁₇ N _x (x ≈ 3). Furthermore, the aforementioned R-Fe-B magnetic powder and Sm-Fe-N magnetic powder may be mixed, and it is acceptable for one or both of them to be magnetically anisotropic magnetic powders.

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.

To achieve saturation magnetization of a sample, it is preferable that the sample thickness T be 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 will be formed to 1.28 mm.

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 it 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 of damage to the green body when ejecting the green body compressed 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.

Here, the handling characteristics of each sample of the green material were evaluated (○: good, no chipping; △: fair, some chipping; ×: poor, many chips). Handling characteristics refer to the ease of handling the ring-shaped green material, which was created by mixing rare-earth magnet powder and a binder, filling a mold with the compound, and compression molding it. The condition of cracks and chips occurring in the green material when removing it from the mold using an ejector pin 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.

Next, the green material removed from the mold was placed in an oven and cured (thermal-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 points using a micrometer to evaluate the thickness variation (maximum value - minimum value) and thickness accuracy. The evaluation results are shown in Table 1. In Table 1, samples with a thickness variation greater than 0.08 mm were marked as unacceptable and their thickness accuracy was indicated by ×. Samples with a variation between 0.08 and 0.06 mm were marked as average and their thickness accuracy was indicated by △. Samples with a variation less than 0.06 mm were marked as good and their thickness accuracy was indicated by ○.

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 material) from the mold using ejector pins. Samples 1 and 2, in particular, showed significant chipping, making them unsuitable. Such chipping poses a problem in subsequent processes, where, if the chipping extends to the track formation area, track formation becomes impossible. Furthermore, because Nd-Fe-B type magnet powder is used as the rare-earth magnet powder, it is prone to rusting. Therefore, a rust-preventive film needs to be applied to the sample surface to prevent oxidation. However, the areas where chipping occurred will not form a uniform rust-preventive film, potentially leading to rust formation in these areas. In contrast, samples 4 and 5 showed no chipping when ejecting the samples (green material) from the mold, demonstrating good handling properties.

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. Samples 3-5 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. Large variations in thickness, when used as a magnetic encoder, can lead to variations in the magnitude of the magnetic flux detected by the magnetic sensor that detects the magnetic track, resulting in a decrease in the angular accuracy of the magnetic encoder.

Table 1 shows the evaluation results for handling and thickness accuracy, indicating that samples 4 and 5 both 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 of samples 1-3.

Here, when the ratio R of the thickness T (mm) of the green material (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, the ratios are approximately 2.82 for sample 1, 3.92 for sample 2, 4.44 for sample 3, and 6.67 for samples 4-5. Therefore, considering the evaluation results of handling and thickness accuracy in Table 1, the sample thickness T 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.

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, jamming is likely to occur 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, cracks and chips may occur in the green body, which is undesirable.

In the above-described embodiment, an axial magnetic encoder with a vernier type in which two magnetic tracks are formed concentrically on a ring-shaped rare-earth bonded magnet was explained. However, the same method can be applied to magnetic encoders with only one magnetic track. Furthermore, the same method can be applied to radial-type magnetic encoders in which magnetic tracks are formed on the outer surface of a cylindrical rare-earth bonded magnet.

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 (5)

A process of mixing rare earth magnet powder and resin, compressing it 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. 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. 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. 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. 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.