Permanent magnets and rotational position sensors

Abstract

To provide a permanent magnet and a rotational position sensor that can achieve both a reduction in the weight of the permanent magnet and an improvement in magnetic flux density.SOLUTION: A permanent magnet 1 comprises a magnet body 10 having an annular shape. The magnet body has an N pole 10n and an S pole 10s facing each other with a center axis CL of the magnet body therebetween. An inner peripheral surface 10a of the magnet body is provided with a pair of recesses 11 hollowed toward an outer peripheral surface 10b of the magnet body. The pair of recesses are arranged at an intermediate part 10c between the N pole and the S pole of the magnet body, and face each other with the center axis of the magnet body therebetween. The recess is formed of a pair of first wall surfaces 11a and a second wall surface 11b. The first wall surface is a flat surface extending in a radial direction with the center axis as the center. The second wall surface is an arc-shaped curved surface with the center axis as the center, and connects the pair of first wall surfaces with each other.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a permanent magnet and a rotational position sensor. Background Art

[0002] Conventionally, there are annular magnets used for sensors and the like. Patent Document 1 discloses a liquid level detection device comprising: a sensor housing; a holder rotatably held by the sensor housing; an arm fixing portion provided on the holder; a float arm whose proximal end side is fixed to the holder by the arm fixing portion; a float provided at a distal end of the float arm and displaced following the liquid level of the liquid stored in a tank; a magnet provided on the holder; and a Hall element provided on the sensor housing and configured to detect displacement of the magnet of the holder. Prior Art Documents Patent Documents

[0003] Patent Document 1 Japanese Unexamined Patent Application Publication No. 2018-205136 Summary of the Invention Problem to be Solved by the Invention

[0004] It is desirable for a permanent magnet to be capable of achieving both weight reduction and improved magnetic flux density. If the weight of the permanent magnet is reduced, miniaturization of sensors using the permanent magnet can be achieved.

[0005] An object of the present invention is to provide a permanent magnet and a rotational position sensor that can achieve both weight reduction and improved magnetic flux density in the permanent magnet. Means for Solving the Problem

[0006] The present invention provides a permanent magnet comprising a ring-shaped magnet body, the magnet body having an N pole and a S pole facing each other with respect to the central axis of the magnet body, the inner circumferential surface of the magnet body having a pair of recesses recessed to the outer circumferential surface of the magnet body, the pair of recesses positioned midway between the N pole and the S pole of the magnet body and facing each other with respect to the central axis of the magnet body, the recesses comprising a pair of first wall surfaces and a second wall surface, the first wall surface being a plane extending radially with respect to the central axis, and the second wall surface being a curved surface in the shape of an arc with respect to the central axis, connecting the pair of first wall surfaces. [Effects of the Invention]

[0007] The permanent magnet according to the present invention has a pair of recesses located midway between the north and south poles of the magnet body. Each recess is composed of a pair of first walls and a second wall. The first walls are planes extending radially around the central axis, and the second walls are curved surfaces in an arc shape around the central axis, connecting the pair of first walls. The permanent magnet according to the present invention achieves the effect of both weight reduction and improved magnetic flux density. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a plan view of a permanent magnet according to the embodiment. [Figure 2] Figure 2 is a plan view of a permanent magnet relating to a comparative example. [Figure 3] Figure 3 is a perspective view of a permanent magnet related to a comparative example. [Figure 4] Figure 4 shows the ratio of magnetic flux density per unit volume. [Figure 5] Figure 5 is a plan view of the permanent magnet related to the first example of study. [Figure 6] Figure 6 is a plan view of the permanent magnet related to the second example of study. [Figure 7] Figure 7 is a plan view of the permanent magnet related to the third example of consideration. [Figure 8]Figure 8 is a cross-sectional view of the rotational position sensor according to this embodiment. [Modes for carrying out the invention]

[0009] The permanent magnet and rotational position sensor according to embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to these embodiments. Furthermore, the components in the following embodiments include those that are easily conceivable by those skilled in the art or that are substantially identical.

[0010] [Embodiment] The embodiments will be described with reference to Figures 1 to 8. This embodiment relates to a permanent magnet and a rotational position sensor. Figure 1 is a plan view of a permanent magnet according to the embodiment, Figure 2 is a plan view of a permanent magnet according to a comparative example, Figure 3 is a perspective view of a permanent magnet according to a comparative example, Figure 4 is a diagram showing the magnetic flux density ratio per unit volume, Figure 5 is a plan view of a permanent magnet according to the first study example, Figure 6 is a plan view of a permanent magnet according to the second study example, Figure 7 is a plan view of a permanent magnet according to the third study example, and Figure 8 is a cross-sectional view of a rotational position sensor according to the embodiment.

[0011] As shown in Figure 1, the permanent magnet 1 of this embodiment has a magnet body 10. The magnet body 10 has an annular shape. The magnet body 10 of this embodiment has a pair of recesses 11. As will be explained below, the magnet body 10 of this embodiment can achieve both weight reduction and improved magnetic flux density in the permanent magnet 1.

[0012] The basic configuration of the magnet will be described with reference to Figures 2 and 3. The shape of the magnet body 10 in this embodiment is such that a recess 11 is provided compared to the magnet body 110 of the comparative example shown in Figure 2. Figures 2 and 3 show the permanent magnet 100 of the comparative example. The type of permanent magnet 100 is, for example, a ferrite magnet. The permanent magnet 100 of the comparative example forms a magnet unit together with the yoke 20. The annular magnet body 110 is magnetized along the magnetization direction MG so that it has an S pole 110s and an N pole 110n. The S pole 110s and the N pole 110n face each other with the central axis CL between them.

[0013] As shown in Figure 3, the comparative example magnet body 110 has a cylindrical portion 111 and a roughly C-shaped projection 112. In the following description, the direction of the central axis CL of the annular magnet body will be referred to as the axial direction X. The projection 112 protrudes from one end face of the cylindrical portion 111 in the axial direction X. When viewed from the axial direction X, the shape of the projection 112 is C-shaped. In other words, the shape of the projection 112 is that of a ring with a part cut out. The notch 112a of the projection 112 is located in the intermediate portion 110c between the S pole 110s and the N pole 110n. In other words, the notch 112a is located at the switching point where the polarity changes in the magnet body 110. The projection 112 protrudes from the inner circumference end of the cylindrical portion 111.

[0014] The magnet body 110 has an inner circumferential surface 110a and an outer circumferential surface 110b. The inner circumferential surface 110a and the outer circumferential surface 110b are arranged concentrically, sharing a central axis CL. The inner circumferential surface 110a is the surface formed by the cylindrical portion 111 and the projection portion 112. That is, the inner diameter of the projection portion 112 is equal to the inner diameter of the cylindrical portion 111. On the other hand, the outer diameter of the projection portion 112 is smaller than the outer diameter of the cylindrical portion 111. In other words, the thickness of the projection portion 112 is smaller than the thickness of the cylindrical portion 111. The outer circumferential surface 110b is the surface of the cylindrical portion 111. The magnet body 110 is formed such that the radius R1 of the inner circumferential surface 111a is equal at any position in the circumferential direction. The cylindrical portion 111 is formed such that the radius R2 of the outer circumferential surface 111b is equal at any position in the circumferential direction.

[0015] The yoke 20 is mounted on the outer circumferential surface 111b of the cylindrical portion 111. The yoke 20 is formed of a magnetic material such as iron. The yoke 20 has a cylindrical shape and covers substantially the entire outer circumferential surface 111b. An end portion of the yoke 20 is provided with an abutting portion 20a that abuts against an end face of the cylindrical portion 111. The yoke 20 concentrates the magnetic force leaking outward from the magnet body 110 toward the center direction and blocks the influence of an external magnetic field.

[0016] Here, in an annular magnet, magnetic loss at the intermediate portion 110c where magnetism is switched is large, so it is difficult to secure magnetic force at the center. When detecting magnetic flux density with a detection element disposed inside the magnet body 110, it is preferable that sufficient magnetic flux density at the center can be achieved.

[0017] In the permanent magnet 1 of the present embodiment, as shown in FIG. 1, a recess 11 is provided in the intermediate portion 10c where polarity is switched. Magnetic loss is reduced by notching the intermediate portion 10c. Further, in the present embodiment, the shape of the recess 11 is optimized so as to achieve both weight reduction of the magnet body 10 and improvement of magnetic flux density.

[0018] As shown in FIG. 1, the magnet body 10 of the embodiment is magnetized along the magnetization direction MG so as to have an N pole 10n and an S pole 10s. The N pole 10n and the S pole 10s face each other across the central axis CL. The magnet body 10 has a cylindrical cylindrical portion 12 and a pair of arc-shaped protruding portions 13. The protruding portions 13 protrude in the axial direction X from one end face of the cylindrical portion 12. The protruding portions 13 protrude from an inner peripheral end of the cylindrical portion 12. One protruding portion 13 is disposed on the N pole 10n side, and the other protruding portion 13 is disposed on the S pole 10s side.

[0019] The magnet body 10 has an inner peripheral surface 10a and an outer peripheral surface 10b. The inner peripheral surface 10a and the outer peripheral surface 10b are provided concentrically sharing the central axis CL. The inner peripheral surface 10a is a surface formed by the cylindrical portion 12 and the protruding portion 13. The outer diameter of the protruding portion 13 is smaller than the outer diameter of the cylindrical portion 12. The cylindrical portion 12 is formed such that the value of the radius R2 of the outer peripheral surface 10b is equal at any position in the circumferential direction.

[0020] The inner peripheral surface 10a of the magnet body 10 is provided with a pair of recesses 11 recessed toward the outer peripheral surface 10b side of the magnet body 10. The pair of recesses 11 are arranged at an intermediate portion 10c between the N pole 10n and the S pole 10s of the magnet body 10. The pair of recesses 11 face each other across the central axis CL of the magnet body 10. When viewed from the axial direction X, the pair of recesses 11 are symmetrical with respect to an imaginary line IL orthogonal to the central axis CL. The imaginary line IL is a straight line that passes through the N pole 10n and the S pole 10s of the magnet body 10 and extends in the radial direction.

[0021] One recess 11 is constituted by a pair of first wall surfaces 11a and one second wall surface 11b. The inner peripheral surface 10a of the magnet body 10 is notched into an arc shape by the pair of first wall surfaces 11a and the second wall surface 11b. The first wall surface 11a is a flat surface extending in the radial direction centered on the central axis CL. The second wall surface 11b is an arc-shaped curved surface centered on the central axis CL, and connects the pair of first wall surfaces 11a. The shape of the recess 11 is line-symmetrical with respect to an intermediate line ML between the N pole 10n and the S pole 10s.

[0022] A distance L1 shown in FIG. 1 is a radial distance from one second wall surface 11b to the other second wall surface 11b. Further, a distance L2 is the maximum width of the recess 11 in the magnetization direction MG. The distance L2 is a linear distance from a connection portion between one first wall surface 11a and the second wall surface 11b to a connection portion between the other first wall surface 11a and the second wall surface 11b.

[0023] The comparison results of the magnet body 10 of the permanent magnet 1 of this embodiment with the comparative example magnet body 110 and the example magnet bodies 30, 40, and 50 will be explained. As shown in Figure 4, the example magnet bodies 30, 40, and 50 each have a pair of recesses 31, 41, and 51. The shape of the recesses 31, 41, and 51 differs from the shape of the recess 11 of the embodiment.

[0024] Figure 5 shows the magnet body 30 of the first study example. The shape of the magnet body 30 is the same as the magnet body 110 of the comparative example, but with a recess 31 added. The magnet body 30 is magnetized along the magnetization direction MG so that it has an N pole 30n and an S pole 30s. The N pole 30n and the S pole 30s face each other with the central axis CL in between.

[0025] The magnet body 30 has a cylindrical portion 32 and a pair of substantially arc-shaped protrusions 33. The protrusions 33 project from one end face of the cylindrical portion 32 in the axial direction X. The magnet body 30 has an inner circumferential surface 30a and an outer circumferential surface 30b. The inner circumferential surface 30a and the outer circumferential surface 30b are arranged concentrically, sharing a central axis CL. The inner circumferential surface 30a is formed by the cylindrical portion 32 and the protrusions 33. On the inner circumferential surface 30a of the magnet body 30, the value of radius R1 of the portion excluding the recess 31 is equal to the value of radius R1 of the comparative example magnet body 110. The value of radius R2 of the outer circumferential surface 30b of the magnet body 30 is equal to the value of radius R2 of the magnet body 110.

[0026] The inner circumferential surface 30a of the magnet body 30 is provided with a pair of recesses 31 that are recessed on the side of the outer circumferential surface 30b. The pair of recesses 31 are located in the intermediate portion 30c between the north pole 30n and the south pole 30s. The pair of recesses 31 face each other with respect to the central axis CL. The pair of recesses 31 are symmetrical with respect to the imaginary line IL.

[0027] The shape of the recess 31 when viewed from the axial direction X is an arc shape. This arc shape is, for example, the shape of a part of a circle passing through the central axis CL. The distance L1 from the bottom of one recess 31 to the bottom of another recess 31 is greater than twice the radius R1. The value of distance L1 of the magnet body 30 is equal to the value of distance L1 of the magnet body 10 in the embodiment shown in Figure 1. The shape of the recess 31 is symmetric with respect to the midpoint line ML between the north pole 30n and the south pole 30s.

[0028] Figure 6 shows the magnet body 40 of the second study example. The shape of the magnet body 40 is the same as the magnet body 110 of the comparative example, but with a recess 41 added. The magnet body 40 is magnetized along the magnetization direction MG so that it has an N pole 40n and an S pole 40s. The N pole 40n and the S pole 40s face each other with the central axis CL in between.

[0029] The magnet body 40 has a cylindrical portion 42 and a substantially C-shaped projection 43. The projection 43 protrudes from one end face of the cylindrical portion 42 in the axial direction X. The magnet body 40 has an inner circumferential surface 40a and an outer circumferential surface 40b. The inner circumferential surface 40a and the outer circumferential surface 40b are arranged concentrically, sharing a central axis CL. The inner circumferential surface 40a is formed by the cylindrical portion 42 and the projection 43. On the inner circumferential surface 40a of the magnet body 40, the value of radius R1 of the portion excluding the recess 41 is equal to the value of radius R1 of the comparative example magnet body 110. The value of radius R2 of the outer circumferential surface 40b of the magnet body 40 is equal to the value of radius R2 of the magnet body 110.

[0030] The inner circumferential surface 40a of the magnet body 40 is provided with a pair of recesses 41 that are recessed on the side of the outer circumferential surface 40b. The pair of recesses 41 are located in the intermediate portion 40c between the north pole 40n and the south pole 40s. The pair of recesses 41 face each other with respect to the central axis CL. The pair of recesses 41 are symmetrical with respect to the imaginary line IL.

[0031] The shape of the recess 41 when viewed from the axial direction X is rectangular. A recess 41 is composed of a pair of first wall surfaces 41a and a second wall surface 41b. The first wall surfaces 41a are planes perpendicular to the imaginary line IL connecting the N pole 40n and the S pole 40s. The pair of first wall surfaces 41a are parallel and face each other in the magnetization direction MG. The second wall surface 41b is a plane extending parallel to the imaginary line IL and connects the pair of first wall surfaces 41a. The second wall surface 41b is perpendicular to the intermediate line ML between the N pole 40n and the S pole 40s. The length L1 of the diagonal of the rectangle formed by the pair of recesses 41 is equal to the distance L1 of the magnet body 10 in the embodiment. In the recess 41, a distance L2 is provided between the pair of first wall surfaces 41a. The distance L2 is the straight-line distance in the magnetization direction MG. The distance L2 of the magnet body 40 is equal to the distance L2 of the magnet body 10 in the embodiment.

[0032] Figure 7 shows the magnet body 50 of the third example. The shape of the magnet body 50 is the same as the magnet body 110 of the comparative example, but with a recess 51 added. The magnet body 50 is magnetized along the magnetization direction MG so that it has an N pole 50n and an S pole 50s. The N pole 50n and the S pole 50s face each other with the central axis CL in between.

[0033] The magnet body 50 has a cylindrical portion 52 and a pair of substantially arc-shaped protrusions 53. The protrusions 53 project from one end face of the cylindrical portion 52 in the axial direction X. The magnet body 50 has an inner circumferential surface 50a and an outer circumferential surface 50b. The inner circumferential surface 50a and the outer circumferential surface 50b are arranged concentrically, sharing a central axis CL. The inner circumferential surface 50a is formed by the cylindrical portion 52 and the protrusions 53. On the inner circumferential surface 50a of the magnet body 50, the value of radius R1 of the portion excluding the recess 51 is equal to the value of radius R1 of the comparative example magnet body 110. The value of radius R2 of the outer circumferential surface 50b of the magnet body 50 is equal to the value of radius R2 of the magnet body 110.

[0034] The inner circumferential surface 50a of the magnet body 50 is provided with a pair of recesses 51 that are recessed on the side of the outer circumferential surface 50b. The pair of recesses 51 are located in the intermediate portion 50c between the north pole 50n and the south pole 50s. The pair of recesses 51 face each other with respect to the central axis CL. The pair of recesses 51 are symmetrical with respect to the imaginary line IL.

[0035] Each recess 51 is composed of a pair of first wall surfaces 51a and a second wall surface 51b. The first wall surfaces 51a are planes perpendicular to the imaginary line IL connecting the north pole 50n and the south pole 50s. The pair of first wall surfaces 51a are parallel and face each other. The second wall surface 51b is a curved surface in the shape of a circular arc centered on the central axis CL, and connects the pair of first wall surfaces 51a.

[0036] The distance L1 shown in Figure 7 is the radial distance from one second wall surface 51b to another second wall surface 51b. The value of distance L1 for the magnet body 50 in the third study example is equal to the value of distance L1 for the magnet body 10 in the embodiment. In the recess 51, a distance L2 is provided between the pair of first wall surfaces 51a. The value of distance L2 for the magnet body 50 is equal to the value of distance L2 for the magnet body 10 in the embodiment.

[0037] Figure 4 shows the magnetic flux density ratio RB per unit volume for magnet bodies 10, 30, 40, 50, and 110. Volume V is the volume of magnet bodies 10, 30, 40, 50, and 110. Magnetic flux density B is, for example, the magnetic flux density at the central axis CL of magnet bodies 10, 30, 40, 50, and 110. The magnetic flux density BU per unit volume is calculated by the following equation (1). The magnetic flux density ratio RB per unit volume is calculated by the following equation (2). Here, BU0 is the magnetic flux density BU per unit volume generated by the comparative example magnet body 110. BU on the right side of equation (2) is the magnetic flux density BU per unit volume generated by magnet bodies 10, 30, 40, 50, and 110. BU = B / V (1) RB = BU / BU0 (2)

[0038] As shown in Figure 4, the magnetic flux density ratio RB per unit volume of the magnet body 30 in the first study example is 1.073, which is an improvement over the comparative example. In the magnet body 40 in the second study example, the magnetic flux density ratio RB per unit volume is 0.993, which is a slight decrease over the comparative example. In the magnet body 10 of the embodiment, the magnetic flux density ratio RB per unit volume is 1.134, which is the largest improvement over the comparative example. In the magnet body 50 in the third study example, the magnetic flux density ratio RB per unit volume is 1.107, which is the second highest improvement over the comparative example, after the embodiment.

[0039] Thus, the magnetic flux density BU per unit volume is improved in the magnet body 30 of the first study example, the magnet body 10 of the embodiment, and the magnet body 50 of the third study example compared to the comparative example. On the other hand, the magnetic flux density BU per unit volume is decreased in the magnet body 40 of the second study example compared to the comparative example.

[0040] In the second example, the decrease in magnetic flux density BU per unit volume in the magnet body 40 is thought to be due to the small amount of notch at the position of the intermediate line ML. In the magnet bodies 10, 30, and 50 of the first example, third example, and embodiment, the radial thickness is reduced at the position of the intermediate line ML. That is, in the magnet bodies 10, 30, and 50, a constricted section with the minimum cross-sectional area is provided at the position of the intermediate line ML. This is thought to have led to an improvement in magnetic flux density BU per unit volume.

[0041] Comparing the first study example, the third study example, and the embodiment, the magnet body 30 of the first study example shows a relatively small improvement in magnetic flux density BU per unit volume. This is thought to be due to the difference in the shape of the curved surfaces of the recesses 31, 51, and 11. In the recess 31 of the first study example, the center of the arc shape of the curved surface is offset from the central axis CL of the magnet body 30. On the other hand, in the recesses 11 and 51 of the embodiment and the third study example, the center of the arc shape of the curved surface coincides with the central axis CL. From this, it is considered advantageous to make the center of the arc shape of the curved surface in the recesses 11 and 51 the central axis CL.

[0042] Comparing the recesses 11 and 51 of the embodiment and the third study example, the recess 11 of the embodiment shows a higher rate of improvement in magnetic flux density BU per unit volume. The difference between the embodiment and the third study example lies in the extension direction of the first wall surfaces 11a and 51a. In the embodiment, the first wall surface 11a extends in the radial direction of the magnet body 10. On the other hand, the first wall surface 51a of the third study example extends in a direction perpendicular to the magnetization direction MG. From this, it is considered that having a first wall surface 11a that extends in the radial direction, as in the recess 11 of the embodiment, leads to an improvement in magnetic flux density BU per unit volume.

[0043] The magnetic flux density B shown above is an example of a simulation result obtained by setting common distances L1 and L2 for the magnet bodies 10, 30, 40, 50, and 110. The depth of the recess 11 in the radial direction centered on the central axis CL may be, for example, half the thickness of the magnet body 10. Distance L2 may be, for example, greater than the radius R1 of the inner circumferential surface 10a. Distance L2 may be, for example, smaller than the radius R2 of the outer circumferential surface 10b. Distance L2 may be half the value of distance L1. Distance L2 may be less than half the length of the outer diameter of the magnet body 10.

[0044] Figure 8 is a cross-sectional view of a rotary position sensor 60 to which the permanent magnet 1 of this embodiment is applied. The illustrated rotary position sensor 60 is a liquid level sensor. The rotary position sensor 60 is installed, for example, in the fuel tank of an automobile. The rotary position sensor 60 includes a permanent magnet 1, a housing 61, a holder 63, and a detection element 64. The housing 61 has a shaft portion 62 that is inserted into the permanent magnet 1, and the shaft portion 62 rotatably supports the permanent magnet 1.

[0045] The holder 63 has an annular recess into which the permanent magnet 1 is inserted. The holder 63 is rotatably supported by the shaft portion 62 while holding the permanent magnet 1. The detection element 64 is an element that detects magnetic flux density, for example, a Hall element. The detection element 64 is positioned on the shaft portion 62 and detects the magnetic flux density caused by the permanent magnet 1. The detection element 64 is positioned inside the permanent magnet 1. The position of the detection element 64 is, for example, the position of the central axis of the permanent magnet 1.

[0046] The holder 63 holds the float arm 65. A float that floats on the liquid surface is connected to the tip of the float arm 65. As the float moves up and down in response to changes in the liquid level, the permanent magnet 1 rotates. The signal indicating the magnetic flux density detected by the detection element 64 is output to, for example, an external device. The external device calculates the liquid level based on the signal obtained from the detection element 64.

[0047] The permanent magnet 1 of this embodiment can improve the magnetic flux density BU per unit volume. Therefore, the permanent magnet 1 of this embodiment can enable miniaturization and weight reduction of the rotational position sensor 60.

[0048] As described above, the permanent magnet 1 of this embodiment includes a magnet body 10 having an annular shape. The magnet body 10 has an N pole 10n and an S pole 10s that face each other with respect to the central axis CL of the magnet body 10. The inner circumferential surface 10a of the magnet body 10 is provided with a pair of recesses 11 that are recessed on the side of the outer circumferential surface 10b of the magnet body 10. The pair of recesses 11 are located in the intermediate portion 10c between the N pole 10n and the S pole 10s of the magnet body 10. The pair of recesses 11 face each other with respect to the central axis CL of the magnet body 10. The recesses 11 are composed of a pair of first wall surfaces 11a and a second wall surface 11b. The first wall surface 11a is a plane extending radially with respect to the central axis CL. The second wall surface 11b is a curved surface in the shape of an arc with respect to the central axis CL and connects the pair of first wall surfaces 11a.

[0049] According to this embodiment, it is possible to achieve both weight reduction of the permanent magnet 1 and improvement of the magnetic flux density B. For example, the volume of the magnet body 10 required to achieve the desired magnetic flux density can be reduced, thereby enabling weight reduction of the permanent magnet 1 and cost reduction of the magnet body 10. Furthermore, the permanent magnet 1 of this embodiment has an optimized magnetic flux density BU per unit volume, and can exhibit a stronger magnetic flux density than a ring magnet of the same diameter and material.

[0050] The rotational position sensor 60 of this embodiment includes a permanent magnet 1, a housing 61, and a detection element 64. The housing 61 has a shaft portion 62 that is inserted into the permanent magnet 1, and the shaft portion 62 rotatably supports the permanent magnet 1. The detection element 64 is positioned on the shaft portion 62 and detects magnetic flux density. The rotational position sensor 60 of this embodiment achieves miniaturization and weight reduction by having a permanent magnet 1 that achieves both weight reduction and improved magnetic flux density.

[0051] In this embodiment, the outer diameter and inner diameter of the magnet body 10 are arbitrary. Also, the values ​​of the distances L1 and L2 are arbitrary and are set to obtain appropriate values ​​for the magnetic flux density BU per unit volume.

[0052] The contents disclosed in the above embodiments can be combined and implemented as appropriate. [Explanation of Symbols]

[0053] 1: Permanent magnet 10: Magnet body, 10a: Inner circumferential surface, 10b: Outer circumferential surface, 10c: Middle part 10n: N pole, 10s: S pole 11: recess, 11a: first wall surface, 11b: second wall surface 20: York 30: Magnet body of the first example under consideration 30a: Inner surface, 30b: Outer surface, 30c: Middle part 30n: N pole, 30s: S pole 31: recess, 32: cylindrical part, 33: protrusion 40: Magnet body of the second example, 40a: Inner surface, 40b: Outer surface 40n:N pole, 40s:S pole 41: recess, 41a: first wall surface, 41b: second wall surface 42: Cylindrical section, 43: Protruding section 50: Magnet body of the third example, 50a: Inner surface, 50b: Outer surface 50n: N pole, 50s: S pole 51: recess, 51a: first wall surface, 51b: second wall surface 52: Cylindrical section, 53: Protruding section 60: Rotational position sensor, 61: Housing, 62: Shaft, 63: Holder 64: Detection element, 65: Float arm 100: Permanent magnets in comparison 110: Magnet body, 110a: Inner circumferential surface, 110b: Outer circumferential surface, 110c: Middle part 110n: N pole, 110s: S pole 111: Cylindrical section, 112: Protruding section, 112a: Notched section CL: Central axis, IL: Virtual line, MG: Magnetization direction, ML: Intermediate line R1: radius of the inner surface, R2: radius of the outer surface

Claims

1. It has a magnet body with a ring shape, The magnet body has an N pole and an S pole that face each other across the central axis of the magnet body, The inner circumferential surface of the magnet body is provided with a pair of recesses that are recessed toward the outer circumferential surface of the magnet body. The pair of recesses are positioned midway between the north and south poles of the magnet body and face each other across the central axis of the magnet body. The recess is composed of a pair of first wall surfaces and a second wall surface. The first wall surface is a plane that extends radially with respect to the central axis, The second wall surface is a curved surface in the shape of an arc with respect to the central axis, and connects the pair of first wall surfaces. A permanent magnet characterized by the following features.

2. A permanent magnet according to claim 1, A housing having a shaft portion inserted into the permanent magnet, the shaft portion rotatably supports the permanent magnet, A detection element is arranged on the shaft portion to detect magnetic flux density, A rotational position sensor equipped with the following features.

Images