Linear actuator and apparatus

Abstract

To provide an actuator whose efficiency is improved in a wide range without lowering the efficiency in a specific region within a drive range while suppressing increase in size and weight.SOLUTION: A linear DC motor 100 includes an armature unit 110 and a field unit 120 that move relatively. The armature unit 110 includes a cylindrical coil 111, and the field unit 120 includes: first permanent magnets 121a, 121b disposed outside the coil with a main magnetizing direction directed toward the coil 111; a center yoke 122 having a shape extending in a moving direction and having a cross-sectional area orthogonal to the moving direction smaller at the center than at both ends in the moving direction and disposed in a state of being inserted into the coil 111; and a second permanent magnets 121c disposed so as to surround the center of a center yoke 122 with the main magnetizing direction directed toward the coil 111.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a linear actuator and a device.

Background Art

[0002] Linear actuators with various configurations are known. Among them, a linear DC motor (hereinafter referred to as "LDM") can transmit static power to a driven body in a non-contact manner, and thus is widely used in drive mechanisms that require high-precision positioning.

[0003] LDMs are classified into a single-pole type and a multi-pole type according to the configuration of their field parts. The single-pole type LDM has advantages such as easy drive control because the drive of the armature part can be performed in a single phase, and less ripple in thrust. On the other hand, the single-pole type LDM is greatly affected by magnetic saturation in the field part. Therefore, when trying to increase the drive efficiency (hereinafter simply referred to as "efficiency") of the single-pole type LDM, generally, it is necessary to increase the size of the magnet and yoke that make up the field part, so the entire LDM tends to become larger and heavier.

[0004] Regarding such problems, Patent Document 1 discloses a configuration in which the magnets in the field part are a group of magnets in a Halbach array where the magnetic flux density near the surface increases. Also, Patent Document 2 focuses on the fact that generally, a single-pole type LDM has higher efficiency near the center than near the ends in a finite drive range, and discloses a configuration in which the effective magnetic flux near the center is allocated to the end side by making the shape of the yoke near the center thinner.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] According to the technology disclosed in the above Patent Document 1, it is possible to improve the efficiency while suppressing the enlargement of the yoke. However, since the magnet group of the Halbach array is in an unstable arrangement state where they repel each other, means for fixing the magnet group is separately required. As a result, the entire motor becomes larger in size, and the assembly difficulty increases.

[0007] On the other hand, according to the technology disclosed in the above Patent Document 2, the necessary thrust can be obtained up to the end of the driving range without increasing the size of the entire motor. However, the efficiency in the vicinity of the central part of the driving range decreases, and depending on the shape of the motor, the efficiency may not increase at the end side of the driving range.

[0008] The present invention has been made in view of the above circumstances, and an object thereof is to provide an actuator that improves efficiency over a wide range without reducing the efficiency in a specific region within the driving range while suppressing enlargement and weight increase.

Means for Solving the Problems

[0009] The linear actuator according to the present invention is a linear actuator having an armature portion and a field portion that linearly move relative to each other, wherein the armature portion has a cylindrical coil, and the field portion includes a first permanent magnet disposed outside the coil with the main magnetization direction facing the coil, a center yoke having a shape extending in the relative movement direction between the armature portion and the field portion, a cross-sectional area perpendicular to the movement direction being smaller at the central part than at both ends in the movement direction, and being disposed in a state of being inserted into the coil, and a second permanent magnet disposed so as to surround the central part of the center yoke with the main magnetization direction facing the coil.

Effects of the Invention

[0010] According to the present invention, it is possible to provide an actuator that improves efficiency in a wide area without reducing efficiency in a specific area within the driving range while suppressing an increase in size and weight.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present embodiment, as the linear actuator according to the present invention, a unipolar type linear DC motor (hereinafter referred to as "LDM") will be taken up.

[0013] FIG. 1 is a perspective view showing a schematic configuration of a unipolar type LDM 100 according to an embodiment, and a part thereof is shown broken to clarify the internal configuration. FIG. 2 is a partial cross-sectional view taken along the arrow A-A shown in FIG. 1.

[0014] The LDM100 includes an armature part 110 and a field part 120 that transmit power to each other and linearly move relative to each other. The armature part 110 has a cylindrical coil 111. The field part 120 has first permanent magnets 121a, 121b, a second permanent magnet 121c, a center yoke 122, back yokes 123a, 123b, and side yokes 124a, 124b. The center yoke 122 is composed of a first center yoke member 122a and a second center yoke member 122b.

[0015] The first center yoke member 122a is a stepped shaft member having shaft parts with different diameters in the S-axis direction, with the central axis extending in the length direction being the S-axis. Hereinafter, for convenience of explanation, four parts with different diameters arranged in the S-axis direction are referred to as the first shaft part 122a1, the second shaft part 122a2, the third shaft part 122a3, and the fourth shaft part 122a4 in order. Here, the part with the largest diameter in the first center yoke member 122a is the third shaft part 122a3. In FIG. 2, the hatching (cross-sectional view) of the first center yoke member 122a, the first permanent magnets 121a, 121b, and the second permanent magnet 121c is omitted.

[0016] If the diameters of the first shaft part 122a1, the second shaft part 122a2, and the third shaft part 122a3 of the first center yoke member 122a are 'De1', 'Dc', and 'Ds' respectively (see FIG. 3), the relationship De1 < Dc < Ds holds. If the diameter of the fourth shaft part 122a4 is 'De2', in the first center yoke member 122a, the relationship De1 < De2 < Dc holds. However, it is not limited to this, and De1 = De2 may also hold. The first center yoke member 122a can be manufactured by, for example, removal processing such as lathe processing or plastic processing such as header processing.

[0017] The second permanent magnet 121c has a cylindrical shape and is attached to the first center yoke member 122a so as to surround the second shaft portion 122a2 of the first center yoke member 122a. And a coil 111 having a cylindrical shape is disposed so as to surround the outer peripheral surface of the second permanent magnet 121c with a predetermined gap from the outer peripheral surface of the second permanent magnet 121c. The second center yoke member 122b has a cylindrical shape and is attached to the first center yoke member 122a so as to surround the first shaft portion 122a1 of the first center yoke member 122a.

[0018] The first shaft portion 122a1 and the fourth shaft portion 122a4 of the first center yoke member 122a are held by the side yokes 124a and 124b. The side yokes 124a and 124b are fixed to the back yokes 123a and 123b with a predetermined interval in the S-axis direction. In other words, the back yokes 123a and 123b are sandwiched by the side yokes 124a and 124b in the S-axis direction. The first permanent magnets 121a and 121b are attached to the back yokes 123a and 123b and are disposed at two positions so as to sandwich the coil 111 facing the outer peripheral surface of the coil 111.

[0019] As will be described later with reference to FIG. 3(a), the outer diameters of the second permanent magnet 121c, the third shaft portion 122a3 of the first center yoke member 122a, and the second center yoke member 122b are substantially equal, and the armature portion 110 and the field portion 120 are relatively movable in the S-axis direction.

[0020] The first permanent magnets 121a and 121b each have a rectangular parallelepiped shape, are unipolar magnetized between a pair of two faces, and are fixed to the back yokes 123a and 123b in a state where the main magnetization direction is directed toward the coil 111 outside the coil 111. For the first permanent magnets 121a and 121b, for example, general neodymium magnets, samarium cobalt magnets, ferrite magnets, etc. can be used.

[0021] The second permanent magnet 121c is a permanent magnet having a main magnetization direction that is monopole magnetized (radially magnetized) between its inner peripheral surface and outer peripheral surface, and is generally referred to as a radial ring magnet or the like. The main magnetization direction of the second permanent magnet 121c is a direction from the inner peripheral surface toward the outer peripheral surface (a direction toward the coil 111).

[0022] For the first center yoke member 122a, the second center yoke member 122b, the side yokes 124a, 124b, and the back yokes 123a, 123b, a magnetic material such as soft iron can be used, for example.

[0023] As an example, the LDM100 can be assembled as follows. First, the second permanent magnet 121c is fitted onto the second shaft portion 122a2 of the first center yoke member 122a, and the second center yoke member 122b is fitted onto the first shaft portion 122a1 of the first center yoke member 122a. Subsequently, the fourth shaft portion 122a4 of the first center yoke member 122a is fitted into the side yoke 124a, and the coil 111 is inserted into the first center yoke member 122a from the side of the first shaft portion 122a1 of the first center yoke member 122a. Then, the first shaft portion 122a1 of the first center yoke member 122a and the side yoke 124b are fitted together. Separately, the first permanent magnets 121a, 121b are attached to the back yokes 123a, 123b, and the back yokes 123a, 123b and the side yokes 124a, 124b are fitted together. Thereby, the LDM100 can be assembled.

[0024] The center yoke 122, the second permanent magnet 121c, and the first permanent magnets 121a, 121b are arranged between the side yokes 124a, 124b in the S-axis direction and between the back yokes 123a, 123b in the direction orthogonal to the S-axis. As a result, the LDM100 has a structure in which the center yoke 122, the side yokes 124a, 124b, and the back yokes 123a, 123b form a closed magnetic circuit, and a strong magnetic flux acts on the coil 111 arranged inside the closed magnetic circuit.

[0025] Here, the LDM according to a known reference example to be compared with the LDM100 according to the embodiment will be described. FIG. 7 is a partial cross-sectional view showing the schematic configuration of the LDM700 according to the reference example, and is shown in the same method and manner as FIG. 2.

[0026] The LDM700 has a coil 711, permanent magnets 721a, 721b, a center yoke 722, back yokes 723a, 723b, and side yokes 724a, 724b. The LDM700 and the LDM100 have the same main sizes and specifications such as the outer shape (size) and the drivable range, and the LDM100 can be used in place of the known LDM700.

[0027] The coil 711, back yokes 723a, 723b, and side yokes 724a, 724b of the LDM700 are the same as the coil 111, back yokes 123a, 123b, and side yokes 124a, 124b of the LDM100, and the description thereof will be omitted. Also, the permanent magnets 721a, 721b of the LDM700 are the same as the first permanent magnets 121a, 121b of the LDM100, and the description thereof will be omitted.

[0028] The LDM700 is different from the LDM100 in that it includes a center yoke 722 instead of the center yoke 122 and the second permanent magnet 121c provided in the LDM100. The center yoke 722 is a single component and has a cylindrical shape with a constant diameter between the side yokes 124a, 124b in the S-axis direction. As a feature affecting the driving performance, the LDM700 is different from the LDM100 in that it does not include the second permanent magnet 121c provided in the LDM100. The performance difference between the LDM100 and the LDM700 will be described later with reference to FIG. 4.

[0029] Returning to the description of the LDM100. FIG. 3(a) is a partial exploded perspective view of the LDM100. FIG. 3(b) is a cross-sectional view showing the arrangement of the center yoke 122 and the second permanent magnet 121c, and is shown in a cross-section including the central axis S of the center yoke 122.

[0030] As described above, the first center yoke member 122a is integrally formed by the first shaft portion 122a1, the second shaft portion 122a2, the third shaft portion 122a3, and the fourth shaft portion 122a4 having different diameters. After fitting the second permanent magnet 121c to the second shaft portion 122a2 and the second center yoke member 122b to the first shaft portion 122a1, the fourth shaft portion 122a4 is fitted to the side yoke 124a and the side yoke 124b is fitted to the first shaft portion 122a1. Thereby, the components shown in FIG. 3 can be assembled. The center yoke 122 and the side yokes 124a and 124b are made of a magnetic material and are attracted by the second permanent magnet 121c. Therefore, the components are held in a stable assembled state without particularly performing adhesion, screwing, or the like between these members.

[0031] As shown in FIG. 3(b), ring-shaped buffer members 125a and 125b (not shown in other drawings) are disposed between the second permanent magnet 121c and the third shaft portion 122a3 of the first center yoke member 122a and the second center yoke member 122b. This is in consideration of the fact that permanent magnets including the second permanent magnet 121c are generally made of brittle materials. That is, for example, even if an external force acts on the LDM100, the impact force applied from the third shaft portion 122a3 of the first center yoke member 122a and the second center yoke member 122b to the second permanent magnet 121c is weakened by the buffer members 125a and 125b. Thereby, breakage of the second permanent magnet 121c is prevented, the product life of the LDM100 can be extended, and the reliability can be improved. In addition, since the buffer members 125a and 125b absorb the shape error of the components in the manufacturing process of the LDM100, an effect that the yield in the manufacturing process can be improved is also obtained.

[0032] As the buffer members 125a and 125b, for example, urethane foam or the like is used. The buffer members 125a and 125b are applied in the LDM100 with a clearance of about 0.2 mm or less in the S-axis direction after compression, and the influence on the field magnet can be ignored.

[0033] As shown in FIG. 3(b), the diameter of the third shaft portion 122a3 of the first center yoke member 122a, the outer diameter of the second center yoke member 122b, and the outer diameter of the second permanent magnet 121c fitted to the second shaft portion 122a2 are substantially equal to 'Ds'. Note that "substantially equal" means that the minute differences in the diameters or outer diameters of the respective components are within the error range of component manufacturing and assembly.

[0034] The combination of the center yoke 122 and the second permanent magnet 121c in the LDM100 (hereinafter referred to as the "center yoke member group") has a cylindrical shape extending in the S-axis direction, and can have substantially the same outer shape as the center yoke 722 in the conventional LDM700. Thereby, the center yoke member group of the LDM100 has shape compatibility with respect to the center yoke 722 of the conventional LDM700 and can be used in place of the center yoke 722. That is, the LDM100 can perform driving with improved efficiency without increasing the size of the LDM700. As will be described with reference to FIG. 4 below.

[0035] FIG. 4 is a graph showing the average effective magnetic flux density with respect to the coil offset value of the LDM100 and 700. Note that FIG. 4 also shows graphs showing the average effective magnetic flux density with respect to the coil offset value of the LDM800 and 850, which will be described later with reference to FIG. 8, but the description thereof will be given later.

[0036] The coil offset value on the horizontal axis of FIG. 4 is the offset amount of the coil center with respect to the field center in the relative movement direction (S-axis direction) between the armature portion and the field portion in the LDM100 and 700. That is, the coil offset value corresponds to the stroke value from the approximate center of the driving range, and the range in which the armature portion and the field portion can move relatively is indicated by the upper limit value and the lower limit value of the coil offset value.

[0037] In the center yoke member group, the portion disposed between the side yokes 124a and 124b can be divided, for convenience, into three parts in the S-axis direction: both end portions (Side in FIG. 3(b)) and a central portion between the both end portions (Center in FIG. 3(b)). In the center yoke member group, one end portion is constituted by a single member of the third shaft portion 122a3, and the other end portion is constituted by two members of the first shaft portion 122a1 and the second center yoke member 122b, but the magnetic characteristics of both end portions can be regarded as substantially the same.

[0038] The average effective magnetic flux density of the vertical axis is the component in the effective direction (the direction orthogonal to the current direction and the S-axis direction, hereinafter referred to as the "effective direction") for generating the Lorentz force among the magnetic flux densities by the coil 111 at the offset position, integrated and averaged over the entire coil 111. The average effective magnetic flux density can be regarded as the proportionality constant of the thrust with respect to the current in the LDMs 100 and 700. Since the larger this value is, the higher the efficiency becomes, it can be used as an index representing the efficiency.

[0039] It can be seen that the LDM 100 has a higher average effective magnetic flux density and thus higher efficiency mainly in the vicinity of the center of the driving range where the stroke value is near 0 (zero) compared with the LDM 700. Also, it can be seen that the effective magnetic flux density is large and the efficiency is high even in the driving range where the absolute value of the stroke value is large (near both ends). That is, the efficiency is improved in almost the entire driving range. Next, the reason for this will be explained.

[0040] FIG. 5(a) is a schematic diagram showing the main magnetic flux in the center yoke 122 of the LDM 100. FIG. 5(b) is a schematic diagram showing the main magnetic flux in the center yoke 722 of the LDM 700. The arrows shown by thick lines in FIGS. 5(a) and 5(b) schematically represent the main magnetic flux in the field magnet portion. Note that in FIGS. 5(a) and 5(b), the illustration of the magnetic flux in each side yoke and each back yoke is omitted.

[0041] In the LDM700, the magnetic pole surfaces of the rectangular parallelepiped-shaped permanent magnets 721a and 721b face the outer peripheral surface of the coil 711, and the center yoke 722 is arranged such that its outer peripheral surface faces the inner peripheral surface of the coil 711 with a predetermined gap. As a result, the magnetic flux emitted from the permanent magnets 721a and 721b heads towards the coil 711 and is attracted by the center yoke 722. In this way, since the magnetic flux emitted from the permanent magnets 721a and 721b mainly acts on the coil 711 in the effective direction, the LDM700 can generate thrust.

[0042] In the center yoke 722, as the magnetic flux from the permanent magnets 721a and 721b enters from the central part towards the end part in the S-axis direction, the absolute value of the magnetic flux density increases. Here, generally, the magnetization of the magnetic material constituting the yoke saturates, so it becomes difficult for further magnetic flux to enter as the magnetic flux density increases. As a result, the magnetic flux from the permanent magnets 721a and 721b becomes less likely to act on the coil 711 in the effective direction from the central part towards the end part of the center yoke 722 in the S-axis direction, and the efficiency decreases.

[0043] Considering these phenomena, in the LDM700, at both ends of the center yoke 722 in the S-axis direction, the magnetization is closer to saturation than in the central part, so the degree of necessity of the magnetic material is large. Therefore, it is considered inappropriate to reduce both ends. On the other hand, in the central part of the center yoke 722 in the S-axis direction, there is room for reduction because the magnetization has a margin compared to the end parts. However, the efficiency decreases with the reduction. Also, when trying to increase the amount of magnetic flux acting on the coil 711 in the LDM700, that is, to increase the absolute value of the magnetic flux density to increase the magnetic flux density in the effective direction and improve the efficiency, it is considered that the effect is greater in the central part than in both end parts in the S-axis direction.

[0044] Therefore, in LDM100, the outer peripheral portion of the central part in the S-axis direction in the center yoke 722 of LDM700 is reduced, and a second permanent magnet 121c that generates magnetic flux is arranged in the reduced cylindrical portion. Here, as described above, a radial ring magnet whose main magnetization direction is from the inner peripheral surface to the outer peripheral surface is used as the second permanent magnet 121c. The second permanent magnet 121c is fitted with a small gap in the S-axis direction with respect to the third shaft portion 122a3 of the first center yoke member 122a and the second center yoke member 122b, and the main magnetization direction faces substantially the coil 111 at any portion on the circumference.

[0045] As compared with LDM700, as shown in Fig. 5(a), it can be seen that in LDM100, the effective magnetic flux (magnetic flux in the effective direction) with respect to the coil 111 increases in the region near the central part in the S-axis direction, and as a result, it is possible to improve the efficiency. Also, since both ends in the S-axis direction of the center yoke 122 are equivalent to (not changed from) both ends in the S-axis direction of the center yoke 722 of LDM700, the effective magnetic flux with respect to the coil 111 remains at the same level or slightly decreases.

[0046] Here, since the coil 111 of LDM100 has a constant length in the S-axis direction, even when the coil 111 is at an offset position moved from the central part to both end sides in the driving direction, the effective magnetic flux increases in the region close to the central part in the S-axis direction of the center yoke 122. That is, the range in which the effective magnetic flux density obtained by the integral average over the entire coil 111 improves is longer in the S-axis direction than the range in which the effective magnetic flux with respect to the coil 111 increases in the field magnet portion 120. Therefore, in LDM100, the efficiency can be improved over a wide range in the S-axis direction.

[0047] As described above, according to the LDM100, without increasing the outer shape of the conventional LDM and without reducing the efficiency within the driving range, the efficiency can be improved over a wide range within the driving range. In the LDM100, a part of the center yoke is reduced with respect to the conventional configuration (LDM700), and permanent magnets are arranged so as to fill the reduced part. However, since the magnetic material of the yoke and the permanent magnets generally have similar specific gravity values, there is no increase in weight. Furthermore, by using the first center yoke member 122a as a multi-stage shaft member, assembly is facilitated.

[0048] Next, a more desirable configuration of the LDM100 will be described in comparison with an inappropriate similar configuration (comparative example) of the LDM100. FIG. 8(a) is a partial cross-sectional view showing the configuration of the LDM800 according to Comparative Example 1. FIG. 8(b) is a partial cross-sectional view showing the configuration of the LDM850 according to Comparative Example 2.

[0049] The side yokes, back yokes, permanent magnets, and coils of the LDM800 and 850 are the same as the side yokes 124a and 124b, back yokes 123a and 123b, first permanent magnets 121a and 121b, and coil 111 of the LDM100, respectively. Therefore, the reference numerals are omitted in FIG. 8 for these, and the description here is also omitted.

[0050] The LDM800 has a structure in which the outer peripheral portions at both ends in the S-axis direction of the center yoke are removed, and the radial ring magnets 823c1 and 823c2 are arranged in the removed portions. The LDM850 has a structure in which the central portion in the S-axis direction of the center yoke is removed, and the radial ring magnet 823c is arranged in the removed portion. Here, it is assumed that the length of the radial ring magnet 823c in the S-axis direction is greater than the threshold value L with respect to a predetermined threshold value L. On the other hand, it is assumed that the length of the second permanent magnet 121c of the LDM100 according to the embodiment in the S-axis direction is equal to or less than the threshold value L. The threshold value L will be described later. Also, the radial ring magnets 823c1, 823c2, and 823c each have a main magnetization direction from the inner peripheral surface toward the outer peripheral surface, similar to the second permanent magnet 121c.

[0051] The graph showing the average effective magnetic flux density with respect to the coil offset value in LDM800 and 850 is shown in FIG. 4 above. The average effective magnetic flux density of LDM800 is smaller than that of LDM700 according to the reference example throughout the entire range of the coil offset value (that is, throughout the entire driving range). This is due to the removal of the highly necessary end portion in the S-axis direction in the center yoke. Therefore, it is necessary to avoid removing the end portion of the center yoke in the S-axis direction, and the arrangement position of the radial ring magnet in the center yoke needs to be at the center rather than at the end in the S-axis direction.

[0052] The average effective magnetic flux density of LDM850 is larger than that of LDM700 according to the reference example near the center of the coil offset value (near the center of the driving range), but it is not improved as much as LDM100 according to the embodiment. Also, the average effective magnetic flux density of LDM850 decreases significantly on the end side of the driving range. This is due to the fact that when the length of the radial ring magnet 823c is made larger than a predetermined threshold value L, the magnetic materials on both end sides necessary for the center yoke are removed. Therefore, it is desirable that the cross-sectional area in the plane orthogonal to the S-axis at the center of the first center yoke member 122a in LDM100, that is, the length of the second shaft portion 122a2 in the S-axis direction, be equal to or less than the threshold value L. The threshold value L varies depending on the shape of the LDM. For example, it can be the length of the coil 111 in the S-axis direction, thereby suppressing the efficiency reduction at the end portion in the S-axis direction. In LDM100, the length of the second permanent magnet 121c in the S-axis direction is slightly smaller than the length of the coil 111 in the S-axis direction (see FIG. 2).

[0053] Next, an application example of LDM100 to a device will be described. LDM100 can be used as a driving device such as a focus lens or a zoom lens with a limited driving range in an imaging device such as a digital camera. FIG. 6 is an exploded perspective view showing the schematic configuration of an imaging device 600 including LDM100.

[0054] The imaging device 600 is composed of an imaging device main body 610 and a lens barrel 620 that is detachable from the imaging device main body 610. A camera-side mount 612 is provided on the front surface of the imaging device main body 610, and a lens-side mount 622 is provided on the back surface of the lens barrel 620. These can be connected and disconnected, for example, by bayonet engagement. Inside the imaging device main body 610, an imaging element 611 such as a CMOS sensor is arranged. The subject light beam that has passed through the lens barrel 620 forms an image on the imaging surface of the imaging element 611, and the imaging element 611 converts the optical image formed on the imaging surface into an electrical signal (image signal).

[0055] The lens barrel 620 includes a plurality of lenses (not shown), a base member 623, a first lens holder 625, a second lens holder 626, two LDM100s, and two guide bars 624.

[0056] The plurality of lenses form an image of the subject light beam on the imaging surface of the imaging element 611. The base member 623 constitutes the framework of the lens barrel 620. The second lens holder 626 holds at least one of the plurality of lenses and a diaphragm (not shown), and is fixed to the base member 623. The first lens holder 625 holds a lens focus lens and a zoom lens (both not shown). The armature part 110 (coil 111) of the LDM100 is fixed to one of the base member 623 and the first lens holder 625, and the field magnet part 120 of the LDM100 is fixed to the other.

[0057] The drive control of the LDM100 is performed by the control unit (not shown) of the imaging device 600 controlling the current flowing through the coil 111. By driving the LDM100, the first lens holder 625 as the driven body can be relatively moved with respect to the fixed base member 623 in the optical axis direction, and the functions of autofocus and power zoom are realized. The two guide bars 624 are fixed to the base member 623 so that their longitudinal directions are substantially parallel to the optical axis, and guide the first lens holder 625 in the optical axis direction.

[0058] Incidentally, LDM100 is configured in a shape suitable for a lens barrel 620 that generally has a cylindrical shape by including a lens group. Specifically, as can be seen from FIG. 2 when viewed in the S-axis direction, in LDM100, the portion where the coil 111 protrudes from the side yoke 124a is small. Also, the normal line of the surface of the first permanent magnet 121a facing the coil 111 and the normal line of the surface of the first permanent magnet 121b facing the coil 111 are not parallel and intersect at a certain angle (for example, an obtuse angle close to 180 degrees). That is, LDM100 (side yokes 124a, 124b) has a substantially U-shaped when viewed in the S-axis direction. By configuring LDM100 in this way, when LDM100 is arranged outside the optical path along the outer periphery of the optical path of the lens barrel 620, the storage efficiency inside the lens barrel 620 can be increased and the outer diameter of the lens barrel 620 can be minimized. In this way, a lens barrel 620 that is small, lightweight, and has high driving efficiency can be realized.

[0059] Incidentally, the linear DC motor according to the present invention is not limited to the lens barrel of an imaging device, and can be used in a mechanical structure part that performs linear relative movement in various devices. That is, for a driven body arranged to be linearly movable in a device, either the armature part or the magnetic field part of the linear DC motor can be connected, and the driven body can be driven by passing an electric current through the coil.

[0060] Also, the shape of the linear DC motor does not necessarily have to be substantially U-shaped when viewed in the S-axis direction. For example, the linear DC motor may be configured such that the normal line of the surface of the first permanent magnet 121a facing the coil 111 and the normal line of the surface of the first permanent magnet 121b facing the coil 111 are parallel.

[0061] The disclosure of this embodiment includes the following configurations. (Configuration 1) A linear actuator having an armature part and a field part that linearly move relative to each other, wherein the armature part has a cylindrical coil, and the field part includes a first permanent magnet disposed outside the coil with the main magnetization direction facing the coil, a center yoke having a shape extending in the relative movement direction between the armature part and the field part, a cross-sectional area orthogonal to the movement direction being smaller at the central part than at both ends in the movement direction, and being disposed in a state of being inserted into the coil, and a second permanent magnet disposed so as to surround the central part of the center yoke with the main magnetization direction facing the coil. A linear actuator characterized by having these components. (Configuration 2) The linear actuator according to Configuration 1, wherein the second permanent magnet has a cylindrical shape and the main magnetization direction is from the inner peripheral surface toward the outer peripheral surface. (Configuration 3) The center yoke includes a first center yoke member configured as a multi-stage shaft member having a first shaft part, a second shaft part, and a third shaft part with different diameters along the movement direction, and a second center yoke member having a cylindrical shape. The second permanent magnet and the second shaft part are fitted to form the central part, and the second center yoke member and the first shaft part are fitted to form one of the both ends, and the third shaft part forms the other of the both ends. The linear actuator according to Configuration 2, characterized by this structure. (Configuration 4) In the first center yoke member, the diameter of the second shaft part is larger than the diameter of the first shaft part, and the diameter of the third shaft part is larger than the diameter of the second shaft part. The linear actuator according to Configuration 3, wherein the diameter of the third shaft part is substantially equal to the outer diameter of the second center yoke member and the outer diameter of the second permanent magnet. (Configuration 5) The linear actuator according to Configuration 3 or 4, wherein in the first center yoke member, the length of the second shaft part in the movement direction is equal to or less than the length of the coil in the movement direction. (Configuration 6) A linear actuator according to any one of Configurations 3 to 5, characterized in that a buffer member is disposed between the second permanent magnet and the third shaft portion of the first center yoke member, and between the second permanent magnet and the second center yoke member, respectively, in the moving direction. (Configuration 7) A linear actuator according to any one of Configurations 1 to 6, characterized in that the field magnet portion includes a back yoke that holds the first permanent magnet so that the first permanent magnet faces the coil, and a side yoke that sandwiches the center yoke in the moving direction and holds the back yoke. (Configuration 8) A linear actuator according to Configuration 7, characterized in that the first permanent magnet and the back yoke are disposed at two positions so as to sandwich the coil. (Configuration 9) A device comprising: the linear actuator according to Configuration 1; a driven body that is linearly movably disposed and is connected to either the armature portion or the field magnet portion of the linear actuator; and drive control means for driving the driven body by passing a current through the coil.

[0062] As described above, the present invention has been described in detail based on its preferred embodiments. However, the present invention is not limited to these specific embodiments, and various forms within the scope not departing from the gist of the present invention are also included in the present invention. Furthermore, each of the above-described embodiments merely shows one embodiment of the present invention, and it is also possible to appropriately combine the embodiments.

Description of Reference Numerals

[0063] 100 LDM (Linear DC Motor) 110 Armature portion 111 Coil 120 Field magnet portion 121a, 120b First permanent magnet 121c Second permanent magnet 122 Center yoke 122a First center yoke member 122b Second center yoke member 123a, 123b Back yoke 124a, 124b Side yoke 125a, 125b Buffer member 600 Imaging device

Claims

1. A linear actuator having an armature portion and a field portion that linearly move relative to each other, wherein the armature portion has a cylindrical coil, and the field portion includes: a first permanent magnet arranged outside the coil with the main magnetization direction facing the coil; a center yoke having a shape extending in the relative movement direction between the armature portion and the field portion, with a cross-sectional area perpendicular to the movement direction being smaller at the central portion than at both end portions in the movement direction, and being arranged in a state of being inserted into the coil; a second permanent magnet arranged so as to surround the central portion of the center yoke with the main magnetization direction facing the coil. The linear actuator is characterized by having these components.

2. The linear actuator according to claim 1, wherein the second permanent magnet has a cylindrical shape and the main magnetization direction is from the inner peripheral surface toward the outer peripheral surface.

3. The center yoke includes: a first center yoke member configured as a multi-stage shaft member having a first shaft portion, a second shaft portion, and a third shaft portion with different diameters along the movement direction; a second center yoke member having a cylindrical shape; the second permanent magnet and the second shaft portion are fitted together to form the central portion; the second center yoke member and the first shaft portion are fitted together to form one of the both end portions, and the third shaft portion forms the other of the both end portions. The linear actuator according to claim 2 is characterized by this configuration.

4. In the first center yoke member, the diameter of the second shaft portion is larger than the diameter of the first shaft portion, and the diameter of the third shaft portion is larger than the diameter of the second shaft portion. The linear actuator according to claim 3, wherein the diameter of the third shaft portion is substantially equal to the outer diameter of the second center yoke member and the outer diameter of the second permanent magnet.

5. The linear actuator according to claim 3 or 4, wherein in the first center yoke member, the length of the second shaft portion in the movement direction is less than or equal to the length of the coil in the movement direction.

6. The linear actuator according to claim 3 or 4, wherein buffer members are arranged between the second permanent magnet and the third shaft portion of the first center yoke member, and between the second permanent magnet and the second center yoke member, respectively, in the movement direction.

7. The field portion includes: A back yoke that holds the first permanent magnet so that the first permanent magnet faces the coil; A side yoke that sandwiches the center yoke in the moving direction and holds the back yoke, the linear actuator according to claim 3 or 4, characterized by comprising:

8. The linear actuator according to claim 7, characterized in that the first permanent magnet and the back yoke are arranged at two positions so as to sandwich the coil.

9. The linear actuator according to claim 1; A driven body that is linearly movably arranged and is connected to either the armature part or the field part of the linear actuator; A device characterized by comprising drive control means for driving the driven body by passing a current through the coil.

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