Actuator

Abstract

An actuator (1) equipped with a motor (5) and an encoder unit (40) includes: a first structure portion (19) that determines the relative angles between a plurality of coils (13) of a stator core (11) and a housing (12); a second structure portion (29) that determines the relative angles between the magnetic poles of a rotor core (12) and the fixed reference region (P) of a rotating disk (41) provided on a shaft (23); a third structure portion (39) that determines the relative angle between the shaft (23) and the rotating disk (41); and a fourth structure portion (48) that determines the relative angle between the housing (12) and a detection portion (42).

Description

Actuator

[0001] The present disclosure relates to actuators.

[0002] In an actuator including an encoder and a motor, such as a servo motor, it is necessary to attach the rotating disk of the encoder to the hollow shaft of the motor (for example, JP 2017-203645 A).

[0003] Japanese Patent Application Publication No. 2017-203645

[0004] The positional relationship between the rotating disk and the hollow shaft determines the relative angle between the rotating disk and the rotor of the motor.

[0005] Furthermore, since the housing of the motor stator and the stator core are generally fitted together by a cylindrical surface, the relative angles between the housing and each phase of the coil wound around the stator core are not fixed.

[0006] Furthermore, since the hollow shaft and the rotor core of the motor rotor are generally fitted with a cylindrical surface, the relative angle between the fixed part of the rotating disk of the encoder provided on the hollow shaft of the rotor and the magnetic pole of the magnet mounted on the rotor core is also not fixed.

[0007] For this reason, when the rotating disk is attached, the angle between the motor coil and magnetic pole as viewed from the Z phase (the angle of the Z phase when the coil and magnetic pole are aligned in the specified positions) cannot be adjusted.

[0008] Therefore, it is desirable to accurately adjust the Z-phase angle by a method of determining the angle of the rotating disk by DC excitation.

[0009] According to a first aspect of the present disclosure, there is provided an actuator comprising: a stator including a housing and a stator core fixed within the housing and wound with a plurality of coils; a rotor arranged facing the stator, the rotor including a rotor core forming a plurality of magnetic poles and a shaft coupled to the rotor core; and an encoder unit including a rotating disk coupled to the shaft, a detection unit that detects rotation of the rotating disk, and a fixed unit fixed to the housing; a first structural unit that determines a relative angle between the plurality of coils of the stator core and the housing; a second structural unit that determines a relative angle between the magnetic poles of the rotor core and a fixed reference portion of the rotating disk provided on the shaft; a third structural unit that determines a relative angle between the shaft and the rotating disk; and a fourth structural unit that determines and adjusts the relative angle between the housing and the detection unit.

[0010] The objects, features, and advantages of the present disclosure will become more apparent from the following description of the embodiments taken in conjunction with the accompanying drawings.

[0011] 7A is an axial cross-sectional view of an actuator based on a first embodiment; FIG. 7B is a radial cross-sectional view of a housing; FIG. 7C is a radial cross-sectional view of a stator core; FIG. 7D is a perspective view showing a hollow shaft and a rotary disk; FIG. 7E is an axial cross-sectional view of an actuator for explaining a modified example of a second structural portion; FIG. 7F is another axial cross-sectional view of the actuator shown in FIG. 1; FIG. 7G is an axial cross-sectional view of an actuator based on another embodiment; FIG. 7H is a front view of an encoder fixing portion; FIG. 7I is a partial axial cross-sectional view of an actuator based on another embodiment; FIG. 7J is a radial cross-sectional view of the actuator shown in FIG. 7A.

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Corresponding components are designated by common reference numerals throughout the drawings. FIG. 1 is an axial cross-sectional view of an actuator based on a first embodiment. The actuator 1 shown in FIG. 1 mainly includes a motor 5, such as a servo motor, consisting of a stator 10 and a rotor 20, and an encoder 40 connected to the motor 5. The encoder 40, which includes a rotating disk 41, is, for example, an incremental encoder, and outputs A-phase, B-phase, and Z-phase signals. The encoder 40 may be an optical encoder or a magnetic encoder.

[0013] The stator 10 includes a stator core 11 disposed within a housing 12. The stator core 11 includes a plurality of coils 13 arranged around the central axis of the motor 5. The rotor 20 is rotatably disposed inside the stator core 11. The rotor 20 includes a rotor core 21 having a plurality of magnets 22, for example, permanent magnets, disposed on its outer circumferential surface, and a hollow shaft 23 that rotates integrally with the rotor core 21 on the central axis of the motor 5. The hollow shaft 23 is rotatably disposed in the housing 12 via bearings 51 and 52.

[0014] Fig. 2A is a radial cross-sectional view of the housing. As shown in Fig. 2A, a pair of keyways 16 are formed on the inner peripheral surface of the housing 12. These keyways 16 are formed at positions opposite each other in the diametrical direction of the housing 12. The number of keyways 16 is preferably one or more. Alternatively, multiple keyways 16 may be formed at non-equidistant intervals around the circumference of the housing 12.

[0015] 2B is a radial cross-sectional view of the stator core. As shown in FIG. 2B , a plurality of coils 13 are fixed at equal intervals on the inner peripheral surface of the stator core 11. A plurality of grooves 17 are formed on the outer peripheral surface of the stator core 11 at positions corresponding to the central portions of the plurality of coils 13. Furthermore, a pair of grooves 17 corresponding to the diameter of the stator core 11 has a parallel key 15 inserted therein, extending parallel to the axial direction of the hollow shaft 23. The number of parallel keys 15 is preferably one or more, and in FIG. 2B , the parallel keys 15 are inserted into a pair of grooves 17 that are opposed in the diameter direction. However, the parallel keys 15 may also be inserted into another groove 17 located at a position corresponding to the key groove 16. Furthermore, the grooves 17 may be formed at a position other than the central portion of the coils 13, for example, at a position corresponding to the position between two adjacent coils 13.

[0016] The positions of one or more key grooves 16 formed in the housing 12 are known in advance, and the positional relationship between the grooves 17 and the coils 13 is also known in advance. Therefore, by inserting one or more parallel keys 15 into the grooves 17 of the stator core 11 so as to engage with such key grooves 16, it is possible to determine the relative angle between the multiple coils 13 of the stator core 11 and the housing 12. Therefore, the one or more parallel keys 15, the one or more key grooves 16, and the one or more grooves 17 serve as a first structural portion 19 that determines the relative angle between the multiple coils 13 of the stator core 11 and the housing 12. Note that a similar effect can be achieved even if the parallel key 15 and the groove 17 are integrated and a portion of the parallel key 15 protrudes from the groove 17 as a convex portion.

[0017] Generally, the rotor core 21 and hollow shaft 23 of the rotor 20 are separate members that are joined together. However, in the first embodiment, as can be seen with reference to FIG. 1 , the hollow shaft 23 and the rotor core 21 are integrally formed. In other words, the hollow shaft 23 and the rotor core 21 are a single member.

[0018] 3 is a perspective view showing the hollow shaft and the rotating disk. As shown in FIG. 3, in order to accurately detect the Z-phase using the rotating disk 41, a fixed reference portion P for the rotating disk 41, such as a mark, is formed on a part of the outer circumferential surface of the hollow shaft 23. The rotating disk 41 is attached to the hollow shaft 23 so that the fixed reference portion P for the rotating disk and a Z-phase slit S formed in advance in the rotating disk 41 correspond to each other. Typically, the rotating disk 41 is attached to the hollow shaft 23 as described below so that the circumferential position of the fixed reference portion P for the rotating disk and the circumferential position of the Z-phase slit S coincide with each other.

[0019] As described above, the hollow shaft 23 and the rotor core 21 are integrally molded, so the relative angle between the magnetic poles of the rotor core 21 and the fixed reference portion P provided on the hollow shaft 23 is automatically and fixedly determined. Therefore, the integrally molded hollow shaft 23 and rotor core 21 serve as a second structural portion 29 that determines the positional relationship between the fixed reference portion P for the rotating disk of the hollow shaft 23 and the magnetic poles of the magnets 22 mounted on the rotor core 21.

[0020] 4 is an axial cross-sectional view of the actuator for explaining a modified example of the second structural portion. In the actuator 1' shown in FIG. 4, the rotor core 21 and the hollow shaft 23 are separate members. One or more grooves (not shown) are formed on the inner peripheral surface of the rotor core 21 and the outer peripheral surface of the hollow shaft 23, respectively.

[0021] One or more parallel keys 26 extending parallel to the axial direction of the hollow shaft 23 are fitted into grooves in the rotor core 21 and the hollow shaft 23, thereby joining the rotor core 21 and the hollow shaft 23. The number and positions of the parallel keys 26 are the same as those of the parallel keys 15 described above.

[0022] Because the rotor core 21 and hollow shaft 23 are coupled to each other by these parallel keys 26 and grooves, the relative angle between the magnetic poles of the rotor core 21 and the fixed reference point P provided on the hollow shaft 23 is automatically and fixedly determined. Therefore, the grooves formed on the inner circumferential surface of the rotor core 21, the grooves formed on the outer circumferential surface of the hollow shaft 23, and the parallel keys 26 function as the above-mentioned second structural portion 29. Note that a similar effect can be achieved if the parallel keys 26 and the corresponding grooves are integrated and part of the parallel keys 26 protrudes from the grooves as a convex portion.

[0023] As shown in FIG. 1 , the hollow shaft 23 includes an expanded portion 23A between the rotary disk 41 and the motor 5, whose thickness expands radially inward. The expanded portion 23A is formed at one end of the hollow shaft 23 on the encoder 40 side. Due to the presence of this expanded portion 23A, the inner diameter D1 of the hollow shaft 23 is larger than the inner diameter D2 of the expanded portion 23A. In one embodiment, (D1-D2) / D1 is preferably 0.1 to 0.2. The length and thickness of the expanded portion 23A in the axial direction of the hollow shaft 23 are large enough to accommodate the rotary disk fixing portion 35, which will be described later.

[0024] 1, the rotating disk 41 of the encoder 40 is fixed to the end surface of the extension portion 23A by a rotating disk fixing portion 35. Typically, the rotating disk fixing portion 35 is a combination of a plurality of first screws 35 extending parallel to the axial direction of the hollow shaft 23 and a plurality of holes formed in the end surface of the extension portion 23A to correspond to the plurality of first screws 35. However, other forms of rotating disk fixing portion 35 that can fix the rotating disk 41 to the extension portion 23A may also be used.

[0025] Therefore, in the first embodiment, the rotary disk 41 of the encoder 40 can be easily connected to the end surface of the hollow shaft 23. Note that the number of the first screws 35 and the number of the holes may be different, and the first screws 35 do not have to be arranged at equal intervals in the circumferential direction of the hollow shaft 23.

[0026] Figure 5 is another axial cross-sectional view of the actuator shown in Figure 1. In Figure 5, at least one wire C, such as a cable, tubing, or the like for supplying power, signals, or materials, passes through the actuator 1 through the internal space of the hollow shaft 23. The at least one wire C is fixed near both ends of the actuator 1 by fixing portions (not shown in Figures 1 and 5).

[0027] As can be seen from Figures 1 and 5, the thickness of the expansion portion 23A is sufficiently smaller than the inner diameter of the hollow shaft 23, so even if the expansion portion 23A is formed, there is no need to reduce the number of filaments C, and the slack in these filaments C can be maintained.

[0028] 1 and 5, the rotary disk 41 is fixed to the end surface of the extension portion 23A via the boss 25 by a plurality of first screws 35. The boss 25 is preferably attached to the rotary disk 41 by adhesive or the like. The rotary disk 41 may also be fixed directly to the end surface of the extension portion 23A. In this case, the rotary disk 41 of the encoder can be more easily connected to the hollow shaft 23. It will be understood that the extension portion 23A, the hole formed in the extension portion 23A, and the first screws 35 serve as a third structural part 39 that determines the relative angle between the hollow shaft 23 and the rotary disk 41.

[0029] FIG. 6 is an axial cross-sectional view of an actuator according to another embodiment. The actuator 1a according to this embodiment has, instead of the extended portion 23A, an extended portion 23B whose thickness extends radially outward of the hollow shaft 23 between the rotating disk 41 and the motor 5. As shown in FIG. 6, the inner diameter D1 of the hollow shaft 23 is equal to the inner diameter D2' of the extended portion 23B. In other words, even with the extended portion 23B, the inner diameter of the hollow shaft 23 does not change throughout the entire hollow shaft 23. The rotating disk 41 of the encoder 40 is fixed to the end face of the extended portion 23B by a plurality of first screws 35 extending parallel to the axial direction of the hollow shaft 23.

[0030] In this case, the inner diameter of the extended portion 23B is equal to the inner diameter of the other portions of the hollow shaft 23, so one or more filaments C can be passed through the hollow shaft 23 with more room than in the embodiment shown in Fig. 1. As described above, the scope of the present disclosure also includes a case in which the rotary disk 41 is fixed directly to the end face of the extended portion 23B without the boss 25. Furthermore, the scope of the present disclosure also includes a case in which both the extended portion 23A and the extended portion 23B are formed at one end of the hollow shaft 23.

[0031] 1, a detector 42 provided on a substrate 49 of the encoder 40 uses a known method to detect an absolute position PA1 within one rotation of the hollow shaft 23 and a total number of rotations PB1. The detected information is stored in a memory (not shown), for example, a volatile memory.

[0032] 7 is a front view of the encoder fixing portion. As can be seen from FIGS. 1 and 7, a plurality of elongated holes 46 are formed in the substrate 49 of the encoder fixing portion 43 of the encoder 40. In each of the elongated holes 46, the portion extending in the circumferential direction of the hollow shaft 23 is longer than the portion extending in the radial direction of the hollow shaft 23. The elongated holes 46 are typically oval or elliptical, but may be other shapes.

[0033] A plurality of second screws 45 are inserted into a plurality of elongated holes 46 in the substrate 49 and fixed to the housing 12. For this purpose, holes for the plurality of second screws 45 are formed in the housing 12. The second screws 45 preferably extend parallel to the axial direction of the hollow shaft 23. Because the elongated holes 46 are not circular, after the second screws 45 are threaded into them, the position of the encoder fixing portion 43 can be finely adjusted in the circumferential direction within the range of the circumferential length of the elongated holes 46.

[0034] Because the detecting portion 42 is in a predetermined position relative to the fixing portion 43 of the encoder 40, the detecting portion 42 can be disposed at a desired position relative to the housing 12. Therefore, the plurality of second screws 45, the elongated holes 46, and the holes in the housing 12 serve as a fourth structural portion 48 that determines the relative angle between the housing 12 and the detecting portion 42 and adjusts the relative angle. The fourth structural portion 48 allows room for adjusting the position of the detecting portion 42 in the rotational direction after the encoder 40 is attached.

[0035] Figure 8A is a partial axial cross-sectional view of an actuator based on another embodiment, and Figure 8B is a radial cross-sectional view of the actuator shown in Figure 8A. For the purpose of simplicity, the encoder 40 is not shown in Figure 8A. The housing 12 of the actuator 1b shown in Figure 8A has a plurality of through-holes formed parallel to the axial direction of the hollow shaft 23. The stator core 11 has a flange 11a that extends radially outward from the hollow shaft 23 to the outer circumferential surface of the housing 12.

[0036] 8A, the flange 11a is configured to engage with a notched portion of the housing 12. In other words, the flange 11a is sandwiched between the housing 12 from the upstream and downstream sides in the axial direction of the hollow shaft 23.

[0037] The flange 11a of the stator core 11 also has a plurality of through holes formed therein that correspond to the through holes of the housing 12. In other words, the plurality of through holes of the flange 11a communicate with the plurality of through holes of the housing 12, respectively.

[0038] The tie rods TL are positioned in the respective through holes of the flange 11 a. The length of the tie rods TL is longer than the length of the flange 11 a in the axial direction of the hollow shaft 23, and shorter than the length of the housing 12 in the axial direction of the hollow shaft 23. Therefore, both ends of each tie rod TL do not reach both ends of the housing 12 in the axial direction of the hollow shaft 23.

[0039] 8B, the plurality of tie rods TL are arranged at equal intervals in the circumferential direction of the hollow shaft 23. However, the plurality of tie rods TL may also be arranged at non-equidistant intervals in the circumferential direction of the hollow shaft 23.

[0040] 8A , a plurality of tie bolts TB are inserted into the through holes from both ends of the housing 12 in the axial direction of the hollow shaft 23, and are coupled to the plurality of tie rods TL, respectively. For this purpose, it is preferable that each of the plurality of tie rods TL has an internal thread formed on both ends. In this way, the plurality of tie bolts TB and the plurality of tie rods TL fix the housing 12 and the stator core 11 together.

[0041] Therefore, the relative angle between the coils 13 of the stator core 11 and the housing 12 is fixedly determined. In other words, the flange 11a, the tie rod TL, and the tie bolt TB serve as a first structural part 19. Note that the tie rod TL may not be present, and the tie bolt TB may be configured to screw into threads formed on the inner surface of the through hole of the flange 11a and the through hole of the housing 12.

[0042] Furthermore, in an embodiment not shown, a plurality of tie rods TL may be used, each of which is longer than the length of the housing 12 in the axial direction of the hollow shaft 23 and has male threads formed on both ends. In this case, the male thread portions of the plurality of tie rods TL protrude from both ends of the through hole, and nuts are threaded onto the male thread portions instead of the tie bolts TB.

[0043] Alternatively, in another embodiment (not shown), the tie rods TL may be eliminated, and multiple bolts longer than the length of the housing 12 in the axial direction of the hollow shaft 23 may be used. The bolts are then threaded into female threads formed in advance in the through holes of the flange 11 a and the housing 12.

[0044] In these embodiments (not shown), the relative angle between the coil 13 of the stator core 11 and the housing 12 can be fixed in the same manner as described above. Therefore, the nuts and bolts also serve as the first structural part 19.

[0045] As described above, in the present disclosure, the relative angle between the housing 12 and the plurality of coils 13 of the stator core 11 is determined by the first structural portion 19, the relative angle between the magnetic poles of the rotor core 21 and the fixed reference portion of the rotating disk 41 provided on the shaft 23 is determined by the second structural portion 29, the relative angle between the shaft 23 and the rotating disk 41 is determined by the third structural portion 39, and the relative angle between the housing 12 and the detection portion 42 is determined and can be adjusted by the fourth structural portion 48. Therefore, the angle of the Z phase of the encoder unit can be accurately adjusted.

[0046] Aspects of the Present Disclosure According to a first aspect, there is provided an actuator comprising a stator, the stator including a housing and a stator core fixed within the housing and wound with a plurality of coils, and a rotor arranged facing the stator, the rotor including a rotor core forming a plurality of magnetic poles and a shaft coupled to the rotor core, and an encoder unit including a rotating disk connected to the shaft, a detection unit that detects rotation of the rotating disk, and a fixed unit fixed to the housing, and further comprising: a first structural unit that determines the relative angle between the plurality of coils of the stator core and the housing, a second structural unit that determines the relative angle between the magnetic poles of the rotor core and a fixed reference portion of the rotating disk provided on the shaft, a third structural unit that determines the relative angle between the shaft and the rotating disk, and a fourth structural unit that determines and adjusts the relative angle between the housing and the detection unit. According to a second aspect, in the first aspect, the third structural portion includes a boss attached to the rotating disk or a plurality of first screws that fix the rotating disk to the shaft, and a plurality of screw holes formed in the shaft that engage with the first screws. According to a third aspect, in the first or second aspect, the fourth structural portion includes a plurality of elongated holes formed in the fixing portion of the encoder unit and a plurality of second screws that are inserted into the elongated holes, respectively. According to a fourth aspect, in any of the first to third aspects, the first structural portion includes a plurality of keyways formed in the stator core and the housing, and a plurality of parallel keys that engage with each of the keyways. According to a fifth aspect, in any of the first to third aspects, the first structural portion includes a plurality of keyways formed in one of the housing and the stator core, and a plurality of protrusions provided on the other of the housing and the stator core that engage with each of the keyways.According to a sixth aspect, in the first aspect, the first structural portion includes a flange provided on the stator core and engaging with the housing, a plurality of through holes formed in the housing and the flange and communicating with each other, and a plurality of tie bolts threadedly engaging with the plurality of through holes. According to a seventh aspect, in any of the first to sixth aspects, the second structural portion includes a plurality of keyways formed in the rotor core and the shaft, and a plurality of parallel keys engaging with each of the plurality of keyways. According to an eighth aspect, in any of the first to sixth aspects, the second structural portion includes a plurality of keyways formed in one of the rotor core and the shaft, and a plurality of protrusions provided on the other of the rotor core and the shaft and engaging with each of the plurality of keyways. According to a ninth aspect, in any of the first to sixth aspects, the rotor core and the shaft are integrally formed in the second structural portion. According to a tenth aspect, in any of the first to ninth aspects, the encoder is an optical encoder or a magnetic encoder.

[0047] Effects of the Aspects In the first aspect, the angle of the Z phase of the encoder unit can be accurately adjusted. In the second aspect, the relative angle between the shaft and the rotating disk can be fixedly determined. In the third aspect, the relative angle between the housing and the detector is determined, and the relative angle is adjusted after the encoder is installed. In the fourth aspect, the relative angle between the multiple coils of the stator core and the housing can be fixedly determined. In the fifth aspect, the relative angle between the multiple coils of the stator core and the housing can be fixedly determined. In the sixth aspect, the relative angle between the multiple coils of the stator core and the housing can be fixedly determined. In the seventh aspect, the angle between the magnetic poles of the rotor core and the shaft can be fixedly determined. In the eighth aspect, the angle between the magnetic poles of the rotor core and the shaft can be fixedly determined. In the ninth aspect, the second structural section can be easily configured.

[0048] Although the embodiments of the present disclosure have been described in detail, the present disclosure is not limited to the individual embodiments described above. Various additions, substitutions, modifications, partial deletions, etc. are possible to these embodiments without departing from the gist of the invention or the concept and spirit of the present invention derived from the content of the claims and their equivalents. For example, in the above-described embodiments, the order of each operation and the order of each process are shown as examples and are not limited to these. The same applies when numerical values ​​or mathematical formulas are used in the description of the above-described embodiments. Furthermore, appropriate combinations of several of the above-described embodiments are within the scope of the present disclosure.

[0049] REFERENCE SIGNS LIST 1, 1a Actuator 5 Motor 10 Stator 11 Stator core 11a Flange 12 Housing 13 Coil 15 Parallel key 19 First structural part 20 Rotor 21 Rotor core 22 Magnet 23 Hollow shaft 23A, 23B Extension part 25 Boss 26 Parallel key 29 Second structural part 30 Reducer 31 Low speed tube 32 Output shaft 35 First screw 39 Third structural part 40 Encoder 41 Rotary disk 42 Detection part 43 Encoder fixing part 46 Slot 48 Fourth structural part 49 Board 51, 52 Bearings 61, 62 Fixing part C Wire body P Fixing reference part S Slit for Z phase TL Tie rod TB Tie bolt

Claims

1. A stator is provided. The stator includes a housing and a stator core fixed within the housing and wound with a plurality of coils; moreover, A rotor is disposed facing the stator, The rotor includes a rotor core that forms a plurality of magnetic poles, and a shaft that is coupled to the rotor core. moreover, An encoder unit is provided. The encoder unit includes a rotating disk connected to the shaft, a detection portion that detects rotation of the rotating disk, and a fixed portion that is fixed to the housing, moreover, a first structural portion that determines a relative angle between the plurality of coils of the stator core and the housing; A second structural portion that determines a relative angle between a magnetic pole of the rotor core and a fixed reference portion of the rotary disk provided on the shaft; a third structure that determines a relative angle between the shaft and the rotating disk; and a fourth structure that defines and adjusts the relative angle between the housing and the sensing portion.

2. 2. The actuator of claim 1, wherein the third structural portion includes a boss attached to the rotating disk or a plurality of first screws fixing the rotating disk to the shaft, and a plurality of screw holes formed in the shaft that engage with the plurality of first screws.

3. 3 . The actuator according to claim 1 , wherein the fourth structural portion includes a plurality of elongated holes formed in the fixed portion of the encoder unit, and a plurality of second screws respectively inserted into the plurality of elongated holes.

4. The actuator according to claim 1 or 2, wherein the first structural portion includes a plurality of keyways formed in the stator core and the housing, and a plurality of parallel keys engaging with the plurality of keyways, respectively.

5. 3. The actuator according to claim 1, wherein the first structural portion includes a plurality of key grooves formed in one of the housing and the stator core, and a plurality of protrusions provided in the other of the housing and the stator core and engaging with each of the plurality of key grooves.

6. The first structural portion is a flange provided on the stator core and adapted to engage with the housing; a plurality of through holes formed in the housing and the flange and communicating with each other; and a plurality of tie bolts threadably engaging the plurality of through holes.

7. The actuator according to claim 1 or 2, wherein the second structural portion includes a plurality of keyways formed in the rotor core and the shaft, and a plurality of parallel keys engaging with the plurality of keyways, respectively.

8. 3. The actuator according to claim 1, wherein the second structural portion includes a plurality of key grooves formed in one of the rotor core and the shaft, and a plurality of protrusions provided in the other of the rotor core and the shaft and adapted to engage with each of the plurality of key grooves.

9. The actuator according to claim 1 , wherein the rotor core and the shaft are integrally formed in the second structural portion.

10. The actuator according to claim 1 or 2, wherein the encoder is an optical encoder or a magnetic encoder.