Micro actuator

Abstract

Micro actuator, which includes: a splined shaft (1) which is designed as a cylinder with a hollow part and has an axially extending slot opening (12, 13); a wedge nut (2) which engages with the splined shaft (1) and guides the splined shaft in the axial direction; two end caps (5) which are attached to openings at both ends of the splined shaft (1); a magnetic rod (30) which is held at both ends in the hollow part of the splined shaft (1) by the end caps; and a punch (31) which is in loose engagement with a circumference of the magnet bar (30) in the hollow part of the splined shaft (1) to form a linear motor together with the magnet bar (30) and is connected to the wedge nut through the slot opening (12, 13).

Description

TECHNICAL FIELD

[0001] The present invention relates to an actuator with a built-in drive device in which a spline shaft and a keyway can move relative to each other in response to an input signal. More specifically, the present invention relates to a micro actuator that can be downsized to approximately 10 mm in diameter and can be used for various industrial devices. STATE OF THE ART

[0002] To date, a linear actuator that converts an electrical input signal into and outputs a linear motion of a moving part mainly includes a guide mechanism for linearly guiding the moving part while supporting an external force, and a drive mechanism for causing the moving part to advance and retract along the guide mechanism. Various known examples of the drive mechanism include a linear actuator that uses a hydraulic cylinder or an air cylinder as a drive source and transmits a linear reciprocating motion of a piston unchanged to the moving part, and an actuator that uses a motor as the drive source and converts a rotary motion of the motor into a linear motion by using a ball screw.Furthermore, a linear actuator is also known which uses a linear motor as a drive source, which has a stator and a moving element, and which transmits a linear reciprocating movement of the linear motor unchanged to the moving part.

[0003] Although there are various combinations of the drive mechanism and the guide mechanism according to the required axial thrust of the moving part, load-bearing capacity, moving speed, precision, etc., the simple combination of any drive mechanism and a guide mechanism results in a linear actuator with relatively large dimensions, resulting in relatively poor usability.

[0004] JP 2003-278873 A describes a linear actuator with a relatively compact structure in which the drive mechanism is incorporated into the guide mechanism. In this linear actuator, a ball screw consisting of a screw shaft and a ball nut is housed in a cylindrical housing having a hollow portion, and the screw shaft is rotatably supported in the hollow portion. A motor is attached to one end of the cylindrical housing, and this motor allows arbitrary rotation to be transmitted to the screw shaft. Furthermore, an axially extending keyway is provided in the outer peripheral surface of the housing; a cylindrical moving member is fittedly engaged with the outer surface of the housing via the keyway, and the cylindrical moving member can move freely in the axial direction of the housing while being locked from rotation relative to the housing.Furthermore, the housing has a slot-like opening that runs in the axial direction, and the ball nut of the ball screw is connected to the moving part via this opening.

[0005] In the actuator having the above-described structure, when the motor rotates to transmit arbitrary rotation to the screw shaft housed in the hollow portion of the housing, the ball nut moves within the hollow portion as the screw shaft rotates, and the cylindrical moving member on the outer side of the housing moves axially along with the ball nut. Therefore, it is possible to move the cylindrical moving member along the axis of the housing in an arbitrary direction and by an arbitrary amount according to the direction or amount of rotation of the motor.

[0006] Further relevant prior art is disclosed in the following documents: JP H09- 182 408 A and US 5 476 324 A.

[0007] Patent document 1: JP 2003- 278 873 A DESCRIPTION OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] However, accommodating the ball screw in the housing leads to the following problems: Since the ball nut, which is threadedly engaged with the screw shaft, is provided with an endless ball recirculation path, its diameter cannot be reduced drastically, and the diameter of the housing, which accommodates the ball nut in the hollow part, is inevitably large. Furthermore, the motor for driving the screw shaft is mounted at one end in the longitudinal direction of the housing via a support, so the difference between the stroke distance of the cylindrical moving part and the overall length of the linear actuator is inevitably relatively large. Even if the required stroke distance is small according to the intended purpose of the moving part, the overall length of the linear actuator cannot be significantly reduced.Furthermore, the spindle shaft must be supported at both its ends by bearings that must be fixed in one place while being housed in the hollow part of the housing, so that the process of assembling the spindle shaft with the housing requires considerable effort and a lot of time, and moreover, the number of components involved is large, which leads to relatively high costs. MEASURES TO RESOLVE THE PROBLEMS

[0009] To solve the above-described problem, a micro actuator having the features of claim 1 is provided. Further advantageous embodiments are defined in the dependent claims.

[0010] The present invention has been developed in view of the above-described problems, and therefore has an object to provide a micro actuator that can be made as compact as a whole, for example, with a diameter of 10 mm, by reducing a difference between a stroke distance of a moving part and a total length, and by reducing a sectional area extending in a direction orthogonal to a stroke direction.

[0011] Another advantage of the present invention is to provide a micro actuator that has a smaller number of components and can be easily assembled.

[0012] To achieve the above-described advantages, a micro actuator according to the present invention comprises: a spline shaft formed as a cylinder having a hollow portion and a slotted opening extending in the axial direction; a keyway fittingly engaged with the spline shaft and guiding the spline shaft in the axial direction; two end caps fixed to openings at both ends of the spline shaft; a magnet rod supported at both ends within the hollow portion of the spline shaft by the end caps; and a plunger loosely fittingly engaged with the periphery of the magnet rod within the hollow portion of the spline shaft, thereby forming a linear motor with the magnet rod and connected to the keyway via the slotted opening.

[0013] Both ends of the magnetic rod engage with the end caps attached to the end openings of the spline shaft, thereby holding the magnetic rod in the hollow part of the spline shaft. The punch includes a coil portion; when an electrical signal is applied to the coil portion, the punch receives an axial compressive load in the hollow part of the spline shaft while loosely engaging the magnetic rod. Therefore, the keyway, which is connected to the punch via a slotted opening of the spline shaft, is subjected to axial compression on the outside of the spline shaft.

[0014] In the micro actuator according to the present invention having the above-described construction, the end openings of the spline shaft are blocked by the two end caps, thereby supporting the magnetic rod in the hollow part of the spline shaft, and the linear motor as the drive mechanism is completely housed in the spline shaft as the guide mechanism. This results in making it possible to reduce the difference between the stroke length of the keyway and the overall length of the actuator. Furthermore, due to the use of a so-called rod-type linear motor in which the plunger is in loose fitting engagement with the periphery of the magnetic rod, a sufficient axial thrust load can be obtained even when the plunger is downsized, thereby making it possible to select a small diameter of the spline shaft that houses the plunger in its hollow part.As a result, according to the present invention, it is possible to obtain a linear actuator which is significantly reduced in size, i.e. a micro actuator.

[0015] Furthermore, as described above, in the micro actuator according to the present invention, the magnetic rod constituting the linear motor is fixed to the spline shaft through two end caps, and the assembling process is substantially completed only by connecting the punch to the spline groove, so that the number of components is relatively small and the assembly can be easily performed, which makes it possible to carry out the manufacturing at a lower cost. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a general perspective view of a micro actuator according to a first embodiment of the present invention. Fig. 2 is a vertically sectioned, longitudinal perspective view of the Fig. 1 shown micro actuator. Fig. 3 is a horizontally cutaway longitudinal perspective view of the Fig. 2 shown micro actuator. Fig. 4 is a perspective view of the Fig. 1, cut away in the direction perpendicular to the longitudinal direction at the center of the keyway. Fig. 5 is a perspective view of a micro actuator according to a second embodiment of the present invention. Fig. 6 is a perspective view seen from the back of the Fig. 5 shown micro actuator. Fig. 7 is a perspective view of a micro actuator according to a third embodiment of the present invention. DESCRIPTION OF REFERENCE SYMBOLS

[0016] 1 ... splined shaft, 2 ... splined nut, 3 ... linear motor, 5 ... end cap, 12, 13 ... slotted opening, 30 ... magnetic rod, 31 ... punch. BEST MODE FOR CARRYING OUT THE INVENTION

[0017] Hereinafter, the micro actuator according to the present invention will be described in detail with reference to the accompanying drawings.

[0018] The Fig. 1 to 4 show a micro actuator according to the first embodiment of the present invention. Fig. 1 is a perspective view, Fig. 2 is a longitudinal perspective view cut in the vertical direction, Fig. 3 is a longitudinal perspective view cut in the horizontal direction, and Fig. 4 is a perspective view cut perpendicular to the longitudinal direction at the center of a spline nut, which will be explained in more detail below. This micro actuator is provided with a spline shaft 1 having a hollow part 10 and a substantially cylindrical shape, and a spline nut 2 that is fittedly engaged with the outer side of the spline shaft 1 via a large number of balls. It can reciprocate the spline nut 2 in the axial direction of the spline shaft 1 using the axial thrust generated by a linear motor 3 housed in the hollow part 10 of the spline shaft 1.

[0019] The spline shaft 1 includes the hollow part 10 and is cylindrical in shape, with the outer peripheral surface having two ball rolling grooves 11 that are offset by 180° from each other. Furthermore, the spline shaft 1 has two axially extending slotted openings 12, 13 that are offset by 90° with respect to the ball rolling grooves 11. The slotted openings 12, 13 are arranged facing each other with the hollow part 10 therebetween, providing communication between the inside and outside of the spline shaft 1. In an actual prototype, the outer diameter of the spline shaft 1 is approximately 7 mm, and its inner diameter is approximately 5 mm.

[0020] The spline nut is essentially formed as a cylinder whose inner diameter is slightly larger than that of the spline shaft 1; it is in fitting engagement with the outer surface of the spline shaft 1 via a large number of balls 4 rolling in the ball rolling grooves 11 of the spline shaft 1. The spline nut 2 comprises a nut main body 20 and two end plates 21 fixed to both axial end surfaces of the nut main body 20.

[0021] In the inner peripheral surface of the nut main body 20, load rolling grooves 22 are provided opposite the ball rolling grooves 11 of the spline shaft 1, and the balls 4 roll between the ball rolling grooves 11 and the ball rolling grooves 22 while absorbing the load. Furthermore, the nut main body 20 has ball return paths 23 parallel to the load rolling grooves 22, and the ball return paths 23 are arranged to overlap the load rolling grooves 22 in the radial direction. The end plates 21 have direction change paths 24 that allow the balls to arrive and depart between the load rolling grooves 22 and the ball return paths 23 of the nut main body 21. When the wedge nut 2 is assembled, when the two end plates 21 are fixed to the end surfaces in the axial direction of the nut main body 20, the wedge nut 2 is provided with endless circulation paths 25 for the balls 4. Fig. 2, the balls 4 are arranged only in one of the two endless circulation paths 25, which are formed to be arranged at positions facing each other with the spline shaft 1 therebetween, but in fact, the balls 4 are actually formed to occupy the entire circumference of the endless circulation path, in which no balls 4 are shown.

[0022] Therefore, when the wedge nut 2 is moved along the spline shaft 1, the balls 4 circulate through the endless circulation paths 25 while absorbing the load between the nut main body 20 and the spline shaft 1, thereby allowing the wedge nut 2 to move continuously along the spline shaft 1. The wedge nut 2 is provided with a screw hole at a location on the back, as shown in Fig. 1, and this screw hole is screw-engaged with a fixing screw, thereby enabling the spline nut 2 to be fixed to another machine, device, or the like. Therefore, in this micro actuator, the spline nut 2 is fixed in place, and the spline shaft 1, supported by the spline nut 2, is caused to perform an advancing movement and a returning movement in the axial direction. In an actual prototype, steel balls with a diameter of 0.4 mm are used as the balls 4.

[0023] Within the hollow part 10 of the spline shaft 1, a magnetic rod 30 is housed, which serves as the stator of the linear motor 3. In the magnetic rod 30, the N-poles and S-poles of permanent magnets are arranged alternately in the axial direction. It can be constructed by inserting a large number of permanent magnets into a steel pipe, or by forming a round rod, and then generating magnetic poles by magnetization.

[0024] Two end caps 5 are fitted with holes at both ends in the axial direction of the spline shaft 1, thereby axially closing the hollow portion 10 of the spline shaft 1. Each of these end caps 5 has a retaining hole 50 at its center for fitting engagement with the end portion of the magnet rod 30; by attaching the two end caps 5 to the spline shaft 1, the magnet rod 30 is held in the hollow portion 10 of the spline shaft 1 like a central crank, and its position is adjusted parallel to the axial direction of the spline shaft 1, that is, in the direction of movement of the spline nut 2. In the drawings, reference numeral 51 denotes fastening screws for attaching the end caps to the spline shaft 1.

[0025] Furthermore, a punch 31 constituting the linear motor 3 is loosely fitted to the periphery of the magnetic rod 30 with a small gap therebetween. This punch is made of aluminum, which has superior thermal conductivity, and has a hollow portion through which the magnetic rod passes. The inner peripheral surface of the hollow portion accommodates excitation coils (not shown) as a stator. The excitation coils comprise a coil group in which three coils of a U-phase, a V-phase, and a W-phase form a group. The excitation coils of all phases have an annular configuration and face the outer peripheral surface of the magnetic rod 3 across the small gap. The arrangement pitch of the excitation coils of the different phases is set smaller than the arrangement pitch of the permanent magnets of the magnetic rod.A magnetic flux extending from the S pole to the N pole exists in the magnetic rod 30, and the coil portion includes a magnetic pole sensor (not shown) for detecting the magnetic flux density. Therefore, the positional relationship of the magnetic poles (the N and S poles) of the magnetic rod with respect to the excitation coil can be determined from a detection output signal from this magnetic pole sensor. A controller that controls the power supply to the excitation coils receives the detection signal from the magnetic pole sensor and calculates an optimal electric current according to the positional relationship between the excitation coils and the magnetic poles of the magnetic rod, which current is supplied to the excitation coils.This results in an attractive force and a repulsive force being generated between the excitation coils and the magnetic poles of the permanent magnets due to the synergistic effect of the electric current flowing through the excitation coils and the magnetic flux generated by the permanent magnets, causing the punch 31 to be advanced in the axial direction of the magnetic rod 30.

[0026] As in Fig. 3, a portion of the punch 31 is inserted into a slotted opening 12 of the spline shaft 1 and is connected to the spline nut 2 by fastening screws 32. This results in the fact that when the punch 31 is advanced in the axial direction of the magnetic rod 30, the spline nut 2 is advanced in the axial direction of the spline shaft 1.

[0027] Furthermore, as in the Fig. 1 and Fig. 3, a linear scale 33 is arranged parallel to the axial direction of the spline shaft 1 in the other opening 13 of the spline shaft 1. The linear scale 33 is formed as a magnetic scale in which N and S magnetic poles are alternately arranged in a metal wire with a circular cross-section. This linear scale is fixed to a location within the slotted opening 13 using a foundation 34.

[0028] Furthermore, as in Fig. 4, a read head 35 for detecting the magnetic poles of the linear scale 33 is inserted into the slotted hole 13 from the outside of the spline shaft 1, and the read head 35 is fixed to the spline nut 2 via a support 36. As a result, when the spline nut 2 moves along the spline shaft 1, the read head 35 outputs pulse signals corresponding to changes in the magnetic poles as it moves along the linear scale 33, and it is possible to detect the amount of movement of the spline nut 2 with respect to the spline shaft 1 by monitoring the pulse signals. It is not essential that the linear scale 33 be a magnetic scale; it may also be provided with, for example, a ladder-like pattern detected by an optical method.

[0029] In this micro actuator having the above-described construction, the linear motor 3 as the drive mechanism can be completely incorporated into the hollow part of the spline shaft 1, thereby making it possible to obtain a highly compact actuator while unifying the drive mechanism and the guide mechanism. Specifically, with respect to the entire axial length of the spline shaft 1, the magnet rod 30 constituting the linear motor 3 is supported at both ends by the two end caps 5, so that there is no large difference between the stroke distance of the spline nut 2 and the overall length of the spline shaft 1, thereby making it possible to significantly reduce the overall axial length of the actuator. Furthermore, it has been found that the number of components is small, and assembly can be carried out very easily, thus enabling low-cost manufacturing.

[0030] In an actual prototype of the actuator, the total length is 35 mm, the maximum diameter of the wedge nut 2 including the support 36 is 12.2 mm, and the stroke distance of the wedge nut 2 is 17 mm. The maximum movement speed of the wedge nut 2 is 762 (mm / s).

[0031] While in the Fig. 1 to 4, two slotted holes 12, 13 are provided in the spline shaft 1, it is also possible to provide a single slotted hole in the spline shaft 1 by appropriately modifying the position of the arrangement of the linear scale 33, thereby making it possible to achieve a further reduction in manufacturing costs.

[0032] The Fig. 5 and Fig. 6 show a micro actuator according to the second embodiment of the present invention, in which, as described above, only one slot opening 14 is provided for the spline shaft 1. Fig. 5 is a perspective view, and Fig. 6 is a perspective view when viewed from the opposite side.

[0033] In Fig. 5, the slotted hole 14 extending in the axial direction is provided at a position in the outer peripheral surface of the spline shaft 1, and the spline nut 2 is connected to the plunger 31 constituting the linear motor 3 via the slotted hole 14.

[0034] Furthermore, as in Fig. 6, a flat surface 60 is provided on the outer peripheral surface of the spline shaft at a position 180° out of phase with the slot opening, and a band-shaped linear scale 61 is attached to the flat surface 60. As in the first embodiment, the linear scale 61 is formed as a magnetic scale. A read head (not shown) is attached to the spline nut 2 at a position opposite to the linear scale 61; when the spline nut 2 moves along the spline shaft 1, the read head outputs pulse signals corresponding to changes in the magnetic poles as it moves along the linear scale 6.

[0035] Otherwise, this embodiment is designed with the same construction as the first embodiment, so that in the Fig. 5 and Fig. 6, those components which are designed in the same way as in the first embodiment are designated by the same reference numerals, and their description is omitted.

[0036] In the micro actuator according to the second embodiment having the above-described structure, only one slotted hole 14 is provided for the spline shaft 1. This improves the rigidity of the spline shaft 1 compared to the first embodiment, making it possible to increase the accuracy with respect to the movement of the spline nut 2; moreover, machining of the spline shaft 1 is facilitated, making it possible to manufacture it at a lower cost. Furthermore, since the linear scale 61 is formed as a band-like member attached to the flat surface 60 provided on the spline shaft 1, the linear scale can be arranged more easily compared to the wire-like linear scale 33 according to the first embodiment, thereby making it possible to increase the measurement accuracy with respect to the movement of the spline shaft 1.

[0037] Next, Fig. 7 a micro actuator according to the third embodiment of the present invention.

[0038] In the micro actuator according to the third embodiment, in order to increase the axial thrust load exerted by the linear motor 3, a plurality of groups of excitation coils, each of which comprises three excitation coils of the three phases, are mounted on a punch 37 that is in loosely fitted engagement with the magnetic rod 30. Therefore, the punch 37 is longer in the axial direction of the magnetic rod 30 than the punch 31 according to the second embodiment. Furthermore, to reliably support the elongated punch 37, two spline nuts 2 are provided in fitting engagement with the spline shaft 1 at a distance in the axial direction, each of the spline nuts 2 being connected to one of the longitudinal ends of the punch 37.The two wedge nuts 2 are therefore connected to each other via the plunger 37; when the linear motor 3 is in operation, the spline shaft 1, which holds the magnetic rod 30, moves relative to the wedge nuts 2, whereby the distance between the wedge nuts 2 remains the same.

[0039] Otherwise, this embodiment is designed in the same way as the second embodiment, so that in Fig. 7 such components, which are the same as in the second embodiment, are designated by the same reference numerals, and their description is omitted.

[0040] In the micro actuator according to the third embodiment having the above-described construction, the number of coil groups arranged on the plunger is arbitrarily selected. This results in the ability to arbitrarily adjust the axial thrust for the reciprocating movement of the spline shaft relative to the spline nuts, thereby allowing the actuator to be used in a variety of applications. Furthermore, it is only necessary to select the axial length of the spline shaft and plunger depending on the required axial thrust load and the reciprocating distance of the spline shaft, and it is not necessary to change the basic structure of the spline nut, so the actuator can be manufactured inexpensively and used in a variety of applications.Therefore, whether the required stroke distance is 50 cm or 1 m, the micro actuator according to the present invention provides the advantages of reducing the number of components, facilitating assembly, and reducing manufacturing costs.

Claims

[1] Micro actuator, which provides: a splined shaft (1) formed as a cylinder with a hollow portion and having a slotted opening (12, 13) extending in the axial direction; a wedge nut (2) which is in fitting engagement with the splined shaft (1) and guides the splined shaft in the axial direction; two end caps (5) attached to openings at both ends of the splined shaft (1); a magnetic rod (30) which is held at both ends in the hollow part of the splined shaft (1) by the end caps; and a plunger (31) which is in loose fitting engagement with a periphery of the magnetic rod (30) in the hollow part of the spline shaft (1) to form a linear motor together with the magnetic rod (30), and is connected to the spline nut through the slotted opening (12, 13). [2] Micro actuator according to claim 1, in which a linear scale (33) is attached to the slot opening (12, 13) so as to extend in the longitudinal direction thereof, and a reading head (35) which is inserted into the slotted opening (12, 13) and reads the linear scale (33) is attached to the wedge nut (2). [3] Micro actuator according to claim 2, in which two slotted openings (12, 13) are provided at locations opposite to each other, the hollow part of the splined shaft (1) being arranged therebetween, and a slotted opening (12) connects the punch (31) and the wedge nut (2) together, and another slotted opening arranges the linear scale (33). [4] Micro actuator according to claim 1, in which a ball rolling groove (11) is provided in an outer peripheral surface of the splined shaft (1), and the nut part (20) is assembled with the splined shaft (1) via a large number of balls (4) which roll in the ball rolling groove (11). [5] Microactuator according to claim 1, in which an axially extending flat surface (60) to which a linear scale (61) is attached is provided on a part of an outer peripheral surface of the splined shaft (1), and a head for reading the linear scale is attached to the wedge nut. [6] A micro actuator according to claim 1, wherein two wedge nuts (2) are in fitting engagement with the spline shaft at a distance in the axial direction, the wedge nuts (2) being connected to both ends in the longitudinal direction of the plunger.

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