ELECTRIC ACTUATOR

Abstract: An electric actuator comprising an actuator body (12), a drive unit (16), a feed screw shaft (32) for transmission of a driving torque of a rotary drive source (14), a displacement member (40) fitted on the feed screw shaft (32) to surround the same and adapted to displace axially under a threading action of a male screw portion (36) and a female screw portion (42), sensor mount grooves (24a, 24b) provided on a side of the actuator body (12), and a sensor (25) mounted at a predetermined location in the sensor mount grooves (24a, 24b).

Description: TECHNICAL FIELD The present invention relates to an electric actuator according to the preamble of claim 1. BACKGROUND ART The electric actuator has been hitherto used, for example, as a means for transporting a workpiece. As shown in FIGS. 9 and 10, such an electric actuator comprises a lengthy frame 1 formed with an opening to function as a main actuator body, and a block-shaped electric motor 2 secured to one end of the frame 1. The electric actuator includes a ball screw 3 and a motor shaft of the electric motor 2 which are coaxially coupled to one another by using an unillustrated coupling member. A movable member 4 is fixed to an unillustrated nut member which is engaged with the ball screw 3. Long grooves 5a, 5b, which are provided for attaching sensors, are formed on a side surface of the frame 1 along the longitudinal direction of the frame 1. The sensor 6 is installed to a predetermined portion of the long groove 5a, 5b. A lead wire 7, which is electrically connected to the sensor 6, is led into the long groove 5a, 5b. One end of the lead wire 7 is connected to an external equipment such as a controller. A detection signal, which is detected by the sensor 6, is inputted into the external equipment. A part of the opening of the frame 1 is closed with a cover plate 8. Slits 9a, 9b are formed at the remaining portions which are not closed with the cover plate 8 (see FIG. 10). The slits 9a, 9b communicate with the outside, and they are formed along the axial direction of the frame 1. In this arrangement, the nut member, which is engaged with the ball screw 3, is linearly displaced along the slits 9a, 9b in accordance with the driving action of the electric motor 2. Thus, the movable member 4, which is fixed to the nut member, is displaced in an integrated manner. However, in the case of the electric actuator concerning the conventional technique described above, the slits 9a, 9b, which serve as the space for moving the nut member, are formed at the opening of the frame 1. Therefore, any dust or the like enters the inside of the frame 1 from the outside via the slits 9a, 9b, and it adheres to the ball screw 3. As a result, an inconvenience arises in that the driving force-transmitting function of the ball screw 3 is lowered, and it becomes impossible to allow the movable member 4 to make smooth reciprocating motion. Further, for example, if any maintenance work is performed, the following inconvenience arises. That is, an operator contacts with the lead wire 7 exposed from the long groove 5a, 5b to the outside, and the lead wire is hitched thereby. As a result, the position of the sensor 6 having been installed at the predetermined portion of the long groove 5a, 5b is deviated. U.S. patent 3 693 759 discloses an actuator according to the preamble of claim 1. In particular, this electromechanical thrust actuator includes a cylinder having a cavity with a longitudinal axis, a piston received in the cavity for displacement relative to the cylinder along the axis and an electromechanical drive means disposed in the cavity connecting to the cylinder and the piston for axially displacing the piston relative to the cylinder. While the cavity housing the drive means is completely closed so that no dust can enter, there are no sensors provided for detecting the position of the piston. Similar actuators are disclosed in the documents EP-A-0 726 118 and US-A-4 307 799. An object of the present invention is to provide an electric actuator which makes it possible to prevent the inside of the main actuator body from invasion of dust or the like so that any decrease in the driving force- transmitting function is avoided, and which makes it possible to avoid any positional deviation of the sensor, which would be otherwise caused by the stretching action effected on the lead wire. DISCLOSURE OF THE INVENTION According to the present invention, there is provided an electric actuator comprising the features of claim 1. Advantageous embodiments of the invention are evident from the dependent claims. The rotary driving force-transmitting member, which is operated in accordance with the driving action of a rotary driving source, is doubly surrounded by a displacement member and a main actuator body. Therefore, it is possible to avoid adhesion of dust or the like to the rotary driving force-transmitting member. Further, according to the present invention, a sensor is attached to a predetermined portion in a long groove provided on a side surface of the main actuator body. The sensor detects the magnetic field of a permanent magnet which makes displacement integrally with the displacement member. Thus, it is possible to detect the amount of displacement of the displacement member. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a front view illustrating an electric actuator according to an embodiment of the present invention; FIG. 2 shows a longitudinal sectional view taken along a line II-II shown in FIG. 1; FIG. 3 shows a sectional view taken along a line III-III shown in FIG. 2; FIG. 4 shows a longitudinal sectional view illustrating a driving unit for constructing the electric actuator shown in FIG. 1; FIG. 5 shows a perspective view illustrating a cover member for constructing the electric actuator shown in FIG. 1; FIG. 6 shows a sectional view illustrating an electric actuator according to another embodiment of the present invention; FIG. 7 illustrates a schematic arrangement depicting a state in which the electric actuator shown in FIG. 6 is assembled to another member; FIG. 8 illustrates a schematic arrangement depicting a state in which the electric actuator shown in FIG. 6 is assembled to still another member; FIG. 9 shows an exploded perspective view illustrating an electric actuator concerning the conventional technique; and FIG. 10 shows a sectional view taken along a line X-X shown in FIG. 9. BEST MODE FOR CARRYING OUT THE INVENTION With reference to FIGS. 1 and 2, reference numeral 10 indicates an electric actuator according to an embodiment of the present invention. The electric actuator 10 comprises a main actuator body 12 having a hollow rectangular parallelepiped-shaped configuration and including a through-hole formed along the axial direction, and a driving unit 16 formed to be substantially flush with the side surface of the main actuator body 12 and arranged with a rotary driving source 14 (see FIG. 4). The main actuator body 12 comprises a first body 18 and a second body 20 each of which has a hollow rectangular parallelepiped-shaped configuration and each of which is formed to have its flush side surface, and a head cover 22 which is coupled to one end of the first body 18. A pair of sensor attachment long grooves 24a, 24b each having a circular arc-shaped configuration are formed substantially in parallel to one another on each of two opposing side surfaces of the first body 18, the second body 20, and the head cover 22 respectively (see FIG. 3). Columnar sensors 25 are installed at predetermined portions of the sensor attachment long grooves 24a, 24b (see FIG. 3). The position of the piston can be detected by detecting the magnetic field of a permanent magnet which is installed to the piston to be moved therewith in an integrated manner as described later on. As shown in FIG. 2, a feed screw shaft (rotary driving force-transmitting member) 32, which is coaxially coupled to a driving shaft 30 of the rotary driving source 14 by the aid of a coupling member 26 and a lock nut 28, is provided at the inside of the second body 20 coupled to the driving unit 16. The feed screw shaft 32 is rotatably supported by a first bearing member 34 provided with a ball bearing 33. A male thread 36 is threaded over a predetermined portion on the outer circumferential surface of the feed screw shaft 32. A displacement member 40 having a piston 38 is externally fitted to the feed screw shaft 32. A female thread 42, which is formed on the inner circumferential surface of the displacement member 40, is engaged with the male thread 36 formed on the outer circumferential surface of the feed screw shaft 32. Therefore, the displacement member 40 is displaced in the direction indicated by the arrow X 1 or X 2 in accordance with the engaging action of the feed screw shaft 32. It is advantageous that the production can be performed inexpensively when the first body 18 and the piston 38 are formed by means of aluminum extrusion forming or resin molding. The piston 38, which makes sliding displacement along a hole 44 of the first body 18, is provided at one end of the cylindrical displacement member 40. A permanent magnet 46 is fitted to the piston 38 by the aid of an annular groove. A spline 48 is formed on the outer circumferential surface of the piston 38. A spline groove 50 corresponding to the spline 48 is formed on the inner wall surface of the hole 44 of the first body 18 on which the piston 38 makes the sliding movement (see FIG. 3). The rotation-preventive function is effected for the displacement member 40 by the displacement of the piston 38 along the hole 44 in accordance with the guiding action of the spline groove 50. The sensor 25, which is installed to the sensor attachment long groove 24a, 24b, detects the magnetic field of the permanent magnet 46 which is displaced integrally with the piston 38. Thus, it is possible to detect the position of the piston 38. A pair of damper members 52a, 52b, which regulate the displacement terminal position and mitigate the shock upon abutment against the piston 38, are arranged at both ends of the first body 18 respectively. A spacer 54 is interposed between the first bearing member 34 and the damper member 52b. The head cover 22 is provided with a rod packing 56 for surrounding the displacement member 40 to prevent the inside of the hole 44 of the first body 18 from invasion of dust or the like, and a bush 58 for making the guiding action when the displacement member 40 is displaced in the direction of the arrow X 1 or X 2 . As shown in FIG. 5, an oblong cover member 60 is detachably attached to a side surface portion of the second body 20 by the aid of a screw 62. The cover member 60 is provided with a pair of lead wire outlets 66a, 66b which communicate with the sensor attachment long grooves 24a, 24b and which are provided for externally leading the lead wires 64 coming from the sensors 25. That is, the cover member 60 is formed with a pair of first holes 68a, 68b which extend coaxially with the sensor attachment long grooves 24a, 24b. The cover member 60 is further formed with a pair of second holes 70a, 70b which are bent externally by a predetermined angle from the first holes 68a, 68b. Fastening tabs 71a, 71b each having an acute angular configuration for fastening the lead wire 64 are formed at the outlets 66a, 66b. In this embodiment, the sensor 25 is installed at the predetermined portion of the sensor attachment long groove 24a, 24b of the main actuator body 12. The lead wire 64, which is led internally along the sensor attachment long groove 24a, 24b, is bent along the first hole 68a, 68b and the second hole 70a, 70b. The lead wire 64 is fastened at its predetermined portion by using the fastening tab 71a, 71b provided at the lead wire outlet 66a, 66b. Thus, the lead wire 64 is held by the cover member 60. As shown in FIG. 4, the driving unit 16 comprises the rotary driving source 14 and a brake mechanism 72, and it is accommodated in a housing 73. The driving shaft 30 of the rotary driving source 14 is rotatably supported by a second bearing member 74 and a third bearing member 76 which are separated from each other by a predetermined spacing distance. The terminal end of the driving shaft 30 extends up to the brake mechanism 72. The rotary driving source 14 includes a rotor 78 which is fixed to the driving shaft 30 to make rotation integrally with the driving shaft 30, and a stator 80 which is fixed to the housing 73. The brake mechanism 72 includes a casing 84 which is arranged with an electromagnetic coil 82, and a braking pad 86 which is provided at the outside of the casing 84 and which is rotated integrally with the driving shaft 30. A pair of interposing plates 88a, 88b are provided and aligned with the braking pad 86 intervening therebetween. The first interposing plate 88a is provided to be pressed toward the second interposing plate 88b by the aid of the resilient force of a spring member 90 fastened to a recess of the casing 84. Therefore, the braking pad 86 is interposed and held at its both surfaces by using the pair of interposing plate 88a, 88b in accordance with the action of the resilient force of the spring member 90. Thus, the braking action is effected to stop the rotation of the driving shaft 30. A gap 89 having a predetermined clearance is formed between the first interposing plate 88a and the casing 84 which surrounds the electromagnetic coil 82. The first interposing plate 88a, on which the pressing force is exerted by the spring member 90, is provided displaceably along the axial direction of the driving shaft 30. When the electric power is applied to the electromagnetic coil 82 to give the ON state starting from the state shown in FIG. 4, the first interposing plate 88a is attracted toward the electromagnetic coil 82 while overcoming the resilient force of the spring member 90. As a result, the braking action is deenergized, and the driving shaft 30 is rotated in the predetermined direction. The electric actuator 10 according to the embodiment of the present invention is basically constructed as described above. Next, its operation, function, and effect will be explained. At first, the pair of sensors 25 are installed to the predetermined portions in the sensor attachment long grooves 24a, 24b. Subsequently, the lead wires 64, which are connected to the sensors 25 are led internally along the sensor attachment long grooves 24a, 24b, and they are installed along the first holes 68a, 68b and the second holes 70a, 70b of the cover member 60. The lead wires 64 are led externally from the lead wire outlets 66a, 66b. In this arrangement, the lead wires 64 are bent by the second holes 70a, 70b which are inclined at the predetermined angle in the cover member 60. The lead wires 64 are fastened by the fastening tabs 71a, 71b provided at the lead wire outlets 66a, 66b. Therefore, as described later on, even when an operator erroneously contacts with the lead wire 64, the lead wire 64 is hitched thereby, then the sensor 25 can be prevented from any deviation of position which would be otherwise caused by the stretching action effected on the lead wire 64, because the lead wire 64 is fastened by the cover member 60. Subsequently, when the unillustrated power source is energized, the rotary driving source 14 makes rotational driving. The rotary driving force of the rotary driving source 14 is transmitted via the coupling member 26 to the feed screw 32 which is coaxially coupled thereto. The displacement member 40 is displaced in the direction indicated by the arrow X 1 or X 2 in accordance with the engaging action between the male thread 36 of the feed screw shaft 32 and the female thread 42 formed on the inner circumferential surface of the displacement member 40. During this process, the spline 48, which is formed on the outer circumferential surface of the piston 38, effects the rotation-preventive function for the displacement member 40. Further, the guiding action is exhibited by the bush 58 provided for the head cover 22 and the first bearing member 34 provided for the second body 20 respectively. The piston 38 abuts against one of the damper members 52a (52b) during the displacement process in which the displacement member 40 is displaced in the direction of the arrow X 1 (X 2 ) in accordance with the engaging action of the feed screw shaft 32. Thus, the displacement terminal position is restricted, and the shock upon the abutment is absorbed. On the other hand, the displacement member 40 is displaced in the direction of the arrow X 2 (X 1 ), and it is restored to the original position, by rotating the rotary driving source 14 in the direction opposite to the above by using an unillustrated switching means. In the embodiment of the present invention, the sensors 25 are conveniently installed to the predetermined portions of the sensor attachment long grooves 24a, 24b formed on the side surface of the main actuator body 12. Accordingly, it is possible to detect the displacement amount of the displacement member 40. In the embodiment of the present invention, the feed screw shaft 32, which is coaxially coupled to the driving shaft 30 of the rotary driving source 14 by the aid of the coupling member 26, is doubly surrounded by the main actuator body 12 and the displacement member 40 externally fitted to the feed screw shaft 32. Accordingly, it is possible to avoid the feed screw shaft 32 from adhesion of dust or the like. Therefore, it is possible to improve the durability by avoiding the increase in sliding resistance of the feed screw shaft 32, which would be otherwise caused by adhesion of dust or the like to the feed screw shaft 32. Further, the rotary driving force of the rotary driving source 14 can be smoothly transmitted to the displacement member 40. Further, in the embodiment of the present invention, the lead wires 64 are inserted along the first holes 68a, 68b and the second holes 70a, 70b provided on the cover member 60, and the parts of the lead wires 64 are fastened by the cover member 60. Accordingly, for example, if an operator contacts the lead wire 64 exposed to the outside from the sensor attachment long groove 24a, 24b, and the lead wire 64 is hitched thereby upon the execution of maintenance operation, then it is possible to avoid the deviation of the position of the sensor 25, which would be otherwise caused by the stretching action effected on the lead wire 64, because the part of the lead wire 64 is fastened by the cover member 60. The embodiment of the present invention adopts the arrangement in which the lead wire 64 is led to the outside via the lead wire outlet 66a, 66b provided for the cover member 60, which is advantageous in that the lead wire 64 is prevented from any contact with the driving unit 16, and it is possible to avoid any damage of the lead wire 64 which would be otherwise caused by the release of heat generated during the driving operation of the driving unit 16. Next, an electric actuator 10a according to another embodiment of the present invention is shown in FIG. 6. The electric actuator 10a differs in the following points. That is, the electric actuator 10a does not include the rotation-preventive means such as the spline 48 and the spline groove 50 as shown in FIG. 3. Cross-sectional configurations of the outer circumferential surfaces of the piston 38 and the hole 44 of the first body 18 are formed to be circular respectively. Another member (as described later on), which is externally provided in an additional manner, is used to exhibit the rotation-preventive function for the displacement member 40. It is noted that the same constitutive components as those of the electric actuator 10 shown in FIG. 1 are designated by the same reference numerals, detailed explanation of which will be omitted. A specified example, in which the electric actuator 10a according to the another embodiment is used, is shown, for example, in FIG. 7. A pair of support pillars 94a, 94b are fixed on a base plate 92. A pin member 96, which is supported between the pair of support pillars 94a, 94b, is used as the center of rotation to support the electric actuator 10a rotatably in the directions indicated by the arrows. A coupling member 98, which is connected to one end of the displacement member 40, is used to support a flat plate 100. In this embodiment, the displacement member 40 makes forward and backward movement in the directions indicated by the arrows in accordance with the driving action of the electric actuator 10a. Accordingly, the plate 100 can be displaced rotationally by a predetermined angle by using the point A as the center of rotation. Alternatively, as shown in FIG. 8, a support block 102 is used to hold the electric actuator 10a substantially in parallel to a base plate 104. A flange 108 is fastened by screws to a linear guide 106 fixed on the base plate 104. The flange 108 is coupled to one end of the displacement member 40. As a result, the displacement member 40 and the flange 108 are displaced in an integrated manner in accordance with the guiding action of a linear guide rail 110. As described above, in the case of the electric actuator 10a according to the another embodiment, the displacement member 40 is prevented from rotation by means of the another additional member such as the guide member. Accordingly, it is unnecessary to provide any rotation-preventive means for the displacement member 40. Therefore, the arrangement described above is advantageous in that the processing operation is simplified, and it is possible to reduce the production cost. Either sharp edge or burr, which is produced during the production steps, is attached to the outer circumferential surface of the feed screw shaft 32 in some cases. It is feared that the sharp edge and the burr may cause any damage on the engaging portion between the feed screw shaft 32 and the displacement member 40, and the smooth rotational operation may be disturbed. In such a case, the feed screw shaft 32 is processed, and then the thread portion of the feed screw shaft 32 is subjected to, for example, chemical polishing, electropolishing, or shot peening. Accordingly, the sharp edge, the burr, the processing scratch and the like can be removed. Further, the surface treatment such as the Tufftride treatment and the electroless plating treatment may be applied to the outer surface of the feed screw shaft 32. Such a treatment is advantageous in that the sliding performance can be further improved. INDUSTRIAL APPLICABILITY According to the present invention, the rotary driving force-transmitting member is doubly surrounded by the displacement member and the main actuator body. Accordingly, it is possible to prevent the rotary driving force-transmitting member from any adhesion of dust or the like. Therefore, it is possible to improve the durability by avoiding the increase in sliding resistance of the rotary driving force-transmitting member, which would be otherwise caused by the adhesion of the dust or the like to the rotary driving force-transmitting member. Further, it is possible to smoothly transmit the rotary driving force of the rotary driving source to the displacement member. Further, the lead wire is fastened by the cover member. Therefore, it is possible to avoid the deviation of the position of the sensor, which would be otherwise caused by the stretching action effected on the lead wire.

Claims: An electric actuator comprising: a cylindrical main actuator body (12); a rotary driving source (14) which is arranged in a driving unit (16) disposed beside said main actuator body (12); a rotary driving force-transmitting member (32) which is coaxially coupled to a driving shaft (30) of said rotary driving source (14), for transmitting rotary driving force of said rotary driving source (14); and a displacement member (40) which surrounds said rotary driving force-transmitting member (32) and which is displaceable along an axial direction of said main actuator body (12) in accordance with an engaging action with said rotary driving force-transmitting member (32), wherein said rotary driving force-transmitting member is composed of a feed screw shaft (32), and said displacement member (40) is displaceable along said axial direction in accordance with an engaging action effected by a male thread (36) formed on an outer circumference of said feed screw shaft (32) and a female thread (42) formed on an inner circumferential surface of said displacement member (40), characterised in that a long groove (24a, 24b) is formed at a side surface portion of said main actuator body (12), and a sensor (25), which is used to detect a magnetic field of a permanent magnet (46) displaceable integrally with said displacement member (40), is provided at a predetermined portion of said long groove (24a, 24b), and that a fastening member (60), which is used to fasten a lead wire (64) of said sensor (25) arranged along said long groove (24a, 24b), is provided at a side surface portion of said main actuator body (12). The electric actuator according to claim 1, wherein said driving unit (16) is provided with a brake mechanism (72) for braking rotational force of said driving shaft (30) of said rotary driving source (14). The electric actuator according to claim 1, wherein said fastening member is composed of a cover member (60), said cover member (60) is formed with a first hole (68a, 68b) communicating with said long groove (24a, 24b), a second hole (70a, 70b) inclined by a predetermined angle from said first hole (68a, 68b) to the outside, and a lead wire outlet (66a, 66b) communicating with said second hole (70a, 70b), and said lead wire outlet (66a, 66b) is provided with a sharp fastening tab (71a, 71b) for fastening said lead wire (64). The electric actuator according to claim 1, wherein a piston (38), which is displaceable integrally with said displacement member (40) along a hole (44) of said main actuator body (12), is provided at one end of said displacement member (40), and a spline (48), which effects a rotation-preventive function for said displacement member (40), is formed on said piston (38). The electric actuator according to claim 4, wherein a pair of damper members (52a, 52b), which are used to restrict an amount of displacement of said displacement member (40) upon abutment of said piston (38), are arranged in said hole (44) of said main actuator body (12). The electric actuator according to claim 2, wherein said brake mechanism (72) includes a braking pad (86) for making rotation integrally with said driving shaft (30) of said rotary driving source (14), a pair of interposing plates (88a, 88b) for interposing said braking pad (86) in accordance with an action of resilient force of a spring member (90), and an electromagnetic coil (82) for deenergizing a braking action by attracting one of said interposing plates (88a (88b)) against said resilient force of said spring member (90) in accordance with application of electric power. The electric actuator according to claim 1, wherein a seal member (56), which is used to surround an outer circumferential surface of said displacement member (40) so that a hole (44) of said main actuator body (12) is prevented from invasion of dust or the like, is provided at one end of said main actuator body (12). The electric actuator according to claim 7, wherein said seal member is composed of a rod packing (56) installed to a head cover (22).

Inventors: NAGAI SHIGEKAZU, SAKURAI SHUUZOU, NAGAI, SHIGEKAZU, SAKURAI, SHUUZOU, NAGAI, SHIGEKAZU, SMC K.K., TSUKUBA GIJUTSU CTR, SAKURAI, SHUUZOU, SMC K.K., TSUKUBA GIJUTSU CTR
Applicants: SMC KK, SMC KABUSHIKI KAISHA
Application year: 1997
Main classification: F16H25
IPC codes: F16D65, B25J9, F16H25, H02K7, F16D55, F16D59