Patent Publication Number: US-2019186605-A1

Title: Actuator

Description:
FIELD 
     The present invention relates to an actuator. 
     BACKGROUND 
     As devices that discharge liquid in syringes at certain flow rates, actuators have been known that use rotational motion-linear motion conversion mechanisms (ball screws). For example, Patent Literature 1 describes a syringe drive unit that discharges liquid in a syringe at a certain flow rate. The syringe drive unit in Patent Literature 1 includes a guide rail, a screw shaft that is provided in parallel with the guide rail and rotated by a motor, and a slider that moves along the guide rail in accordance with rotation of the screw shaft. In accordance with movement of the slider, liquid in the syringe is discharged at a certain flow rate. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2010-229926 A 
     SUMMARY 
     Technical Problem 
     In order to increase accuracy of flow rate of discharged liquid, it is required that no variation occurs in a movement amount of the slider with respect to a rotation amount of the ball screw. It is thus desirable that the ball screw and the guide rail be perfectly in parallel with each other. The ball screw, however, may not be in parallel with the guide rail in no small degree due to errors in assembling respective members, for example. This may cause variation in the movement amount of the slider with respect to the rotation amount of the ball screw. 
     In view of the problem described above, the present invention is made and aims to provide an actuator that can prevent variation in a movement amount of a slider with respect to a rotation amount of a rotational motion-linear motion conversion mechanism. 
     Solution to Problem 
     In order to achieve the aim described above, an actuator according to the present invention includes a frame, a guide rail attached to the frame, a slider guided by the guide rail, a rotational motion-linear motion conversion mechanism that is disposed on a side opposite to the guide rail with the frame interposed therebetween and includes a screw shaft supported by the frame and a nut attached to the screw shaft, and a coupling member that couples the slider and the nut. The coupling member is coupled to the nut via a nut bracket. The coupling member is an annular member along an axial direction of the screw shaft and includes a slider connection portion that has a plate shape and is fixed to the slider, and a nut that has a plate shape and is fixed to the nut bracket. 
     The rotational motion-linear motion conversion mechanism is disposed on the side opposite to the guide rail with the frame interposed therebetween, resulting in a distance from the rotational motion-linear conversion mechanism to the guide rail being larger than that when the rotational motion-linear conversion mechanism and the guide rail are disposed in the same direction with respect to the frame. This larger distance increases the length of the coupling member that couples the slider and the nut, thereby making it easy for the coupling member to be deformed. The coupling member is deformed, thereby making it easy to hold the screw shaft in parallel with the guide rail even if force causing an angle of the screw shaft with respect to the guide rail to be changed acts on the rotational motion-linear motion conversion mechanism. The coupling member functions as a buffering member to prevent a shift in the direction of the screw shaft with respect to the guide rail. The actuator according to the present invention thus can prevent variation in the movement amount of the slider with respect to the rotation amount of the rotational motion-linear motion conversion mechanism. 
     In addition, in the coupling member, the torsional stiffness around an axis of the screw shaft easily becomes smaller than that around the axis orthogonal to the screw shaft. The coupling member is thus easily deformed in such a direction that a shift of the direction of the screw shaft with respect to the guide rail is prevented, and is not easily tilted (pitching in the coupling member does not easily occur). 
     As a desirable embodiment of the present invention, it is preferable that the coupling member is disposed with a clearance between the coupling member and the frame. 
     This makes it hard for the frame to interfere with the coupling member when the coupling member is deformed. The coupling member thus can be easily deformed. 
     As a desirable embodiment of the present invention, it is preferable that the coupling member has a line symmetrical shape with respect to a straight line serving as a symmetrical axis viewed from the axial direction of the screw shaft, the straight line passing through the guide rail and the screw shaft. 
     This makes it hard to generate variation in easiness of deformation of the coupling member according to the direction of acted force. As a result, a shift in the direction of the screw shaft with respect to the guide rail is easily prevented. 
     As a desirable embodiment of the present invention, it is preferable that the nut bracket includes a nut support that has a plate shape and is orthogonal to the screw shaft. 
     The nut bracket thus receives force in the axial direction of the screw shaft transferred from the nut by the nut support that has a plate shape and is orthogonal to the screw shaft. This makes it hard for the direction of force transferred from the nut bracket to the coupling member to be shifted with respect to the axial direction of the screw shaft. 
     As a desirable embodiment of the present invention, it is preferable that the nut bracket supports the nut such that the nut is rotatable around an axis orthogonal to the axial direction of the screw shaft. 
     This prevents stress from occurring in the rotational motion-linear motion conversion mechanism, for example, even when the screw shaft is tilted (pitching of the screw shaft occurs). 
     As a desirable embodiment of the present invention, it is preferable to further include a plunger drive member that is disposed on a side opposite to the slider with the coupling member interposed therebetween and moves together with the coupling member. It is preferable that a length of the plunger drive member in an X direction is equal to or larger than half of a length of the coupling member in the X direction, the X direction being in parallel with a surface of the frame to which the guide rail is attached and being orthogonal to the axial direction of the screw shaft. 
     This reinforces the surface to which the plunger drive member is disposed of the coupling member. The torsional stiffness around the axial direction of the screw shaft thus increases in the coupling member. As a result, the tilting (pitching) of the plunger drive member is prevented. 
     As a desirable embodiment of the present invention, it is preferable that the nut bracket connection portion is in parallel with the slider connection portion. 
     As a result, in the coupling member, the connection portion connected to the slider and the connection portion connected to the nut bracket are planes in parallel with each other. This makes it easy to perform a process to fix the coupling member and the slider and a process to fix the coupling member and the nut bracket. 
     As a desirable embodiment of the present invention, it is preferable that the frame includes a base plate in contact with the guide rail and a reinforcing member disposed on a side opposite to the guide rail with the base plate interposed therebetween. This makes it hard for the base plate to be deformed. In addition, a female screw for fixing the guide rail can be provided to the reinforcing member, thereby allowing the guide rail to be more firmly fixed than a case where the reinforcing member is not provided. As a result, the positioning accuracy of the guide rail increases. 
     Advantageous Effects of Present Invention 
     The present invention can provide the actuator that can prevent variation in the movement amount of the slider with respect to the rotation amount of the rotational motion-linear motion conversion mechanism. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating an actuator according to an embodiment. 
         FIG. 2  is a front view illustrating the actuator according to the embodiment. 
         FIG. 3  is a plan view illustrating the actuator according to the embodiment. 
         FIG. 4  is a left side view illustrating the actuator according to the embodiment. 
         FIG. 5  is a right side view illustrating the actuator according to the embodiment. 
         FIG. 6  is a sectional view taken along A-A in  FIG. 3 . 
         FIG. 7  is an enlarged view of a periphery of a support unit in  FIG. 6 . 
         FIG. 8  is a schematic diagram explaining a state where a coupling member according to the embodiment is deformed. 
         FIG. 9  is a left side view of an actuator according to a first modification. 
         FIG. 10  is a B arrow view of  FIG. 9 . 
         FIG. 11  is a perspective view illustrating a coupling member according to a second modification. 
         FIG. 12  is a perspective view illustrating an actuator according to a third modification. 
         FIG. 13  is a front view illustrating the actuator according to the third modification. 
         FIG. 14  is a plan view illustrating the actuator according to the third modification. 
         FIG. 15  is a bottom view illustrating the actuator according to the third modification. 
         FIG. 16  is a left side view illustrating the actuator according to the third modification. 
         FIG. 17  is a right side view illustrating the actuator according to the third modification. 
         FIG. 18  is a sectional view taken along C-C in  FIG. 14 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     The following describes an embodiment of the present invention in detail with reference to the accompanying drawings. The content described in the following embodiment does not limit the present invention. The constituent elements described below include elements that the skilled person could easily think of and substantially identical elements. The constituent elements described below can be combined as appropriate. 
     Embodiment 
       FIG. 1  is a perspective view illustrating an actuator according to an embodiment.  FIG. 2  is a front view illustrating the actuator according to the embodiment.  FIG. 3  is a plan view illustrating the actuator according to the embodiment.  FIG. 4  is a left side view illustrating the actuator according to the embodiment.  FIG. 5  is a right side view illustrating the actuator according to the embodiment.  FIG. 6  is a sectional view taken along line A-A in  FIG. 3 . An actuator  1  is a device used for discharging liquid in a syringe at a certain flow rate, for example. 
     As illustrated in  FIGS. 1 to 6 , the actuator  1  includes a frame  2 , a syringe fixing member  11 , a plunger drive member  12 , a guide rail  5 , a slider  4 , a motor  9 , a ball screw  6 , a nut bracket  7 , and a coupling member  3 . When liquid in a syringe is discharged using the actuator  1 , the syringe is fixed to the syringe fixing member  11  and a plunger is fixed to the plunger drive member  12 . The syringe fixing member  11  is fixed to the frame  2 . The plunger drive member  12  is fixed to the coupling member  3  that is movable in accordance with rotation of the ball screw  6 . The plunger drive member  12  moves in a direction in which the plunger drive member  12  approaches the syringe fixing member  11 , resulting in liquid in the syringe being discharged. 
     The frame  2  is a member that supports the respective members included in the actuator  1 . The frame  2  is formed of a metal such as a stainless steel, for example. As illustrated in  FIG. 1 , the frame  2  includes a base plate  21 , and side plates  22  and  23 . The base plate  21  and the side plates  22  and  23  are integrally formed by bending a stainless steel plate having a thickness of 1.5 mm, for example. The base plate  21  is a plate-shaped member that is long in a moving direction of the plunger drive member  12 , and supports the guide rail  5  and the syringe fixing member  11 . The side plates  22  and  23  protrude from both ends in a short side direction of the base plate  21  in a direction orthogonal to the base plate  21 . The frame  2  has thus a substantially U-shape when viewed from an axial direction of the ball screw  6 . The side plate  22  includes a first facing surface  221  and a second facing surface  222  (refer to  FIG. 6 ). The first facing surface  221  faces one edge of the coupling member  3  in the axial direction of the ball screw  6  while the second facing surface  222  faces the other edge of the coupling member  3  in the axial direction of the ball screw  6 . The side plate  23  includes a first facing surface  231  and a second facing surface  232 . The first facing surface  231  faces the one edge of the coupling member  3  in the axial direction of the ball screw  6  while the second facing surface  232  faces the other edge of the coupling member  3  in the axial direction of the ball screw  6 . 
     In the following description, an orthogonal coordinate system is used that is composed of an X axis in parallel with the short side direction of the base plate  21 , a Y axis in parallel with the axial direction of the ball screw  6 , and a Z axis orthogonal to the X axis and the Y axis. In the X direction, the direction on the side plate  22  side of the base plate  21  is defined as the negative X direction while the direction on the side plate  23  side of the base plate  21  is defined as the positive X direction. In the Y direction, the direction on the syringe fixing member  11  side of the plunger drive member  12  is defined as the positive Y direction while the direction opposite to the positive Y direction is defined as the negative Y direction. In the Z direction, the direction in which the side plates  22  and  23  protrude from the base plate  21  is defined as the negative Z direction while the direction opposite to the negative Z direction is defined as the positive Z direction. 
     The syringe fixing member  11  and the plunger drive member  12  are disposed on the positive Z direction side of the base plate  21 . The syringe fixing member  11  is fixed to the base plate  21  by welding, for example. The plunger drive member  12  is fixed to the coupling member  3  by welding, for example. 
     As illustrated in  FIGS. 1 and 3 , the base plate  21  is provided with an opening  29 . The opening  29  is a rectangular hole, for example. Work for connecting members at the periphery of the motor  9  is done through the opening  29 , for example. As illustrated in  FIG. 6 , to the base plate  21 , a back plate  25  and a support unit bracket  26  are attached. The back plate  25  is formed of the same material as the base plate  21 , for example. The thickness of the back plate  25  is larger than that of the base plate  21 . The thickness of the back plate  25  is 2 mm, for example. The back plate  25  is fixed to the base plate  21  by welding, for example. The support unit bracket  26  is a plate-shaped member orthogonal to the back plate  25 . The support unit bracket  26  is formed integrally with the back plate  25  by bending a stainless steel plate having a thickness of 2 mm. The back plate  25  and the support unit bracket  26  thus form a substantially L shape when viewed from the X direction. 
     The guide rail  5  is a long member attached to the base plate  21 . The guide rail  5  is fixed to a surface of the base plate  21 , the surface facing away from the back plate  25 . More specifically, the guide rail  5  is fixed to the base plate  21  by fitting fastener members such as bolts passing through the base plate  21  to female screws formed in the back plate  25 . 
     The slider  4  is a member that is guided by the guide rail  5  and movable along the guide rail  5 . As illustrated in  FIG. 1 , the slider  4  holds the guide rail  5  from both sides thereof in the X direction. The slider  4  has therein a plurality of rolling elements in contact with the guide rail  5 . The rolling elements are balls, for example, and rotate while in contact with the guide rail  5 . The slider  4  thus, can perform straight line motion smoothly along the guide rail  5 . The slider  4  and the guide rail  5  form a linear motion guide. More specifically, the multiple rolling elements are circularly disposed and circulate while rotating. The multiple rolling elements form an endless circulation path. 
     As described above, all of the linear motion guide composed of the slider  4  and the guide rail  5 , the syringe fixing member  11 , and the plunger drive member  12  are disposed on the positive Z direction side of the base plate  21 . The linear motion guide is thus disposed in the vicinity of the syringe, thereby making it possible to prevent tilting (pitching and rolling) of the syringe fixing member  11  due to external force. As a result, accuracy of discharged flow rate of liquid in the syringe increases. The pitching is rotation around the X axis. The rolling is rotation around the Y axis. 
     The motor  9  is supported by the frame  2  and is provided with a shaft  91 . The motor  9  is a stepping motor, for example, and can rotate by a certain angle in accordance with a pulse signal supplied from a control device. The shaft  91  is connected to the ball screw  6  via a coupling  92 , for example. The ball screw  6  is a rotational motion-linear motion conversion mechanism that can covert rotational motion into linear motion. The ball screw  6  includes a screw shaft  61  including a main body  610  and a connection portion  611 , and a nut  62  attached to the main body  610 . Specifically, the shaft  91  is coupled to the connection portion  611  provided at one end of the screw shaft  61  via the coupling  92 . The screw shaft  61  has a thread formed on a surface of the main body  610 . The connection portion  611  has a smooth surface. A thread formed on an inner wall of the nut  62  is fitted to the thread formed on the main body  610 . The outer diameter of the connection portion  611  is smaller than that of the main body  610 . 
       FIG. 7  is an enlarged view of a periphery of a support unit in  FIG. 6 . As illustrated in  FIG. 7 , to the support unit bracket  26 , a support unit  8  is attached with fastener members such as bolts. The support unit  8  is a member that supports the ball screw  6  in a rotatable manner. The support unit  8  includes a case  80 , bearings  81  and  82 , and a locking nut  83 . 
     The case  80  is in contact with outer races of the bearings  81  and  82  and supports the bearings  81  and  82 . The bearings  81  and  82  are press-fitted into a hole preliminarily provided to the case  80 , for example. As a result, the outer races of the bearings  81  and  82  are fixed to the case  80 . The bearings  81  and  82  are juxtaposed along the axial direction of the screw shaft  61 . An inner race of the bearing  81  is in contact with a riser  612  between the main body  610  and the connection portion  611  while the inner race of the bearing  82  is in contact with the locking nut  83  attached to the connection portion  611 . The bearings  81  and  82  are positioned by the riser  612  and the locking nut  83 . 
     The nut bracket  7  is the member connected to the nut  62  of the ball screw  6 . As illustrated in  FIG. 6 , the nut bracket  7  includes a base  71 , a nut support  73 , and a reinforcing portion  72 . The base  71 , the nut support  73 , and the reinforcing portion  72  are integrally formed by bending and welding a stainless steel plate, for example. 
     The base  71  is a plate-shaped member in parallel with the base plate  21 , for example. The nut support  73  is a plate-shaped member orthogonal to the Y axis. The nut  62  passes through a hole provided to the nut support  73  and is fixed to the nut support  73  by fastener members such as bolts. The nut bracket  7  thus moves integrally with the nut  62  as the nut  62  moves. The reinforcing portion  72  is a plate-shaped member orthogonal to both of the base  71  and the nut support  73 , and is connected to both of the base  71  and the nut support  73 . The reinforcing portion  72  prevents the nut support  73  from being tilted in such a manner that the nut support  73  changes an angle with respect to the base  71 . This structure makes it hard for the nut support  73  to be deformed even when external force is applied to the nut support  73 , thereby allowing a posture of the nut  62  to be easily maintained. 
     The coupling member  3  couples the slider  4  and the nut  62 . Specifically, the coupling member  3  couples the slider  4  to the nut  62  via the nut bracket  7 . The coupling member  3  is an annular member along the Y direction, for example, as illustrated in  FIG. 1 . The coupling member  3  includes a slider connection portion  31 , a first circumventing portion  32 , a second circumventing portion  33 , and a nut bracket connection portion  34 . The annular member along the Y direction means that the axial direction of the annular member is along the Y direction. The term is used as the same meaning as that described above in the following description. The slider connection portion  31 , the first circumventing portion  32 , the second circumventing portion  33 , and the nut bracket connection portion  34  are integrally formed by bending and welding a stainless steel plate, for example. 
     The slider connection portion  31  is a plate-shaped member in parallel with the base plate  21 . In the X direction, the slider connection portion  31  is longer than the base plate  21 . In other words, when viewed from the Z direction, each of both ends of the slider connection portion  31  in the X direction is located outside the corresponding one of both ends of the base plate  21  in the X direction. The slider connection portion  31  is fixed to the slider  4  by fastener members such as bolts. The coupling member  3  is thus guided by the guide rail  5  together with the slider  4 . Specifically, the slider  4  is fixed on the negative Z direction side of the slider connection portion  31 . On the positive Z direction side of the slider connection portion  31 , the plunger drive member  12  is fixed. 
     Each of the first circumventing portion  32  and the second circumventing portion  33  is a plate-shaped member orthogonal to the slider connection portion  31 . The first circumventing portion  32  and the second circumventing portion  33  protrude from both ends of the slider connection portion  31  in the negative Z direction. The first circumventing portion  32  is located on the negative X direction side of the side plate  22  of the frame  2 . The first circumventing portion  32  faces the side plate  22  with a clearance therebetween that has a width d 1 . The width d 1  is about 1 mm to 2 mm, for example. The second circumventing portion  33  is located on the positive X direction side of the side plate  23  of the frame  2 . The second circumventing portion  33  faces the side plate  23  with a clearance therebetween that has the width dl. As illustrated in  FIG. 4 , a length L 1  of each of the first circumventing portion  32  and the second circumventing portion  33  in the Z direction is longer than a length L 2  between the slider connection portion  31  to the ball screw  6 . The ends opposite to those on the slider connection portion  31  side of the first circumventing portion  32  and the second circumventing portion  33  are located on the negative Z direction side of the ball screw  6 . 
     If the first circumventing portion  32  and the second circumventing portion  33  do not extend outside but extend inside the frame  2 , i.e., the first circumventing portion  32  and the second circumventing portion  33  pass through a slit provided to the base plate  21 , for example, torsional stiffness of the base plate  21  is reduced. Partial loss of sectional area occurs corresponding to the slit through which the first circumventing portion  32  and the second circumventing portion  33  pass in the base plate  21 , thereby reducing the torsional stiffness. As a result, the base plate  21  is easily deformed by external force. This arises a problem in that positioning accuracy of the guide rail  5  is reduced or a life-span is reduced due to stresses occurring in the guide rail  5  and the ball screw  6 , for example. In contrast, in the embodiment, the first circumventing portion  32  and the second circumventing portion  33  circumvent the frame  2 , thereby preventing the base plate  21  from being deformed. In addition, processing steps for making the base plate  21  are reduced, thereby making it easy to manufacture the frame  2 . 
     The nut bracket connection portion  34  is a plate-shaped member in parallel with the slider connection portion  31 , and connects the first circumventing portion  32  and the second circumventing portion  33 . The coupling member  3  thus has a substantially oblong shape when viewed from the Y direction. More specifically, when viewed from the Y direction, the coupling member  3  has a substantially oblong shape having short sides along the X direction and long sides along the Z direction. In other words, when viewed from the Y direction, the coupling member  3  has a line symmetrical shape with respect to a straight line C (refer to  FIG. 4 ) serving as the symmetrical axis. The straight line C passes through the slider  4  and the screw shaft  61 . The nut bracket connection portion  34  is fixed to the base  71  of the nut bracket  7  by fastener members such as bolts. The coupling member  3  thus moves integrally with the nut bracket  7  as the nut  62  moves. 
     As illustrated in  FIGS. 1 and 6 , one edge of the nut bracket connection portion  34  faces the first facing surfaces  221  and  231  while the other edge of the nut bracket connection portion  34  faces the second facing surfaces  222  and  232 . With the movement of the nut  62 , the nut bracket connection portion  34  moves in the negative Y direction by a certain amount, resulting in the nut bracket connection portion  34  being in contact with the first facing surfaces  221  and  231 . In this way, a limit position to which the coupling member  3  can move in the negative Y direction is defined. With the movement of the nut  62 , the nut bracket connection portion  34  moves in the positive Y direction by a certain amount, resulting in the nut bracket connection portion  34  being in contact with the second facing surfaces  222  and  232 . In this way, a limit position to which the coupling member  3  can move in the positive Y direction is defined. The limit positions prevent the nut  62  from dropping off from the screw shaft  61 . 
     When the motor  9  operates, the shaft  91  rotates, thereby rotating the screw shaft  61  of the ball screw  6 . With the rotation of the screw shaft  61 , the nut  62  fitting the thread of the screw shaft  61  moves in the Y direction. The nut bracket  7 , the coupling member  3 , and the slider  4  thus move in the Y direction together with the nut  62 . A movement amount of the plunger drive member  12  in the Y direction is equal to that of the slider  4  in the Y direction. In order to increase accuracy of flow rate of liquid discharged from the syringe provided to the actuator  1 , the movement amount of the slider  4  needs to be highly accurately controlled. For highly accurate control, it is desirable that an axis A 1  of the guide rail  5  and an axis A 2  of the screw shaft  61  be in parallel with each other. The axes A 1  and A 2  are illustrated in  FIG. 1 . It is, however, not easy to constantly hold the axes A 1  and A 2  in parallel with each other. For example, the axis A 1  is not in parallel with the axis A 2  in no small degree due to errors in assembling respective members to the frame  2 . With the movement of the nut  62 , the screw shaft  61  is oscillated in no small degree to the support unit  8  serving as a fulcrum. As a result, the axis A 1  may not be in parallel with the axis A 2 . An angle of the axis A 2  with respect to the axis A 1  may change with time. This may cause variation in the movement amount of the slider  4  with respect to the rotation amount of the ball screw  6 . 
     If the guide rail  5 , the slider  4 , and the ball screw  6  are disposed close to one another and the ball screw  6  is supported such that the screw shaft  61  does not oscillate, the angle of the axis A 1  with respect to the axis A 2  is prevented from being changed with time, but stresses may continue to occur in the guide rail  5 , the slider  4 , and the ball screw  6 . These stresses hinder sooth movement of the slider  4 . As a result, variation may occur in the movement amount of the slider  4  with respect to the rotation amount of the ball screw  6 . 
     In contrast, in the actuator  1  according to the embodiment, the coupling member  3  is deformed, thereby preventing variation in the movement amount of the slider  4  with respect to the rotation amount of the ball screw  6 .  FIG. 8  is a schematic diagram explaining a state where the coupling member according to the embodiment is deformed.  FIG. 8  is a diagram when the actuator  1  is viewed from direction toward the positive Y direction.  FIG. 8  exaggerates the deformation of the coupling member  3  for explanatory purpose. The deformation of the coupling member  3  illustrated in  FIG. 8  differs from the practical deformation thereof in some cases. 
     Even if force causing an angle of the axis A 1  with respect to the axis A 2  to be changed acts on the ball screw  6 , the coupling member  3  is deformed and makes it easy to hold the axes A 1  and A 2  in parallel with each other. Specifically, as illustrated in  FIG. 8 , the coupling member  3  is deformed in such a manner that the ends of the first circumventing portion  32  and the second circumventing portion  33  in the Z direction shift in the X direction. As described above, clearances are each provided between the first circumventing portion  32  and the side plate  22  and between the second circumventing portion  33  and the side plate  23 , thereby allowing the coupling member  3  to be easily deformed. The coupling member  3  is deformed and makes it hard to cause stress in the guide rail  5 , the slider  4 , and the ball screw  6 . The life-spans of the guide rail  5 , the slider  4 , and the ball screw  6  are thus elongated. 
     In order to cause the coupling member  3  to be easily deformed as illustrated in  FIG. 8 , the torsional stiffness around the Y axis is preferably smaller than that around the X axis in the coupling member  3 , for example. In the embodiment, the torsional stiffness around the Y axis is smaller than that around the axis orthogonal to the Y axis in the coupling member  3  because the coupling member  3  has an annular shape along the Y direction. The torsional stiffness of a certain member around a certain axis is expressed as G×J/L where G is the modulus of transverse elasticity of the member, J is the torsion constant of the member, and L is the length in the axial direction of the member. 
     The coupling member  3  is not always required to have the shape described above. The coupling member  3  may not be an annular member. For example, a slit from one end to the other end in the Y direction may be provided to the slider connection portion  31  or the nut bracket connection portion  34 . A slit from one end to the other end in the Y direction may be provided to both of the slider connection portion  31  and the nut bracket connection portion  34 . The coupling member  3  may not have a line symmetrical shape with respect to a straight line, which serves as the symmetrical axis, in parallel with the Z axis. For example, either the first circumventing portion  32  or the second circumventing portion  33  may not be included in the coupling member  3 . As a result, the coupling member  3  may have a substantially U shape when viewed from the Y direction. The length L 1  illustrated in  FIG. 4  may not be always required to be longer than the distance L 2 . The length L 1  is preferably larger than the distance L 2  from point of view that the coupling member  3  is easily deformed by force in the X direction. 
     The frame  2  may not be always required to include the side plates  22  and  23 . The frame  2  may include at least the base plate  21 . If the frame  2  does not include the side plates  22  and  23 , the width dl illustrated in  FIG. 4  means the width of the clearance between the first circumventing portion  32  and the base plate  21  or the width of the clearance between the second circumventing portion  33  and the base plate  21 . 
     As described above, the actuator  1  according to the embodiment includes the frame  2 , the guide rail  5  attached to the frame  2 , the slider  4  guided by the guide rail  5 , the rotational motion-linear motion conversion mechanism (ball screw  6 ) that is disposed on the side opposite to the guide rail  5  with the frame  2  interposed therebetween and includes the screw shaft  61  supported by the frame  2  and the nut  62  attached to the screw shaft  61 , and the coupling member  3  that couples the slider  4  and the nut  62 . 
     The ball screw  6  is disposed on the side opposite to the guide rail  5  with the frame  2  interposed therebetween, resulting in the distance from the ball screw  6  to the guide rail  5  being larger than that when the ball screw  6  and the guide rail  5  are disposed in the same direction with respect to the frame  2 . This increases the length of the coupling member  3  that couples the slider  4  and the nut  62 , thereby making it easy for the coupling member  3  to be deformed. The coupling member  3  is deformed, thereby making it easy to hold the screw shaft  61  in parallel with the guide rail  5  even if force causing an angle of the screw shaft  61  with respect to the guide rail  5  to be changed acts on the ball screw  6 . The coupling member  3  functions as a buffering member to prevent a shift in the direction of the screw shaft  61  with respect to the guide rail  5 . The actuator  1  according to the embodiment thus can prevent variation in the movement amount of the slider  4  with respect to the rotation amount of the rotational motion-linear motion conversion mechanism (ball screw  6 ). 
     In the actuator  1  according to the embodiment, the coupling member  3  is disposed with a clearance with respect to the frame  2 . This structure makes it hard for the frame  2  to interfere with the coupling member  3  when the coupling member  3  is deformed. The coupling member  3  thus can be easily deformed. 
     In the actuator  1  according to the embodiment, the coupling member  3  has a line symmetrical shape with respect to the straight line C serving as the symmetrical axis when viewed from the axial direction (Y direction) of the screw shaft  61 , the straight line C passing through the guide rail  5  and the screw shaft  61 . This structure makes it hard to generate variation in easiness of deformation of the coupling member  3  according to the direction of acted force. As a result, a shift in the direction of the screw shaft  61  with respect to the guide rail  5  is more easily prevented. 
     In the actuator  1  according to the embodiment, the coupling member  3  is an annular member along the axial direction (Y direction) of the screw shaft  61 . In the coupling member  3 , this structure makes it easy for the torsional stiffness around the axis (Y axis) of the screw shaft  61  to be smaller than that around the axis orthogonal to the Y axis. The coupling member  3  is thus easily deformed in such a direction that a shift of the direction of the screw shaft  61  with respect to the guide rail  5  is prevented, and is not easily tilted (pitching of the coupling member  3  does not easily occur). 
     In the actuator  1  according to the embodiment, the coupling member  3  is coupled to the nut  62  via the nut bracket  7 . The nut bracket  7  includes the nut support  73  that has a plate shape and is orthogonal to the screw shaft  61 . The nut bracket  7  thus receives force in the axial direction (Y direction) of the screw shaft  61  transferred from the nut  62  by the nut support  73  that has a plate shape and is orthogonal to the screw shaft  61 . This structure makes it hard for the direction of force transferred from the nut bracket  7  to the coupling member  3  to be shifted with respect to the axial direction (Y direction) of the screw shaft  61 . 
     In the actuator  1  according to the embodiment, the coupling member  3  is an annular member along the axial direction (Y direction) of the screw shaft  61 , and includes the slider connection portion  31  that has a plate shape and is fixed to the slider  4 , and the nut bracket connection portion  34  that is fixed to the nut bracket  7  and has a plate shape in parallel with the slider connection portion  31 . In the coupling member  3 , the connection portion connected to the slider  4  and the connection portion connected to the nut bracket  7  are planes in parallel with each other. This structure makes it easy to perform a process to fix the coupling member  3  and the slider  4  and a process to fix the coupling member  3  and the nut bracket  7 . 
     In the actuator  1  according to the embodiment, the frame  2  includes the base plate  21  in contact with the guide rail  5  and the reinforcing member (back plate  25 ) disposed on the side opposite to the guide rail  5  with the base plate  21  interposed therebetween. This structure makes it hard for the base plate  21  to be deformed. In addition, female screws for fixing the guide rail  5  can be provided to the reinforcing member, thereby allowing the guide rail  5  to be more firmly fixed than a case where the reinforcing member is not provided. As a result, the positioning accuracy of the guide rail  5  increases. 
     First Modification 
       FIG. 9  is a left side view of an actuator according to a first modification.  FIG. 10  is a B arrow view of  FIG. 9 . As illustrated in  FIGS. 9 and 10 , in the first modification, a ball screw  6 A different from the ball screw  6  and a nut bracket  7 A different from the nut bracket  7  are provided. The same constituent elements as described in the embodiment are labeled with the same numerals and duplicated descriptions thereof are omitted. 
     As illustrated in  FIGS. 9 and 10 , the ball screw  6 A includes the screw shaft  61  and a nut  62 A. The nut  62 A is a cylindrical member, for example. The thread formed on the inner wall of the nut  62 A fits the thread formed on the main body  610 . 
     The nut bracket  7 A is the member that supports the nut  62 A such that the nut  62 A can rotate around the X axis. As illustrated in  FIGS. 9 and 10 , the nut bracket  7 A includes a base  71 A and two nut supports  73 A. The base  71 A and the nut supports  73 A are integrally formed by bending a stainless steel plate, for example. 
     The base  71 A is a plate-shaped member in parallel with the base plate  21 , and is connected to the nut bracket connection portion  34 . The nut support  73 A is a plate-shaped member orthogonal to the X axis. The nut support  73 A includes a bearing  75  and a trunnion  76 . The bearing  75  is fixed to the nut support  73 A. One end of the trunnion  76  is press-fitted into the inner race of the bearing  75 , for example. The other end of the trunnion  76  is fixed to the nut  62 A. The nut bracket  7 A thus can support the nut  62 A such that the nut  62 A can rotate around the X axis. 
     As described above, in an actuator  1 A according to the first modification, the coupling member  3  is coupled to the nut  62 A via the nut bracket  7 A. The nut bracket  7 A supports the nut  62 A such that the nut  62 A can rotate around the X axis. This structure prevents stress from occurring in the ball screw  6 A, for example, even when the screw shaft  61  is tilted (pitching of the screw shaft  61  occurs). 
     Second Modification 
       FIG. 11  is a perspective view illustrating a coupling member according to a second modification. In the second modification, a coupling member  3 B different from the coupling member  3  is provided. The same constituent elements as described in the embodiment are labeled with the same numerals and duplicated descriptions thereof are omitted. 
     The coupling member  3 B couples the slider  4  and the nut  62 . Specifically, the coupling member  3 B couples the slider  4  to the nut  62  via the nut bracket  7 . The coupling member  3 B is an annular member along the Y direction, for example, as illustrated in  FIG. 11 . The coupling member  3 B includes a slider connection portion  31 B, a first circumventing portion  32 B, a second circumventing portion  33 B, and a nut bracket connection portion  34 B. The slider connection portion  31 B, the first circumventing portion  32 B, the second circumventing portion  33 B, and the nut bracket connection portion  34 B are connected by welding, for example. 
     The slider connection portion  31 B is a plate-shaped member in parallel with the base plate  21 . The slider connection portion  31 B is a metallic plate such as a stainless steel plate. In the X direction, the slider connection portion  31 B is longer than the base plate  21 . When viewed from the Z direction, each of both ends of the slider connection portion  31 B in the X direction is located outside the corresponding one of both ends of the base plate  21  in the X direction. The slider connection portion  31 B is fixed to the slider  4  by fastener members such as bolts. The coupling member  3 B is thus guided by the guide rail  5  together with the slider  4 . Specifically, the slider  4  is fixed to the slider connection portion  31 B on the negative Z direction side of the slider connection portion  31 B. On the positive Z direction side of the slider connection portion  31 B, the plunger drive member  12  is fixed. 
     Each of the first circumventing portion  32 B and the second circumventing portion  33 B is a plate-shaped member orthogonal to the slider connection portion  31 B. The first circumventing portion  32 B and the second circumventing portion  33 B protrude from both ends of the slider connection portion  31 B in the negative Z direction. Each of the first circumventing portion  32 B and the second circumventing portion  33 B is a metallic plate such as a stainless steel plate having a thickness smaller than that of the slider connection portion  31 B, for example. The first circumventing portion  32 B is located on the negative X direction side of the side plate  22 . The first circumventing portion  32 B faces the side plate  22  with a clearance therebetween that has the width d 1  (refer to  FIG. 4 ). The second circumventing portion  33 B is located on the positive X direction side of the side plate  23 . The second circumventing portion  33  faces the side plate  23  with a clearance therebetween that has the width dl. In the same manner as the first circumventing portion  32  and the second circumventing portion  33  illustrated in  FIG. 4 , the length of each of the first circumventing portion  32 B and the second circumventing portion  33 B in the Z direction is longer than the distance from the slider connection portion  31 B to the ball screw  6 . The ends opposite to those on the slider connection portion  31 B side of the first circumventing portion  32 B and the second circumventing portion  33 B are located on the negative Z direction side of the ball screw  6 . 
     The nut bracket connection portion  34 B is a plate-shaped member in parallel with the slider connection portion  31 B, and connects the first circumventing portion  32  and the second circumventing portion  33 . The coupling member  3 B thus has a substantially oblong shape when viewed from the Y direction. More specifically, when viewed from the Y direction, the coupling member  3 B has a substantially oblong shape having short sides along the X direction and long sides along the Z direction. The nut bracket connection portion  34 B is a metallic plate such as a stainless steel plate having the same thickness as the slider connection portion  31 B, for example. In other words, when viewed from the Y direction, the coupling member  3 B has a line symmetrical shape with respect to a straight line serving as the symmetrical axis, the straight line passing through the slider  4  and the screw shaft  61 . The nut bracket connection portion  34 B is fixed to the base  71  of the nut bracket  7  by fastener members such as bolts. The coupling member  3 B thus moves integrally with the nut bracket  7  as the nut  62  moves. 
     As described above, in the second modification, the thickness of each of the first circumventing portion  32 B and the second circumventing portion  33 B is smaller than that of each of the slider connection portion  31 B and the nut bracket connection portion  34 B. As a result, the first circumventing portion  32 B and the second circumventing portion  33 B having a thinner thickness reduce the torsional stiffness around the Y axis in the whole of the coupling member  3 B while the torsional stiffness of the slider connection portion  31 B and the nut bracket connection portion  34 B is maintained. The titling (pitching) of the slider  4  and nut bracket  7  is thus prevented. 
     The slider connection portion  31 B, the first circumventing portion  32 B, the second circumventing portion  33 B, and the nut bracket connection portion  34 B may not be always required to be connected by welding. For example, pins provided to the slider connection portion  31 B and the nut bracket connection portion  34 B fit depressed portions provided to the first circumventing portion  32 B and the second circumventing portion  33 B, thereby achieving positioning. Thereafter, the slider connection portion  31 B, the first circumventing portion  32 B, the second circumventing portion  33 B, and the nut bracket connection portion  34 B are fastened by bolts, for example. 
     Third Modification 
       FIG. 12  is a perspective view illustrating an actuator according to a third modification.  FIG. 13  is a front view illustrating the actuator according to the third modification.  FIG. 14  is a plan view illustrating the actuator according to the third modification.  FIG. 15  is a bottom view illustrating the actuator according to the third modification.  FIG. 16  is a left side view illustrating the actuator according to the third modification.  FIG. 17  is a right side view illustrating the actuator according to the third modification.  FIG. 18  is a sectional view taken along C-C in  FIG. 14 . In  FIG. 18 , a part of the structure is illustrated as a front view. An actuator  1 C according to the third modification includes a plunger drive member  12 C different from the plunger drive member  12 . The same constituent elements as described in the embodiment are labeled with the same numerals and duplicated descriptions thereof are omitted. 
     As illustrated in  FIG. 12 , when liquid in a syringe  13  is discharged using the actuator  1 C, the syringe  13  is fixed to the syringe fixing member  11  and a plunger  14  is fixed to the plunger drive member  12 C. The plunger drive member  12 C is fixed to the coupling member  3  that is movable in accordance with rotation of the ball screw  6 . The plunger drive member  12 C moves in a direction in which the plunger drive member  12 C approaches the syringe fixing member  11 , resulting in liquid in the syringe  13  being discharged. 
     The plunger drive member  12 C is disposed on the positive Z direction side of the slider connection portion  31 . The plunger drive member  12 C is an annular member an axial direction of which is along the Z direction. When viewed from the Z direction, the outer periphery of the plunger drive member  12 C has a substantially oblong shape. As illustrated in  FIG. 12 , the plunger drive member  12 C includes a plunger contact portion  121 , a facing portion  124 , a first side surface portion  122 , and a second side surface portion  123 . 
     The plunger contact portion  121  is a plate-shaped member orthogonal to the Y direction, and is in contact with the plunger  14  when the syringe  13  and the plunger  14  are attached to the actuator  1 C. The plunger contact portion  121  is fixed to the slider connection portion  31  by welding, for example. The plunger contact portion  121  is provided with a slit  121   a  extending in the Z direction at the center in the X direction. 
     The facing portion  124  is a plate-shaped member in parallel with the plunger contact portion  121 . The length of the facing portion  124  in the X direction is substantially equal to that of the plunger contact portion  121  in the X direction. The facing portion  124  has a hole  124   a  at the center in the X direction. The hole  124   a  overlaps with the slit  121   a  when viewed from the Y direction. 
     The first side surface portion  122  and the second side surface portion  123  are plate-shaped members orthogonal to the X direction. The first side surface portion  122  and the second side surface portion  123  are integrated with the facing portion  124 , for example. The facing portion  124 , the first side surface portion  122 , and the second side surface portion  123  are formed by bending a single plate. The length of the second side surface portion  123  in the Y direction is substantially equal to that of the first side surface portion  122  in the Y direction. The facing portion  124 , the first side surface portion  122 , and the second side surface portion  123  are fixed to the slider connection portion  31  by welding, for example. The ends of the first side surface portion  122  and the second side surface portion  123  in the positive Y direction are fixed to the plunger contact portion  121 . The lengths in the Z direction of the plunger contact portion  121 , the facing portion  124 , the first side surface portion  122 , and the second side surface portion  123  are substantially equal. 
     In the plunger drive member  12 C according to the third modification, the length thereof in the X direction differs from that of the plunger drive member  12  according to the embodiment. As illustrated in  FIG. 16 , a length L 3  of the plunger drive member  12 C in the X direction is equal to or larger than half of a length L 5  of the coupling member  3  in the X direction and equal to or smaller than the length L 5 , for example. The length L 3  is equal to or larger than a length L 4  of the nut bracket  7  in the X direction, for example. In other words, the length L 3  is the length of each of the plunger contact portion  121  and the facing portion  124  in the X direction. 
     As described above, the actuator  1 C according to the third modification includes the plunger drive member  12 C. The plunger drive member  12 C is disposed on the side opposite to the slider  4  with the coupling member  3  interposed therebetween and moves together with the coupling member  3 . The length L 3  of the plunger drive member  12 C in the X direction is equal to or larger than half of the length L 5  of the coupling member  3  in the X direction where the X direction is the direction that is in parallel with the surface (base plate  21 ) of the frame  2  to which the guide rail  5  is attached and is orthogonal to the axial direction of the screw shaft  61 . This structure reinforces the surface (slider connection portion  31 ) to which the plunger drive member  12 C is disposed of the coupling member  3 . The torsional stiffness around the axial direction (Y axis) of the screw shaft  61  thus increases in the coupling member  3 . As a result, the tilting (pitching) of the plunger drive member  12 C is prevented. 
     REFERENCE SIGNS LIST 
       1 ,  1 A,  1 C actuator 
       11  syringe fixing member 
       12 ,  12 C plunger drive member 
       121  plunger contact portion 
       121   a  slit 
       122  first side surface portion 
       123  second side surface portion 
       124  facing portion 
       124   a  hole 
       13  syringe 
       2  frame 
       21  base plate 
       22 ,  23  side plate 
       221 ,  231  first facing surface 
       222 ,  232  second facing surface 
       25  back plate 
       26  support unit bracket 
       29  opening 
       3 ,  3 B coupling member 
       31 ,  31 B slider connection portion 
       32 ,  32 B first circumventing portion 
       33 ,  33 B second circumventing portion 
       34 ,  34 B nut bracket connection portion 
       4  slider 
       5  guide rail 
       6 ,  6 A ball screw (rotational motion-linear motion conversion mechanism) 
       61  screw shaft 
       610  main body 
       611  connection portion 
       612  riser 
       62 ,  62 A nut 
       7 ,  7 A nut bracket 
       71 ,  71 A base 
       72  reinforcing portion 
       73 ,  73 A nut support 
       75  bearing 
       76  trunnion 
       8  support unit 
       80  case 
       81 ,  82  bearing 
       83  locking nut 
       9  motor 
       91  shaft 
       92  coupling 
     A 1 , A 2  axis 
     C straight line