Patent Publication Number: US-2020281673-A1

Title: Actuator device, end effector, and surgical system

Description:
TECHNICAL FIELD 
     The technology disclosed in the present specification relates to: an actuator device applied to, for example, a surgical system; an end effector of the surgical system; and the surgical system. 
     BACKGROUND ART 
     Recently, a robotics technology has made remarkable progress, and the robotics technology has widely spread in workplaces of various industrial fields. A master-slave robot system is used in industrial fields where it is still difficult to perform full autonomous operation under the control of a computer, such as a medical robot. For example, a surgeon uses a master-slave medical robot for endoscopic surgery for an abdominal cavity, a chest cavity, or the like, and can carry out the surgery by remotely operating a slave arm, to which a surgical tool such as a forceps is attached, while viewing an operative field on a 3D monitor screen. 
     Considering invasion to living tissue and efficiency of surgical treatment under the endoscope, it is preferable that external force received by the end effector of the slave from an affected site and the like is presented to a user on the master side. As of this master-slave robot system, several proposals have been made for a medical robot capable of detecting force acting on an end effector such as a gripping portion (gripper). Furthermore, a proposal also has been made for a medical instrument and a medical support arm device capable of detecting contact force (see Patent Document 1, for example). 
     In a surgical robot utilized in an endoscopic surgery, downsizing a configuration of an end effector is essential. Therefore, it is general to employ a driving mechanism in which driving force generated in a driving unit like an actuator or the like arranged apart from the end effector is transmitted by a wire or cable to open/close the end effector. However, in a configuration in which a force sensor is disposed between the end effector and the driving unit that drives the end effector, traction force of the wire to open/close the end effector interferes with, for example, external force applied in a long axis direction of the end effector, and therefore, there are concerns that sensitivity of the force sensor may be degraded or calibration may become difficult. 
     On the other hand, there is a known pair of surgical forceps including: a pair of jaw members respectively having cam slots bored and coupled to each other in an openable/closable manner; and a shaft having an elongated shape and including a cam pin that is positioned at a tip and inserted into the cam slots, in which the pair of surgical forceps opens/closes the jaw members by reciprocating the elongated shaft in a longitudinal direction to make the cam pin slide inside the cam slots (see Patent Document 2, for example). In this type of forceps, when gripping force is increased, frictional force between the cam slots is increased, and traction force via the shaft is largely lost before being transmitted as the gripping force by the jaw members. To obtain desired large gripping force, it is necessary to increase the traction force by an amount compensating for the frictional force. Therefore, there is a problem that output of an actuator that generates the traction force is required to be increased. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open No. 2017-29214 
         Patent Document 2: Japanese Patent Application Laid-Open No. 2008-188440 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     An object of the technology disclosed in the present specification is to provide: an actuator device applied to a surgical system; an end effector of the surgical system; and the surgical system. 
     Solutions to Problems 
     The technology disclosed in the present specification is made in consideration of the problems described above, and according to a first aspect thereof, provided is an actuator device including: 
     a first magnetic body portion; 
     a first system movable in a predetermined direction or an opposite direction of the predetermined direction; 
     a second system including a second magnetic body portion that moves the first system in the predetermined direction by magnetic force generated between the second magnetic body portion and the first magnetic body portion, and a pressurizing portion capable of applying, to the first system, force in the opposite direction of the predetermined direction and including an elastic body and the like; and 
     a driving unit capable of applying, to the second system, force in the predetermined direction or the opposite direction by driving. The more the first system is drawn in the predetermined direction, the more the force in the opposite direction of the elastic portion is increased. Furthermore, the first system includes a supporting portion configured to support an acting portion that acts by a reciprocating motion in the predetermined direction. 
     The second system includes a sliding portion connected to the supporting portion via the elastic portion. The sliding portion has one surface that is oriented in a direction parallel to the predetermined direction and connected to the elastic portion, has the other surface connected to the second magnetic body portion, and is relatively movable in the direction parallel to the predetermined direction by the driving of the driving unit. 
     The supporting portion has a hollow structure. Additionally, the sliding portion is housed inside the hollow structure and relatively movable in a direction parallel to the predetermined direction. 
     Furthermore, a second magnetic body portion attached to the sliding portion in a manner facing the magnetic body portion is further provided, and the magnetic body portion sucks the second magnetic body portion by magnetic force. 
     Additionally, the driving unit includes, for example, a dielectric elastomer and is driven in the predetermined direction by extension/contraction. 
     In a state where the first system is positioned closest to the magnetic body portion, attraction force by magnetic force of the first magnetic body portion and the magnetic force of the second magnetic body portion is larger than restoring force of the elastic portion. Furthermore, in a case where the second system separates the first system from the first magnetic body portion, the driving unit generates driving force in the opposite direction of the predetermined direction, the driving force being larger than a difference between the attraction force by the magnetic force of the first magnetic body portion and the restoring force of the elastic portion. 
     Furthermore, according to a second aspect of the technology disclosed in the present specification, provided is an end effector including: 
     a gripping portion; and an actuator unit that generates traction force to the gripping portion, in which 
     the actuator unit includes 
     a first magnetic body portion, 
     a first system movable in a predetermined direction or an opposite direction of the predetermined direction, 
     a second system including a second magnetic body portion that moves the first system in the predetermined direction by magnetic force generated between the second magnetic body portion and the first magnetic body portion, and a pressurizing portion capable of applying, to the first system, force in the opposite direction of the predetermined direction, and 
     a driving unit capable of applying, to the second system, force in the predetermined direction or the opposite direction by driving. 
     Moreover, according to a third aspect of the technology disclosed in the present specification, provided is a surgical system including: 
     an end effector; 
     an actuator unit that generates traction force to the end effector; and 
     a force sensor arranged closer to a proximal end side than the actuator unit. 
     The force sensor includes, for example, a strain detection element that detects strain of a strain element and includes an FBG sensor. 
     Furthermore, according to a fourth aspect of the technology disclosed in the present specification, provided is a surgical system including: 
     an end effector; and an actuator unit that generates traction force to the end effector, in which 
     the actuator unit includes 
     a first system that is sucked by magnetic force of a magnetic body portion and moves, in a predetermined direction, an acting portion that causes the traction force to act on the gripping portion, and 
     a second system that applies, to the first system, force in an opposite direction of the predetermined direction, and separates the first system from the magnetic body portion. 
     Effects of the Invention 
     According to the technology disclosed in the present specification, it is possible to provide the actuator device applied to the surgical system, the end effector of the surgical system, and the surgical system. 
     Note that the effect described in the present specification is an example, and the effect of the present invention is not limited thereto. Furthermore, there may be a case where the present invention further provides an additional effect other than the above-described effect. 
     Other objects, features, and advantages of the technology disclosed in the present specification will be further described in more detail on the basis of embodiments as described later and the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating an exemplary configuration of a surgical robot  100  to which a technology disclosed in the present specification is applied. 
         FIG. 2  is a view illustrating a modified example of the surgical robot  100 . 
         FIG. 3  is a view illustrating an exemplary configuration of an actuator unit  102 . 
         FIG. 4  is a view illustrating the exemplary configuration of the actuator unit  102 . 
         FIG. 5  is a view illustrating force acting on a first system. 
         FIG. 6  is a diagram illustrating force acting on a second system. 
         FIG. 7  is a diagram illustrating exemplary calculation of generative force in accordance with a displacement amount of the actuator unit  102 . 
         FIG. 8  is a diagram illustrating exemplary calculation of gripping force of a gripping portion  101  in accordance with the displacement amount of the actuator unit  102 . 
         FIG. 9  is a diagram illustrating exemplary calculation of the generative force in accordance with the displacement amount of the actuator unit  102 . 
         FIG. 10  is a view illustrating an exemplary configuration of a force sensor  103 . 
         FIG. 11  is a view illustrating an XY cross section at a position a of a strain element  1001 . 
         FIG. 12  is a view to describe a mechanism of detecting force acting on the strain element  1001 . 
         FIG. 13  is a diagram to describe a method of installing, on the strain element  1001 , a strain detection element utilizing an FBG sensor. 
         FIG. 14  is a diagram illustrating a processing algorithm of a  4 DOF force sensor. 
         FIG. 15  is a view illustrating exemplary implementation of the actuator unit  102 . 
         FIG. 16  is a view illustrating a first system of the actuator unit  102 . 
         FIG. 17  is a view illustrating a second system of the actuator unit  102 . 
         FIG. 18  is a view illustrating exemplary operation of the actuator unit  102 . 
         FIG. 19  is a view illustrating exemplary operation of the actuator unit  102 . 
         FIG. 20  is a view illustrating exemplary operation of the actuator unit  102 . 
         FIG. 21  is a view illustrating exemplary operation of the actuator unit  102 . 
         FIG. 22  is a view illustrating exemplary operation of the actuator unit  102 . 
         FIG. 23  is a view illustrating exemplary operation of the actuator unit  102 . 
         FIG. 24  is a view illustrating exemplary operation of the actuator unit  102 . 
         FIG. 25  is a view illustrating exemplary operation of the actuator unit  102 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment of a technology disclosed in the present specification will be described in detail with reference to the drawings. 
       FIG. 1  schematically illustrates an exemplary configuration of a surgical robot  100  to which the technology disclosed in the present specification is applied. The illustrated surgical robot  100  includes, for example, an arm robot and is provided with, sequentially from a distal end side more than a bend portion  104  such as a joint: a gripping portion  101  as an end effector; an actuator unit  102  that supplies gripping traction force to the gripping portion  101 ; and a force sensor  103  to detect external force acting on the gripping portion  101 . 
     The gripping portion  101  is a pair of surgical forceps and includes a pair of blades  101   a  and  101   b  coupled in an openable/closable manner. When the blades  101   a  and  101   b  are opened/closed by being driven in directions opposing to each other, and can grip living tissue. A coupled portion between the respective blades  101   a  and  101   b  has a mechanical structure that converts traction force in a linear movement direction into gripping force. Therefore, when the traction force in the linear movement direction acts on the gripping portion  101  as indicated by an arrow A in the drawing, the blades  101   a  and  101   b  are closed, and when force in an opposite direction of the arrow A acts on the gripping portion  101 , the blades  101   a  and  101   b  are opened. 
     For example, cam slots are bored on the respective blades  101   a  and  101   b , a cam pin protruding at a tip portion of an elongated shaft is inserted into the cam slots, and the pair of blades can be opened/closed by reciprocating the elongated long shaft in a longitudinal direction to make the cam pin slide inside the cam slots (see Patent Document  2 , for example). Note that the structures of the cam and the slots are not illustrated to simplify the drawing. 
     The actuator unit  102  includes, for example, an acting portion that performs linear movement, and can supply traction force, through the acting portion, for reciprocating the elongate-shaped shaft of the gripping portion  101  as the pair of surgical forceps. 
     For example, large gripping force, such as gripping a needle with strong force during surgery, is necessary when an open/close angle of the gripping portion  101  becomes close to zero degrees. In the present embodiment, the actuator unit  102  generates the large traction force when the open/close angle of the gripping portion  101  becomes close to zero degrees. Note, however, that a detailed configuration of the actuator unit  102  will be described later. 
     The force sensor  103  includes, for example, a six-axis force sensor and can detect: triaxial force acting on the gripping portion  101  provided as the end effector; and torque around the respective axes. A detailed configuration of the force sensor  103  will be described later. 
     The surgical robot  100  according to the present embodiment has the gripping portion  101 , the actuator unit  102 , and the force sensor  103  which are sequentially disposed from the distal end side toward a proximal end. In other words, the force sensor  103  is arranged in a region located between the actuator unit  102  and the proximal end and free from acting of the traction force to generate the gripping force of the gripping portion  101 . According to such a configuration, the traction force by the actuator unit  102  does not reach the force sensor  103 . Since the traction force of the actuator unit  102  does not interfere with the external force applied in a long axis direction of the end effector, sensitivity of the force sensor  103  is not degraded and a detection signal from the force sensor  103  can be easily calibrated. 
       FIG. 2  illustrates a modified example of the surgical robot  100  for comparison with  FIG. 1 . In the surgical robot  100  according to the illustrated modified example, the gripping portion  101 , a bend portion  104 , the force sensor  103 , and the actuator unit  102  are sequentially disposed from a distal end side. Note, however, that constituent elements same as those illustrated in  FIG. 1  are denoted by the same reference signs. 
     Main differences from the exemplary configuration illustrated in  FIG. 1  are that: the bend portion  104  is interposed between a portion including the force sensor  103  and the actuator unit  102  and the gripping portion  101  and; and the force sensor  103  is disposed on the distal end side (or close to the gripping portion  101 ) more than the actuator unit  102 . In the configuration in which the force sensor  103  is arranged between the gripping portion  101  and the actuator unit  102 , the traction force by the actuator unit  102  reaches the force sensor  103 . In other words, the traction force of the actuator unit  102  interferes with the external force applied in the long axis direction of the end effector. Due to this, there is a problem that the sensitivity of the force sensor  103  is degraded and calibration of the force sensor  103  becomes difficult. 
     As described above, according to the configuration of the surgical robot  100  illustrated in  FIG. 1 , the sensitivity of the force sensor  103  can be improved. On the other hand, in a case where the actuator unit  102  is arranged in the vicinity of the distal end, downsizing is required, and therefore, there is a problem that output of an actuator is reduced. For example, large gripping force, such as gripping a needle with strong force during surgery, is necessary when the open/close angle of the gripping portion  101  becomes close to zero degrees. Considering this, the present specification proposes a structure of the actuator unit  102  that can be downsized and is capable of extracting the large gripping force even with little driving force. 
       FIGS. 3 and 4  illustrate an exemplary configuration of the actuator unit  102  proposed in the present specification. Both  FIGS. 3 and 4  illustrate a cross section of the actuator unit  102 . Note, however, that  FIG. 3  illustrates a state where the traction force to generate the gripping force of the gripping portion  101  is not acting (that is, corresponding to a state where the gripping portion  101  is opened), and  FIG. 4  illustrates a state where the traction force is acting (that is, corresponding to a state where the gripping portion  101  is closed). 
     The actuator unit  102  generates traction force in a linear movement direction indicated by the arrow A in  FIG. 3 , and includes: an acting portion  301  that causes the traction force to act on the gripping portion  101 ; a supporting portion  302  supporting the acting portion  301 ; and a sliding portion  303  relatively movable in a direction parallel to the arrow A with respect to the supporting portion  302 . 
     The supporting portion  302  has a hollow cylindrical shape, and an axis of the cylinder is parallel to the arrow A. Furthermore, the sliding portion  303  is housed inside the cylinder and can be relatively moved in the direction parallel to the arrow A with respect to the supporting portion  302  by the sliding portion sliding or slipping along an inner wall of the cylinder. Therefore, a portion including the acting portion  301  and the supporting portion  302  and the sliding portion  303  are basically constrained so as to be relatively moved only in the direction parallel to the arrow A. The sliding portion  303  can also be referred to as an internal component of the supporting portion  302 . 
     The sliding portion  303  has one end surface that is oriented in the arrow A direction and connected to a bottom surface portion of the hollow cylinder on the supporting portion  302  side via an elastic portion  304  including a coil spring or the like. Therefore, when a relative position between the supporting portion  302  and the sliding portion  303  is changed in the linear movement direction indicated by the arrow A or in an opposite direction thereof, restoring force F k  of the elastic portion  304  acts in a direction returning to an original position. The coil spring used for the elastic portion  304  has, for example, a linear characteristic, and the restoring force F k  thereof is directly proportional to a displacement amount Δx from a natural length of the coil spring. Using a spring constant k, it can be expressed as F k =k·Δx. Note, however, that a non-linear spring can also be used as the elastic portion  304 . Furthermore, as far as the force in an opposite direction of a predetermined direction indicated by the arrow A can be applied, the elastic portion  304  is not limited to the one including an elastic member, and a pressurizing portion can also be used as the elastic portion  304 . For example, a magnet that generates attraction force in the opposite direction can also be applied as the elastic portion  304 . 
     Furthermore, a magnetic body portion  306  that includes a permanent magnet or the like and generates magnetic force is disposed at a rear end (proximal end side) of the actuator unit  302 . Additionally, the sliding portion  303  has the other end surface to which a second magnetic body portion  307  is attached in a manner facing the magnetic boy portion  306 . Since the magnetic body portion  306  and the second magnetic body portion  307  are disposed in a manner such that different polarities face each other, attraction force F M  by the magnetic force of the magnetic body portion  306  acts on the sliding portion  303  in the predetermined direction indicated by the arrow A. Therefore, the force F M  in the arrow A direction is applied to the supporting portion  302  via the sliding portion  303  and the elastic portion  304 , and becomes the traction force in the linear movement direction of the acting portion  301 . 
     The attraction force F M  is inversely proportional to the square of a distance between the magnetic body portion  306  and the second magnetic body portion  307 . Due to this, when the magnetic body portion  306  and the second magnetic body portion  307  are closest to each other and the open/close angle of the gripping portion  101  becomes close to zero degrees, the actuator unit  102  can generate large traction force by the magnetic force. Therefore, it is possible to downsize dimensions of the actuator unit  102  (particularly, in the direction orthogonal to the longitudinal direction). 
     Note that an electromagnet including a coil may be used instead of the permanent magnet in one or both of the magnetic body portion  306  and the second magnetic body portion  307  (note, however, that it is necessary to increase the number of turns of the coil, leading to upsize of the magnetic body portion, and also large coil current is required to generate magnetic force as much as the magnetic force of the permanent magnet. Using the permanent magnet is more inexpensive and provides a simple structure). Furthermore, even when either one of the magnetic body portion  306  and the second magnetic body portion  307  is manufactured with a magnetic body instead of a magnet, the attraction force F M  by the magnetic force can be made to act on a range between the sliding portion  303  and the magnetic body portion  306  (or a range between the supporting portion  302  and the magnetic body portion  306 ). For example, a magnetic body may constitute the entire sliding portion  303 , instead of attaching the magnetic body to the other end surface of the sliding portion  303 . 
     Furthermore, the sliding portion  303  is coupled to a driving unit  305  that is linearly moved in the direction parallel to the arrow A. Specifically, the sliding portion  303  includes protruding portions protruding to an upper end and a lower end in the drawing paper. Additionally, these protruding portions are coupled to the driving unit  305  via linear apertures bored on the cylinder portion of the supporting portion  302 , and the driving unit  305  is disposed outside the supporting portion  302 . The driving unit  305  is a linear movement actuator that drives the supporting portion  302  in the direction parallel to the arrow A. Therefore, driving force FA in the direction parallel to the arrow A is applied to the sliding portion  303  from the driving unit  305 . As described later, the driving force FA in the opposite direction of the arrow A acts so as to pull away the sliding portion  303  from the magnetic body portion  306 . 
     In the present embodiment, a dielectric elastomer (DEA) that is one of electro-active polymers (EAP) is used as the driving unit  305  that is the linear movement actuator. Examples of the DEA include a silicon-based polymer, a urethane-based polymer, an acrylic polymer, and the like. The DEA as the driving unit  305  is extended/contracted in the linear movement direction indicated by the arrow A, and the relative position between the portion including the acting portion  301  and the supporting portion  302  and the sliding portion  303  is changed by this configuration. Therefore, the driving force FA by the driving unit  305  includes generative force F DEA  by the DEA. The driving force F DEA  by the driving unit  305  is varied in accordance with voltage applied to the DEA. For example, the driving unit  305  includes the DEA shaped like a hollow cylinder and is disposed so as to house the supporting portion  302  inside the cylinder. 
     The DEA is an example of the linear movement actuator. Besides the DEA, it may be possible to use, as the driving unit  305  that is the linear movement actuator, a conductive polymer actuator, an ion conducting actuator, a macro fiber composite (MFC) actuator, a ferroelectric polymer actuator, a piezo actuator, a voice coil, a micromotor, a pneumatic cylinder, or the like. Note, however, that the present applicant considers that the DEA is preferable because of characteristics as follows: a displacement amount in the linear movement direction can be estimated from changes in dimensions, magnitude of the generative force, and a displacement amount, and changes in a displacement amount and capacitance. Note that, as for a transducer device utilizing a DEA, refer to, for example, Japanese Patent Application No. 2017-133160 already assigned to the present applicant. 
     Assume that main components of the actuator unit  102 , such as the supporting portion  302 , the sliding portion  303 , the driving unit  305 , and the magnetic body portion  306  described above, are accommodated in a housing  310 . 
     The acting portion  301  and the supporting portion  302  are integrally fixed. Force that pushes the acting portion  301  in the linear movement direction indicated by the arrow A is the traction force to the gripping portion  101  coupled to the end (distal end side) of the acting portion  101 . This traction force includes resultant force including: the restoring force F k  by the elastic portion  304 ; the driving force F DEA  by the driving unit  305 ; and the magnetic force F M  by the magnetic body portion  306 . Note, however, that the restoring force F k  is internal force received by the supporting portion  302  from the sliding portion  303  provided as the internal component, and the restoring force is offset inside, and therefore, the restoring force does not contribute to the traction force acting on the outside.  FIG. 4  illustrates the state where the traction force of the actuator unit  102  is acting. Since the acting portion  301  applies the traction force to the gripping portion  101 , the gripping portion  101  is closed. 
     The gripping portion  101  is the pair of surgical forceps that grips living tissue, and includes the pair of blades  101   a  and  101   b  that are opened and closed by being driven in the opposing directions from each other. A coupled portion of the respective blades  101   a  and  101   b  has the mechanical structure that converts the traction force in the linear movement direction into the gripping force. Specifically, the cam slots are bored on respective blades  101   a  and  101   b . Furthermore, the acting portion  301  includes the elongated shaft and the cam pin protrudes from the tip portion of the shaft, and the pair of blades  101   a  and  101   b  can be opened and closed by the cam pin sliding inside the cam slots. That is, when the traction force in the linear movement direction indicated by the arrow A in  FIG. 3  acts on the gripping portion  101 , the blades  101   a  and  101   b  are closed as illustrated in  FIG. 4 . Furthermore, when the force in the opposite direction of the arrow A acts on the gripping portion  101  with the blades  101   a  and  101   b  closed, the blades  101   a  and  101   b  are opened as illustrated in  FIG. 3 . 
     Note, however, that detailed configurations of the gripping portion  101  and the blades  101   a  and  101   b  are not illustrated in the drawing because: the structure of a surgical terminal that is opened/closed by converting the traction force in the linear movement direction into the gripping force is well known; and furthermore, the technology disclosed in the present specification is not limited to the structure of a specific surgical terminal. 
     Operation of the actuator unit  102  will be described in more detail. 
     The actuator unit  102  illustrated in  FIGS. 3 and 4  is structurally separated into: a first system that directly influences the traction force of the gripping portion  101 ; and a second system that does not directly influence the traction force of the gripping portion  101 . In the following, resultant force acting on the first system will be defined as F 1 , and resultant force acting on the second system will be defined as F 2 . 
     The first system includes the acting portion  301  and the supporting portion  302 . Note that the sliding portion  303  is included as the internal component of the supporting portion  302 , but does not belong to the first system. The first system is moved in the linear movement direction indicated by the arrow A and generates the large traction force while utilizing the magnetic force F M  of the magnetic body portion  306  particularly in a region where the open/close angle of the gripping portion  101  becomes close to zero degrees. Note that the restoring force F k  generated by the elastic portion  304  that connects the supporting portion  302  and the sliding portion  303  is the internal force received by the supporting portion  302  from the sliding portion  303  provided as the internal component, and the restoring force is offset inside, and therefore, the restoring force does not contribute to the traction force acting on the outside. 
     On the other hand, the second system includes the sliding portion  303 , the elastic portion  304 , and the second magnetic body portion  307  that is integrated with the sliding portion  303 , and receives the driving force F DEA  from the driving unit  305  and is further applied with the restoring force F k  from the elastic portion  304 . In the second system, if a design is made such that the restoring force F k  by the elastic portion  304  and the magnetic force F M  by the magnetic body portion  306  are cancelled each other, the second magnetic body portion  307  can be pulled away from the magnetic body portion  306  by making the second system slide in the opposite direction of the arrow A by small force F 2 . 
     The magnetic force has a characteristic of nonlinearly attenuating relative to a distance between the magnets (specifically, the magnetic force attenuates in inverse proportion to the square of the distance between the magnets). Therefore, the actuator unit  102  obtains large gripping force on the basis of such a characteristic of the magnets by utilizing the magnetic force of the magnetic body portion  306  when the open/close angle of the gripping portion  101  is close to the zero degrees, and furthermore, the second system is made to slide even by the small driving force F DEA  of the driving unit  305  to open the gripping portion  101 , and a gripped object can be released. 
       FIG. 5  illustrates the force acting on the first system when the actuator unit  102  pulls the gripping portion  101 . In the drawing, the components constituting the first system are surrounded by a thick line  501 . Note, however, that the sliding portion  303  included as the internal component of the supporting portion  302  is surrounded by the thick line  501 , but does not belong to the first system (as described above). The resultant force F 1  of the force acting on the first system becomes the traction force to the gripping portion  101 , and also becomes the gripping force when the open/close angle of the gripping portion  101  is close to the zero degrees. 
     The restoring force F k  is applied to the supporting portion  302  from the elastic portion  304 . Furthermore, the attraction force F M  from the magnetic body portion  306  is applied to the sliding portion  303 . Among these kinds of force, the restoring force F k  is the internal force received by the supporting portion  302  from the sliding portion  303  provided as the internal component, and the restoring force is offset inside. Therefore, in the first system, it can be said the traction force F 1  of the gripping portion  101  corresponds to the attraction force F M  received from the magnetic body portion  306  as represented by Expression (1) below. 
       [Math. 1] 
         F   1   =F   M +( F   k   −F   k )= F   M    (1)
 
     The attraction force F M  acts in the same direction as the traction force indicated by the arrow A, in other words, the attraction force F M  becomes the traction force acting on the gripping portion  101  in the linear movement direction. Therefore, when the open/close angle of the gripping portion  101  becomes close to zero degrees, the first system can generate the large traction force F 1  utilizing the magnetic force F M  and can lock a gripped state. 
       FIG. 7  illustrates exemplary calculation values of the attraction force F M  by the magnetic force of the magnetic body portion  306 , the restoring force F K  of the elastic portion  304 , and the generative force F DEA  of the driving unit (DEA)  305  when the actuator unit  102  attempts to displace the acting portion  301  in the linear movement direction indicated by the arrow A (that is, when the traction force is applied to the gripping portion  101  so as to close the gripping portion). Note that a horizontal axis represents the displacement amount of the acting portion  301  and a vertical axis represents force [N]. Furthermore, a maximum displacement amount of the actuator unit  102  is set to 3 mm, a position where the acting portion  301  is displaced maximally in the opposite direction of the arrow A is set as 0 on the horizontal axis, and the linear movement direction indicated by the arrow A is defined as a positive direction of the horizontal axis. Additionally, the calculation is made while setting an elastic coefficient of the elastic portion  304  as k=4.5 N/mm. 
     The attraction force F M  by the magnetic force of the magnetic body portion  306  is increased in inverse proportion to the distance from the second magnetic body portion  307 . Furthermore, the elastic portion  304  includes the coil spring having, for example, the linear characteristic, and the restoring force F K  thereof is increased in proportion to the distance from where the displacement amount is close to 1.5 mm. Therefore, the more the displacement amount is increased and the smaller the open/close angle of the gripping portion  101  becomes, the more the gripping force is nonlinearly increased. Furthermore, the restoring force F K  of the elastic portion  304  has the linear characteristic, and a magnitude relation with the attraction force F M  by the magnetic force of the magnetic body portion  306  is reversed in the process in which the acting portion  301  is displaced, but an insufficient force is compensated by the generative force F DEA  of the driving unit  305 . It is found that when the generative force F DEA  of the driving unit  305  is in a range of −1 to +1 [N], the actuator unit  102  is operable. 
     A rightmost end of the horizontal axis of the graph illustrated in  FIG. 7  is the maximum displacement position of the actuator unit  102  where the magnetic body portion  306  closely contacts (or is positioned closest to) the second magnetic body portion  307 . The gripping portion  101  should be designed and accurately attached to the end (distal end side) of the acting portion  301  such that the gripping portion  101  is completely closed at this maximum displacement position. Furthermore, the gripping portion  101  can be brought into a grip lock state by selecting a coil spring used for the elastic portion  304  such that the attraction force F M  by the magnetic force of the magnetic body portion  306  becomes larger than the restoring force F K  of the elastic portion  304  at the maximum displacement position of the actuator unit  102 . 
     Note that  FIG. 7  illustrates the exemplary calculation in the case of using the elastic portion  304  in which the restoring force F K  has the linear characteristic. When a coil spring having a non-linear characteristic or the like is used as the elastic portion  304 , it is possible to fit a curve with a displacement curve of the attraction force F M  by the magnetic force of the magnetic body portion  306 . Consequently, it is possible to further reduce the force necessary for the DEA used for the driving unit  305 , and as a result, this can contribute to downsizing the dimensions of the actuator unit  102  (particularly, in the direction orthogonal to the longitudinal direction). 
     When the driving unit  305  is contracted in the linear movement direction indicated by the arrow A, the sum of the attraction force F M  by the magnetic force of the magnetic body portion  306  and the generative force F DEA  of the driving unit  305  becomes the gripping force.  FIG. 8  illustrates exemplary calculation value of the gripping force of the gripping portion  101  when the actuator unit  102  displaces the acting portion  301  in the linear movement direction indicated by the arrow A. Note that a horizontal axis represents the displacement amount of the acting portion  301 , a maximum displacement is set to 3 mm, and a vertical axis represents the force [N]. Furthermore, the maximum displacement amount of the actuator unit  102  is set to 3 mm, the position where the acting portion  301  is displaced maximally in the linear movement direction indicated by the arrow A (see  FIG. 4 ) is set to 0 on the horizontal axis, and the opposite direction of the arrow A is defined as the positive direction of the horizontal axis. Additionally, calculation is made on the basis of the calculation results illustrated in  FIG. 7  while setting the generative force F DEA  of the driving unit  305  to less than 1 N (that is, F DEA &lt;1 [N]). As illustrated, the gripping force is transitional together with the displacement amount of the actuator unit  102 . 
     The sum of the attraction force F M  by the magnetic force of the magnetic body portion  306  and the generative force F DEA  of the driving unit  305  becomes the traction force by the actuator unit  102 , and it is found from  FIG. 8  that force of  7 N or more can be obtained. It should be fully understood that force minimally required for the driving unit  305  including the DEA can be reduced to  1  N or less by compensation with the restoring force F k  of the elastic portion  304  including the coil spring or the like. Therefore, the output of the DEA can be suppressed small and it is possible to downsize the dimensions of the actuator unit  102  (particularly, in the direction orthogonal to the longitudinal direction). 
     Furthermore,  FIG. 6  illustrates the force acting on the second system when the gripping portion  101  is opened to release the gripped object. In the drawing, the components constituting the second system are surrounded by a thick line  601  (the second system includes the sliding portion  303 , the second magnetic body  307 , and the elastic portion  304  as described above). When the resultant force F 2  of the force acting on the second system acts in the opposing direction of the arrow A, the resultant force becomes the force that pulls away, from the magnetic body portion  306 , the second magnetic body portion  307  integrated with the sliding portion  303 , and the gripping portion  101  can be opened by making the second system slide. 
     The sliding portion  303  is applied with: the restoring force F k  from the elastic portion  304 ; the driving force F DEA  by the driving unit  305  (note, however, when the DEA is extended); and the attraction force F M  by which the second magnetic body portion  307  attached to the other end surface of the sliding portion  303  is sucked by the magnetic force of the magnetic body portion  307 . Among these kinds of force, the restoring force F k  and the driving force F DEA  acts in the direction opposite to the traction force indicated by the arrow A (note, however, when DEA is extended), and the attraction force F M  by the magnetic force of the magnetic body portion  306  acts in the direction same as the traction force indicated by the arrow A. Therefore, the resultant force F 2  acting on the second system is as represented by Expression (2) below. 
       [Math. 2] 
         F   2   =F   DEA   +F   k   −F   M    (2)
 
     When F 2 &gt;0, that is, when the sum of the restoring force F k  and the driving force F DEA  is larger than the magnetic force F M , in other words, when the driving force F DEA  is larger than a difference between the magnetic force F M  and the restoring force F k , the second magnetic body portion  307  integrated with the sliding portion  303  is pulled away from the magnetic body portion  306 , and the gripping portion  101  can be opened by making the second system slide. A conditional expression of pulling away the second magnetic body portion  307  from the magnetic body portion  306  is as represented by Expression (3) below. 
       [Math. 3] 
         F   DEA   &gt;F   M   −F   k    (3)
 
     Therefore, when the elastic portion (coil spring)  304  is selected so as to obtain appropriate restoring force F k , the second magnetic body portion  307  can be pulled away from the magnetic body portion  306  with the small driving force F DEA  of the driving unit  305  including the DEA, and the grip lock can be released. 
       FIG. 9  illustrates exemplary calculation values of the attraction force F M  by the magnetic force of the magnetic body portion  306 , the restoring force F K  of the elastic portion  304 , and the generative force F DEA  of the driving unit (DEA)  305  when the actuator unit  102  displaces the acting portion  301  in the opposite direction of the arrow A (that is, when the gripping portion  101  is opened). Note that a horizontal axis represents the displacement amount of the acting portion  301 , a maximum displacement is set to  3  mm, and a vertical axis represents the force [N]. Furthermore, the maximum displacement amount of the actuator unit  102  is set to  3  mm, the position where the acting portion  301  is displaced maximally in the linear movement direction indicated by the arrow A (see  FIG. 4 ) is set to  0  on the horizontal axis, and the opposite direction of the arrow A is defined as the positive direction of the horizontal axis. Additionally, the calculation is made while setting an elastic coefficient of the elastic portion  304  as k=4.5 N/mm. 
     The attraction force F M  by the magnetic force of the magnetic body portion  306  attenuates in inverse proportion to the distance from the second magnetic body portion  307 . Furthermore, the elastic portion  304  includes the coil spring having, for example, the linear characteristic, and the restoring force F K  thereof is decreased in proportion to the distance from where the displacement amount is close to 1.5 mm. Therefore, the more the displacement amount is increased and the larger the open/close angle of the gripping portion  101  is, the more the gripping force is nonlinearly reduced. Furthermore, the restoring force F K  of the elastic portion  304  has the linear characteristic, and a magnitude relation with the attraction force F M  by the magnetic force of the magnetic body portion  306  is reversed in the process in which the acting portion  301  is displaced, but an insufficient force is compensated by the generative force F DEA  of the driving unit  305 . It is found that when the generative force F DEA  of the driving unit  305  is in a range of −1 to +1 [N], the actuator unit  102  is operable. 
     A leftmost end of the horizontal axis of the graph illustrated in  FIG. 9  is the maximum displacement position of the actuator unit  102  where the magnetic body portion  306  closely contacts (or is positioned closest to) the second magnetic body portion  307 . As already described with reference to  FIG. 7 , since the attraction force F M  by the magnetic force of the magnetic body portion  306  becomes larger than the restoring force F K  of the elastic portion  304  at the maximum displacement position of the actuator unit  102 , the gripping portion  101  is brought into the grip lock state by stopping the driving force F DEA  of the driving unit  305 . Therefore, when the driving unit  305  supplies the driving force F DEA  larger than the difference between the magnetic force F M  and the restoring force F k , the grip lock of the gripping portion  101  can be released. 
       FIG. 15  illustrates exemplary implementation of the actuator unit  102 . Furthermore,  FIG. 16  illustrates a portion of the first system of the actuator unit  102  in an extracted manner, and  FIG. 17  illustrates a portion of the second system thereof in an extracted manner. The first system illustrated in  FIG. 16  includes the supporting portion  302  that supports the acting portion  301 . The supporting portion  302  is movable in the linear movement direction (left direction in the drawing paper of  FIG. 16 ) of the actuator unit  102  indicated by the arrow A in  FIG. 1  and in the opposite direction thereof. Furthermore, the second system illustrated in  FIG. 17  includes the sliding portion  303 , the elastic portion  304 , and the second magnetic body portion  307 . The second magnetic body portion  307  moves the first system illustrated in  FIG. 16  in the linear movement direction by the magnetic force generated between the second magnetic body portion  307  and the magnetic body portion  306 . Furthermore, the elastic portion  304  can apply the force to the first system in the opposite direction of the linear movement direction. The sliding portion  303  has one surface (end surface on the distal end side) that is oriented in a direction parallel to the linear movement direction and connected to the elastic portion  304 , and has the other surface (end surface on the proximal end side) connected to the second magnetic body portion  307 . The sliding portion  303  can be relatively moved in the direction parallel to the linear movement direction by the driving of the driving unit  305  (not illustrated in  FIGS. 15 to 17 ). 
     Furthermore,  FIGS. 18 to 25  illustrate how the gripping portion  101  is changed from the closed state to the opened state and again changed to the closed state by the operation of the actuator unit  102 . 
       FIGS. 18 to 22  each illustrate how the gripping portion  101  is opened by linear movement operation of the actuator unit  102  toward the left side in the drawing paper. During steps between  FIGS. 18 and 19 , the driving unit  305  is extended, the second magnet portion  307  is separated from the magnet portion  306  by the resultant force of tensile force F k  of the elastic portion  304  and the driving force F DEA  of the driving unit  305 , and the second system starts linear movement toward the left side in the drawing paper. 
     Then, when the end surface of the sliding portion  303  abuts on a rear end portion of the acting portion  301  at a time point illustrated in  FIG. 20 , the first system and the second system are integrally and linearly moved toward the left side in the drawing paper during the steps between  FIGS. 20 to 22 , and as a result, the gripping portion  101  can be opened as illustrated in  FIG. 22 . 
       FIGS. 22 to 25  illustrate how the gripping portion  101  is by linearly moving the actuator unit  102  to the right side in the drawing paper and generating the traction force. In the state illustrated in  FIG. 22 , when the driving unit  305  stops the driving force F DEA  or switches to the driving force F DEA  directed to the right side in the drawing paper (namely, the magnet portion  306 ), influence of the sucking force F M  by which the magnet portion  306  sucks the second magnet portion  307  with the magnetic force is increased, and the second system starts the linear movement toward the right side in the drawing paper as illustrated in  FIG. 23 . 
     In steps during  FIGS. 24 and 25 , the end surface of the sliding portion  303  is separated from the rear end portion of the acting portion  301 , and only the second system is moved toward the right side in the drawing paper. Furthermore, when the coil spring as the elastic portion  304  exceeds the natural length, the elastic force F k  is applied to the second system toward the left side in the drawing paper, but the attraction force F M  by the magnetic force of the magnet portion  306  is stronger, and therefore, the second system keeps movement toward the right side in the drawing paper. 
     Then, as illustrated in  FIG. 25 , the gripping portion  101  is completely closed at the maximum displacement position where the second magnet portion  307  is adsorbed to the magnet portion  306 . At this maximum displacement position, the gripping portion  101  can be made into the grip lock state by selecting the coil spring used for the elastic portion  304  such that the attraction force F M  by the magnetic force of the magnetic body portion  306  becomes larger than the restoring force F K  of the elastic portion  304 . 
     As described above, according to the actuator unit  102  according to the present embodiment, the large traction force can be generated when the open/close angle of the gripping portion  101  is close to zero degrees. Therefore, the gripping portion  101  can grasp a needle and living tissue with strong force during surgery. In contrast, when the open/close angle of the gripping portion  101  is fixed around zero degrees due to a structural failure or the like, the body tissue is kept gripped, which is dangerous. Accordingly, it is preferable that the actuator unit  102  be equipped with a structure for security assurance. 
     As an example, the magnetic body portion  306  on the proximal end side may have a detachable structure. Specifically, as indicated by reference sign  311  in  FIG. 4 , a wire is attached to the end surface on the proximal end side of the magnetic body portion  306  such that the magnetic body portion  306  can be dropped (or can be pulled away manually from the second magnetic body portion  307 ) by pulling this emergency wire  311 . Consequently, the traction force of the actuator unit  102  is lost, and the gripping portion  101  is opened and the gripped object can be released. 
     Furthermore, in a case of using an electromagnet including a coil as the magnetic body portion  306  instead of a permanent magnet (as described above), the direction of the magnetic force can be changed to the opposing direction by changing a direction of coil current, and the grip lock can be easily released. Furthermore, in the event of structural failure or emergency also, a polarity of the electromagnet is switched to release the grip lock, and the gripped object can be released. In the event of electrical failure, the magnetic force is lost by stopping the current to the coil, and therefore, the grip lock is automatically released. 
     Subsequently, the force sensor  103  applied to the surgical robot  100  illustrated in  FIG. 1  will be described in detail. In the present embodiment, the force sensor  103  is arranged in the region located between the actuator unit  102  and the proximal end and free from acting of the traction force to generate the gripping force of the gripping portion  101  (see  FIG. 1 ). Therefore, since the traction force of the actuator unit  102  does not interfere with the external force applied in the long axis direction of the end effector, the sensitivity of the force sensor  103  is not degraded, and a detection signal from the force sensor  103  can be easily calibrated. 
       FIG. 10  illustrates an exemplary configuration of the force sensor  103 . The illustrated force sensor  103  includes: a strain element  1001  having a hollow cylindrical shape; and strain detection element(s) disposed at one or more places on an outer periphery of the strain element  1001 . Note, however, that a part of a link structure included in the surgical robot  100  can also be used as the strain element  1001 . 
     In the example illustrated in  FIG. 10 , a plurality of strain detection elements for detecting strain in XY directions at the respective different two positions a and b in the long axis direction is attached to the outer periphery of the strain element  1001 . Specifically, at the position a, a pair of strain detection elements  1011   a  and  1013   a  (not illustrated in  FIG. 10 ) to detect a strain amount in the X direction of the strain element  1001  are attached to facing sides of the outer periphery of the strain element  1001 . Furthermore, a pair of strain detection elements  1012   a  and  1014   a  to detect a strain amount in the Y direction of the strain element  1001  are attached to facing sides of the outer periphery of the strain element  1001 . Similarly, at the position b, a pair of strain detection elements  1011   b  and  1013   b  (not illustrated in  FIG. 10 ) to detect the strain amount in the X direction of the strain element  1001  are attached, and also a pair of strain detection elements  1012   b  and  1014   b  to detect a strain amount in the Y direction are attached. 
       FIG. 11  is a view illustrating an XY cross section at the position a of the strain element  1001 . As is clear from the drawing, the pair of strain detection elements  1011   a  and  1013   a  that detect the strain amount in the X direction are attached to the facing sides in the X direction of the outer periphery of the strain element  1001 , and the pair of strain detection elements  1012   a  and  1014   a  that detect the strain amount in the Y direction are attached to the facing sides in the Y direction of the outer periphery of the strain element  1001 . Note that, as for the XY cross section at the position b of the strain element  1001  also, the pair of strain detection elements  1011   b  and  1013   b  that detect the strain amount in the X direction are attached to the facing sides in the X direction of the outer periphery of the strain element  1001 , and the pair of strain detection elements  1012   b  and  1014   b  that detect the strain amount in the Y direction are attached to the facing sides in the Y direction of the outer periphery of the strain element  1001  in a manner similar to  FIG. 11 , although not illustrated. 
     First, a description will be provided referring to  FIG. 12  for a reason why the pair of strain detection elements  1011   a  and  1013   a  (or  1011   b  and  1013   b ) are disposed on the facing sides in the X direction and the pair of strain detection elements  1012   a  and  1014   a  (or  1012   b  and  1014   b ) are disposed on the facing sides in the Y direction at the one detecting position. 
     As illustrated in  FIG. 12(A) , in a case where only one strain detection element  1211  is attached to a cantilever beam  1201 , the strain detection element  1211  is compressed when Z-direction external force F z  is applied to the cantilever beam  1201 , and therefore, the external force F z  can be measured. However, since the strain detection element  1211  is stretched even if the cantilever beam  1201  is bent in either an upper direction or a lower direction in the drawing paper, it is not possible to identify which one of directions, a positive direction or a negative direction (upper or lower direction in the drawing paper) an acting direction of the external force F y  applied in the Y direction is. 
     In contrast, as illustrated in  FIG. 12(B) , in a case of attaching a pair of detection elements  1221  and  1222  on facing sides in the Y direction of the cantilever beam  1201 , when the cantilever beam  1201  is bent upward in the drawing paper, the strain detection element  1221  on one side is compressed and the strain detection element  1222  on the other side is stretched, whereas when the cantilever beam  1201  is bent downward in the drawing paper, the strain detection element  1221  on the one side is stretched and the strain detection element  1222  on the other side is compressed. Therefore, it is possible to identify the acting direction of the external force F y  applied in the Y direction on the basis of the relation between the positive and negative signs of strain amounts detected by the pair of detection elements  1221  and  1222  attached to the facing sides in the Y direction. 
     Accordingly, it is possible to detect the Z-direction external force acting on the strain element  1001  by acquiring the sum of the respective strain amounts detected by the pair of strain detection elements  1011   a  and  1013   a  (or  1011   b  and  1013   b ) attached to the facing sides in the X direction at an arbitrary position in the long axis direction of the strain element  1001 , and also it is possible to calculate X-direction external force acting on the strain element  1001  by acquiring a difference between the respective strain amounts. 
     Furthermore, the strain amount detected by each of the strain detection elements  1011   a  and  1013   a  (or  1011   b  and  1013   b ) includes not only a component caused by acting force but also a component caused by a temperature change, but there are advantages that the component caused by the temperature change is setoff at the time of calculating the X-direction external force by acquiring the difference between the respective strain amounts, and it is not necessary to perform temperature compensation processing. Note that a method of performing the temperature compensation by acquiring a detection value difference between sensors installed on facing sides, for example, a  4 -gauge method using four strain gauges is known in the industry. 
     Similarly, it is possible to detect Z-direction external force acting on the strain element  1001  by acquiring the sum of the respective strain amounts detected by the pair of strain detection elements  1012   a  and  1014   a  (or  1012   b  and  1014   b ) attached to the facing sides in the Y direction at an arbitrary position in the long axis direction of the strain element  1001 , and also it is possible to calculate the Y-direction external force acting on the strain element  1001  by acquiring a difference between the respective strain amounts. Furthermore, the strain amount detected by each of the strain detection elements  1012   a  and  1014   a  (or  1012   b  and  1014   b ) includes not only a component caused by acting force but also a component caused by a temperature change, but there are advantages that the component caused by the temperature change is setoff at the time of calculating the Y-direction external force by acquiring the difference between the respective strain amounts, and it is not necessary to perform the temperature compensation processing (same as described above). 
     Next, a description will be provided for a reason for adopting the configuration in which the strain amounts in the XY directions are detected at the different two positions a and b in the long axis direction of the strain element  100 . 
     It is possible to calculate translational force from a strain amount at one place of the cantilever beam, but it is not possible to calculate a moment. In contrast, it is possible to calculate both a moment and the translational force from strain amounts at two or more places. Therefore, according to the configuration illustrated in  FIG. 10 , X-direction translational force F x  acting on the strain element  1001  and a moment M x  around the X axis can be calculated on the basis of the X-direction strain amount detected at the two positions a and b, and similarly, Y-direction translational force F y  acting on the strain element  1001  and a moment M y  around the Y axis can be calculated on the basis of the Y-direction strain amounts detected at the two positions a and b. Therefore, it can be said that the force sensor  103  is equipped with a sensor having  4  degrees of freedom (DOF) of the moments M x  and M y  around the two axes in addition to the two-direction translational force F x  and F y . 
     In  FIG. 10  and  FIG. 11 , the strain element  1001  is illustrated to have a simple cylindrical shape to simplify the drawings. When the strain element  1001  has a structure suitable as a strain element, the detection performance as the  4 DOF sensor is improved. That is, in a case where the strain element  1001  is formed in a shape in which stress is concentrated at each of the two measurement positions a and b in the long axis direction and the strain element  1001  is easily deformed, the strain amounts can be easily measured by the strain detection elements  1011   a  to  1014   a  and  1011   b  to  1014   b , and improvement in the detection performance as the  4 DOF sensor is expected. 
     Furthermore, as the strain detection element, a capacitive sensor, a semiconductor strain gauge, a foil strain gauge, and the like are also widely known in this industry, and any of these can be used as the strain detection elements  1011   a  to  1014   a  and  1011   b  to  1014   b . Note, however, that in present embodiment, a fiber Bragg grating (FBG) sensor manufactured by utilizing optical fibers are used as the strain detection elements  1011   a  to  1014   a  and  1011   b  to  1014   b . 
     Here, the FBG sensor is a sensor formed by engraving diffraction gratings (gratings) along a long axis of each optical fiber, and it is possible to detect, as a wavelength change in reflection light relative to incident light in a predetermined wavelength band (Bragg wavelength), a change in an interval between the diffraction gratings caused by expansion or contraction along with a change in a strain or a temperature caused by the acting force. Then, the wavelength change detected from the FBG sensor can be converted into a strain, stress, and a temperature change which are to be causes. Since the FBG sensor utilizing the optical fibers has a small transmission loss (hardly carries noise from the outside), detection accuracy can be kept high even under an assumed usage environment. Furthermore, the FBG sensor has advantages of easily coping with sterilization necessary for medical care and coping with a strong magnetic field environment. 
     A description will be provided with reference to  FIG. 13  for a structure of the strain element  1001  that can be easily deformed at the two measurement positions a and b, and a method of installing, on the outer periphery of the strain element  1001 , the strain detection elements  1011   a  to  1014   a  and  1011   b  to  1014   b  utilizing the FBG sensors. 
       FIG. 13  illustrates a YZ cross section and a ZX cross section of the strain element  1001  respectively. In the drawing, the YZ cross section and the ZX cross section of the strain element  1001  are colored in gray. The strain element  1001  is, for example, hollow and has a rotationally symmetric shape around the long axis. The strain element  1001  has a structure in which a recess having a radius gradually reduced is provided in each of the different two measurement positions a and b in the long axis direction. Therefore, when force acts in at least one of directions X or Y, stress is concentrated at each of the two measurement positions a and b, and the strain element  1001  is easily deformed and can be used as a strain element. 
     The strain element  1001  is manufactured by using, for example, stainless steel (steel use stainless: SUS), a Co—Cr alloy, or a titanium material which are known as metal materials excellent in biocompatibility. For example, from a viewpoint of forming a strain element in a partial structure of the surgical robot  100 , it is preferable to manufacture the strain element  1001  using a material having mechanical characteristics, for example, a titanium alloy. The acting force to the end effector such as the gripping portion  101  can be measured with high sensitivity by using the low-rigidity material for the strain element  1001 . Furthermore, the titanium alloy is biocompatible and is also a preferable material in a case of use in medical practice such as a surgical operation. 
     A pair of optical fibers  1302  and  1304  are laid in the long axis direction on the facing sides in the Y direction of the outer periphery of the strain element  1001 . Similarly, a pair of optical fibers  1301  and  1303  are laid in the long axis direction on the facing sides in the X direction of the outer periphery of the strain element  1001 . In short, the four optical fibers  1301  to  1304  are laid in the entire strain element  1001 . 
     The optical fibers  1302  and  1304  laid on the facing sides in the Y direction have ranges that overlap with the two recesses of the strain element  1001  (or in the vicinity of the measurement positions a and b) and have the FBG sensors formed by engraving the diffraction gratings, and the FBG sensors are utilized as the strain detection elements  1012   a ,  1012   b ,  1014   a , and  1014   b , respectively. The portions including the FBG sensors in the optical fibers  1302  and  1304  are indicated by hatching in the drawing. 
     Furthermore, the respective optical fibers  1302  and  1304  are fixed to the outer periphery of the strain element  1001  with an adhesive or the like at both ends  1311  to  1313  and  1314  to  1316  of the portions including the FBG sensors  1012   a ,  1012   b ,  1014   a , and  1014   b  on the surface of the strain element  1001 . Therefore, when the external force acts and bends the strain element  1001  in the Y direction, the respective optical fibers  1302  and  1304  are also integrally deformed, and the portions of the FBG sensors, namely, the strain detection elements  1012   a ,  1012   b ,  1014   a , and  1014   b  are strained. 
     Similarly, the optical fibers  1301  and  1303  laid on the facing sides in the X direction have ranges that overlap with the two recesses of the strain element  1001  (or in the vicinity of the measurement positions a and b) and have the FBG sensors formed by engraving the diffraction gratings, and the FBG sensors are utilized as the strain detection elements  1011   a ,  1011   b ,  1013   a , and  1013   b , respectively. The portions including the FBG sensors in the optical fibers  1301  and  1303  are indicated by hatching in the drawing. 
     Furthermore, the respective optical fibers  1301  and  1301  are fixed to the outer periphery of the strain element  1001  with an adhesive or the like at both ends  1321  to  1323  and  1324  to  1326  of the portions including the FBG sensors  1011   a ,  1011   b ,  1013   a , and  1013   b  on the surface of the strain element  1001 . Therefore, when the external force acts and bends the strain element  1001  in the Y direction, the respective optical fibers  1301  and  1303  are also integrally deformed, and the portions of the FBG sensors, namely, the strain detection elements  1011   a ,  1011   b ,  1013   a , and  1013   b  are strained. 
     In  FIG. 13 , only the portions attached to the outer periphery of the strain element  1001  are illustrated out of the optical fibers  1301  to  1304  used as the strain detection elements  1011   a  to  1014   a  and  1011   b  to  1014   b , and other portions are not illustrated. Actually, it should be understood that each of these optical fibers  1301  to  1304  have a total length of, for example, about  400  millimeters and extend to a detection unit and a signal processing unit (both not illustrated). 
     The detection unit and the signal processing unit are disposed apart from the end effector, for example, in the vicinity of a base of the surgical robot  100 . The detection unit makes light of a predetermined wavelength (Bragg wavelength) enter each of the optical fibers  1301  to  1304 , receives reflection light thereof and detects a wavelength change AA. Then, the signal processing unit calculates two-direction translational force F x  and F y  and two-direction moments M x  and M y  acting on the gripping portion  101  on the basis of wavelength changes detected from the respective FBG sensors provided as the strain detection elements  1011   a  to  1014   a  and  1011   b  to  1014   b  respectively attached to the facing sides in each of the XY directions of the strain element  1001 . 
       FIG. 14  schematically illustrates a processing algorithm for the  4 DOF sensor to calculate, in a detection unit  1401  and a signal processing unit  1402 , the two-direction translational force F x  and F y  and the two-direction moments M x  and M y  acting on the gripping portion  101  provided as the end effector, on the basis of detection results obtained from the FBG sensors respectively formed on the optical fibers  1301  to  1304  laid in the strain element  1001 . 
     The detection unit  1401  detects, on the basis of the reflection light of the incident light in the predetermined wavelength band to each of the optical fibers  1301  to  1304  attached to the respective facing sides in the respective XY directions of the strain element  1001 , respective wavelength changes AAa 1  to AAa 4  in the respective FBG sensors as the strain detection elements  1011   a  to  1014   a  disposed at the position a of the strain element  1001  in a case where the translational force F x  and F y  and moments M x  and M y  act. Note, however, that each of the detected wavelength changes AAa 1  to AAa 4  also include a wavelength change component caused by a temperature change. 
     Furthermore, the detection unit  1401  detects, on the basis of the reflection light of the incident light in the predetermined wave length to each of the optical fibers  1301  to  1304  attached to the respective facing sides in the respective XY directions of the strain element  1001 , respective wavelength changes AAb 1  to AAb 4  in the respective FBG sensors as the strain detection elements  1011   b  to  1014   b  disposed at the position b of the strain element  1001  in a case where the translational force F x  and F y  and moments M x  and M y  act. Note, however, that each of the detected wavelength changes AAb 1  to AAb 4  also includes a wavelength change component caused by a temperature change. 
     Here, the wavelength changes AAa 1  to AAa 4  detected by the detection unit  1401  from the respective optical fibers  1301  to  1304  at the position a are respectively equivalent to strain amounts Δεa 1  to Δεa 4  generated at the position a of the strain element  1001  when the translational force F x  and F y  and moments M x  and M y  act. Furthermore, the wavelength changes Δλb 1  to Δλb 4  detected by the detection unit  1401  from the optical fibers  1301  to  1304  at the position b are respectively equivalent to strain amounts Δεb 1  to Δεb 4  generated at the position b of the strain element  1001  when the translational force F x  and F y  and moments M x  and M y  act (note, however, that this is a case where a wavelength change component caused by a temperature change is ignored). 
     A differential mode unit  1403  subtracts an average value of these eight inputs from each of the above-described eight inputs Δλa 1  to Δλa 4  and Δλb 1  to Δλb 4  received from the detection unit in accordance with Expression (4) below, and outputs obtained values to a latter translational force/moment deriving unit  1404 . Each of the wavelength changes detected at the respective positions a and b includes a wavelength change component Δλ temp  caused by a temperature change together with a wavelength change component caused by strain due to action of translational force F x  and F y  and the moments M x  and M y . The differential mode unit  1403  can cancel the wavelength change component Δλ temp  caused by the temperature change. 
     
       
         
           
             
               
                 
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     Then, the translational force/moment deriving unit  1404  multiplies Δλ diff  received from the differential mode unit  1403  by a calibration matrix K as represented by Expression (5) below and calculates the translational force F x  and F y  and the moments M x  and M y . 
     
       
         
           
             
               
                 
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     Note that the signal processing unit  1402  illustrated in  FIG. 14  and the calibration matrix K used in the calculation in Expression (5) can be derived from, for example, a calibration experiment. In the present embodiment, the force sensor  103  is arranged in the region located between the actuator unit  102  and the proximal end and free from acting of the traction force to generate the gripping force of the gripping portion  101  (see  FIG. 1 ). Therefore, since the traction force of the actuator unit  102  does not interfere with the external force applied in the long axis direction of the end effector, the calibration matrix can be easily calculated. 
     For example, in a case where the surgical robot  100  operates as a slave device in the master-slave robot system, a detection result from the force sensor  103  of the 4DOF described above is transmitted to a master device as feedback information in response to remote control. The feedback information can be utilized on the master device side for various purposes. For example, the master device can perform force sense presentation for an operator on the basis of the feedback information from the slave device. This presentation can contribute to achievement of minimal invasive endoscopic treatment. 
     INDUSTRIAL APPLICABILITY 
     As described above, the technology disclosed in the present specification has been described in detail with reference to the specific embodiment. However, it is obvious that those skilled in the art can make modifications and substitutions of the embodiment without departing from the scope of the technology disclosed in the present specification. 
     The application range of the actuator device and the end effector proposed in the present specification is not limited to the gripping purpose. For example, the large gripping force can be generated with small traction force by applying the actuator device and the end effector proposed in the present specification to various situations where large gripping force is desired to be obtained when an open/close angle is small, such as a stationery (scissors or clips) and a work tool (piers or nippers). 
     Furthermore, the embodiment related to the end effector to which the pair of surgical forceps including the pair of blades coupled in the openable/closable manner is applied has been mainly described in the present specification, but the application range of the technology disclosed in the present specification is not limited thereto. As the end effector, not only the forceps but also a medical instrument such as a pair of tweezers or a cutting instrument that contacts a patient during a surgical operation, or an imaging device such as an endoscope or a microscope may be attached. Furthermore, the pressurizing portion is not limited to the elastic member as far as force in the opposite direction of the predetermined direction can be applied. For example, a magnet that generates attraction force in the opposite direction may be used. 
     In short, the technology disclosed in the present specification has been described with the embodiment as exemplified, but the content of the present specification should not be interpreted in a limited manner. The scope of the technology disclosed in the present specification should be determined in consideration of the claims. 
     Note that the technology disclosed in the present specification can also have the following configurations. 
     (1) An actuator device including: 
     a first magnetic body portion; 
     a first system movable in a predetermined direction or an opposite direction of the predetermined direction; 
     a second system including a second magnetic body portion that moves the first system in the predetermined direction by magnetic force generated between the second magnetic body portion and the first magnetic body portion, and a pressurizing portion capable of applying, to the first system, force in the opposite direction of the predetermined direction; and 
     a driving unit capable of applying, to the second system, force in the predetermined direction or the opposite direction by driving. 
     (2) The actuator device recited in (1) above, in which 
     the pressurizing portion includes an elastic portion. 
     (3) The actuator device recited in (2) above, in which 
     the more the first system is drawn in the predetermined direction, the more the force in the opposite direction of the elastic portion is increased. 
     (4) The actuator device recited in (3) above, in which 
     the first system includes a supporting portion configured to support an acting portion that acts by a reciprocating motion in the predetermined direction. 
     (5) The actuator device recited in (4) above, in which 
     the second system includes a sliding portion connected to the supporting portion via the elastic portion. 
     (6) The actuator device recited in (5) above, in which 
     the sliding portion has one surface that is oriented in a direction parallel to the predetermined direction and connected to the elastic portion, has the other surface connected to the second magnetic body portion, and is relatively movable in the direction parallel to the predetermined direction by being driving of the driving unit. 
     (7) The actuator device recited in (6) above, in which 
     the supporting portion has a hollow structure, and 
     the sliding portion is housed inside the hollow structure and is relatively movable in the direction parallel to the predetermined direction. 
     (8) The actuator device recited in any one of (1) to (7) above, in which 
     the driving unit includes a dielectric elastomer. 
     (9) The actuator device recited in any one of (1) to (8), in which 
     in a state where the first system is positioned closest to the magnetic body portion, attraction force by magnetic force of the first magnetic body portion and magnetic force of the second magnetic body portion is larger than restoring force of the elastic portion. 
     (10) The actuator device recited in any one of (2) to (9) above, in which 
     in a case where the second system separates the first system from the first magnetic body portion, the driving unit generates driving force in the opposite direction of the predetermined direction, the driving force being larger than a difference between attraction force by magnetic force of the first magnetic body portion and restoring force of the elastic portion. 
     (11) The actuator device recited in (4) above, further including 
     a gripping portion that is opened or closed by the reciprocating motion of the acting portion in the predetermined direction. 
     (12) An end effector including: 
     a gripping portion; and an actuator unit that generates traction force to the gripping portion, in which 
     the actuator unit includes 
     a first magnetic body portion, 
     a first system movable in a predetermined direction or an opposite direction of the predetermined direction, 
     a second system including a second magnetic body portion that moves the first system in the predetermined direction by magnetic force generated between the second magnetic body portion and the first magnetic body portion, and a pressurizing portion capable of applying, to the first system, force in the opposite direction of the predetermined direction, and 
     a driving unit capable of applying, to the second system, force in the predetermined direction or the opposite direction by driving. 
     (13) The end effector recite in (12) above, in which 
     the first system includes a supporting portion configured to support an acting portion that causes force in the predetermined direction to act on a gripping portion, and a magnetic body portion that sucks, by magnetic force, the supporting portion in the predetermined direction, and 
     the second system includes the sliding portion connected to the supporting portion via an elastic portion, and a driving unit that drives the sliding portion in a direction parallel to the predetermined direction. 
     (14) The end effector recited in (12) or (13) above, in which 
     the gripping portion converts the traction force in a linear movement direction into gripping force. 
     (15) The end effector recited in any one of (12) to (14), in which 
     the gripping portion includes a pair of surgical forceps or another surgical tool. 
     (16) A surgical system including: 
     an end effector; 
     an actuator unit that generates traction force to the end effector; and 
     a force sensor arranged closer to a proximal end side than the actuator unit. 
     (17) A surgical system including: 
     an end effector; and an actuator unit that generates traction force to the end effector, 
     in which the actuator unit includes 
     a first system that is sucked by magnetic force of a magnetic body portion and moves, in a predetermined direction, an acting portion that causes the traction force to act on the gripping portion, and 
     a second system that applies, to the first system, force in an opposite direction of the predetermined direction, and separates the first system from the magnetic body portion. 
     (18) A surgical robot recited in (17) above, in which 
     the first system includes a supporting portion configured to support the acting portion that causes force in the predetermined direction to act on a gripping portion, and a magnetic body portion that sucks, by magnetic force, the supporting portion in the predetermined direction, and 
     the second system includes the sliding portion connected to the supporting portion via an elastic portion, and a driving unit that drives the sliding portion in a direction parallel to the predetermined direction. 
     (19) The surgical system recited in any one of (16) to (18) above, further including 
     a force sensor arranged closer to a proximal end side than the actuator unit. 
     (20) The surgical system recited in (15) or (19), in which 
     the force sensor includes a strain detection element that detects strain of a strain element and includes an FBG sensor. 
     REFERENCE SIGNS LIST 
     
         
           100  Surgical robot 
           101  Gripping portion 
           101   a ,  101   b  Blade 
           102  Actuator unit 
           103  Force sensor 
           104  Bend portion 
           301  Acting portion 
           302  Supporting portion 
           303  Sliding portion 
           304  Elastic portion 
           305  Driving unit (DEA) 
           306  Magnetic body portion 
           307  Second magnetic body portion 
           310  Housing 
           311  Wire 
           1001  Strain element 
           1011  to  1014  Strain detection element (FBG sensor) 
           1301  to  1304  Optical fiber 
           1401  Detection unit 
           1402  Signal processing unit 
           1403  Difference mode unit 
           1404  Translational force/moment deriving unit