Abstract:
A robot arm including a force sensing apparatus capable of accurately sensing of a force in the axial direction of the robot arm during operation thereof, without being affected by a motion of the robot arm. The robot arm includes: a body that is elastically deformable and has a pipe form extending in an axial direction; an instrument connected to an operational end of the body; a cable that is connected to a terminal end of the instrument and controls an operation of the instrument; and a force sensing apparatus that is attached to a surface of the body and senses a force acting upon the body, wherein the cable at the terminal end of the instrument is configured to move along a direction perpendicular to the axial direction of the body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of Korean Patent Application No. 10-2011-0112876, filed on Nov. 1, 2011, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Example embodiments of the following disclosure relate to a robot arm, including a force sensing apparatus, and more particularly, to a robot arm including a force sensing apparatus capable of accurately sensing a force in an axial direction of the robot arm, without being affected by a motion of the robot arm. 
         [0004]    2. Description of the Related Art 
         [0005]    Robots for diverse uses have been developed and commercialized in line with the advances in robot technology. For example, a remote-controlled surgery robot connected to a surgical part of a patient assists a surgeon in easily conducting minute surgical operations by viewing the surgical part remotely on an endoscope screen. By using the surgery robot, handshake that is caused as the surgeon moves his/her hand may be compensated for, and a remote surgical arm may scale-down and reproduce the motion of the surgeon&#39;s hand, and thus, precise operations may be conducted. 
         [0006]    However, currently commercialized surgery robots provide only image information about a surgical part via an endoscope, and touch information which can be obtained in typical surgical operations is not provided. That is, compared to a surgical operation actually performed by using the hands, it is difficult for a surgeon to accurately figure out the amount of force applied to a surgical part by a surgical instrument attached to a surgery robot arm. Thus, if information about intensity of a contact of the surgical instrument, attached to the surgery robot arm, to the surgical part is provided to the surgeon, the information about the contact together with the image information may be helpful for minute operations needed for cutting, cauterization, suture, and the like, of the surgical part. To this end, it is important to accurately measure forces acting between the surgical instrument attached to the surgery robot arm and the surgical part. 
         [0007]    Accordingly, installment of a delicate force sensing apparatus at an operational end of a surgery robot has been researched. However, currently suggested force sensing apparatuses are not capable of accurately measuring forces in each direction or it is difficult to mount force sensing apparatuses on an arm portion of a surgery robot in a limited space. Additionally, such force sensing apparatuses may malfunction in an environment where a strong electromagnetic field exists. In particular, it is difficult to sense a force acting in an axial direction of a robot arm. In addition, while grippers installed at the operational end of the surgery robot or the like are operating, a reaction force may be generated in the axial direction of the robot arm, which makes it even more difficult to sense a force in the axial direction. 
       SUMMARY 
       [0008]    Provided is a robot arm including a force sensing apparatus capable of accurately sensing a force in an axial direction of the robot arm without being affected by a motion of the robot arm. 
         [0009]    Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
         [0010]    According to an aspect of the present disclosure, a robot arm includes: a body that is elastically deformable and has a pipe form extending in an axial direction; an instrument connected to an operational end of the body; a cable that is connected to a terminal end of the instrument and controls an operation of the instrument; and a force sensing apparatus that is attached to a surface of the body and senses a force action on the body, wherein the cable at the terminal end of the instrument is configured to move along a direction perpendicular to an axial direction of the body. 
         [0011]    The cable may include a pair of cables configured to move at the terminal end of the instrument along the direction perpendicular to the axial direction of the body in opposite directions to each other. 
         [0012]    The robot arm may further include a cover covering the body at the operational end of the body, wherein the instrument is pivotally installed to the cover. 
         [0013]    The instrument may include a pair of grippers, and the cover may include a hinge that pivots the pair of grippers and a torsion spring that is coupled to the hinge so as to provide an elastic force to the pair of grippers in a direction in which the pair of grippers split apart. 
         [0014]    The robot arm may further include: a base frame arranged in the body; and a direction conversion unit that is installed on the base frame and converts a direction of the cable which is parallel to the axial direction to a direction perpendicular to the axial direction. 
         [0015]    The cable may include a pair of cables configured to move at the terminal end of the instrument along the direction perpendicular to the axial of the body in opposite directions to each other, wherein the instrument includes a pair of grippers respectively connected to the pair of cables, wherein the pair of grippers includes a first grippers and a second grippers, and the pair of cables includes a first cable and a second cable, and the direction conversion unit includes a first direction conversion unit and a second direction conversion unit, and the first cable is engaged with the first direction conversion unit to be coupled to a terminal end of the second grippers, and the second cable is engaged with the second direction conversion unit to be coupled to a terminal end of the first grippers, wherein the first direction conversion unit is disposed opposite to the terminal end of the second grippers with respect to a center of the body, and the second direction conversion unit is disposed opposite to the terminal end of the first grippers with respect to the center of the body. 
         [0016]    The first cable may be arranged to move in a direction perpendicular to the axial direction of the body between the first direction conversion unit and the terminal end of the second grippers, and the second cable is arranged to move in the direction perpendicular to the axial of the body between the second direction conversion unit and the terminal end of the first grippers, and the first cable and the second cable are arranged to simultaneously move in opposite directions. 
         [0017]    Each of the first and second direction conversion units may include at least one pulley. 
         [0018]    Each of the first and second direction conversion units may include two pulleys, and the first and second cables may be wound around the two pulleys in opposite directions to each other. 
         [0019]    The force sensing apparatus may include: at least one fiber Bragg gratings (FBGs) attached to the body; a light source providing light to each of the FBGs; and a light detector detecting light reflected by each of the FBGs or light that has passed through each of the FBGs. 
         [0020]    In addition, the force sensing apparatus may include: at least three fiber Bragg gratings (FBGs) attached on the surface of the body; a light source providing light to each of the FBGs; and a light detector detecting light reflected by each of the FBGs or light that has passed through each of the FBGs. 
         [0021]    The at least three FBGs may be arranged to extend in the axial direction of the body. 
         [0022]    The at least three FBGs may be attached to the surface of the body at at least three different positions at predetermined intervals along an azimuth angle direction. 
         [0023]    The force sensing apparatus may further include at least three openings that are formed in the body between each two adjacent FBGs. 
         [0024]    When an operational end of the force sensing apparatus at which the instrument is installed is assumed to be an upper portion of the body, a center of the openings may be arranged at a position lower than a center of the FBGs between each two adjacent FBGs. 
         [0025]    The force sensing apparatus may further include an adhesive to adhere the FBGs to the body, and the adhesive may be coated on the body so as to cover the FBGs overall. 
         [0026]    The body may include: an upper portion and a lower portion separated from each other; at least three elastic beams that connect the upper portion and the lower portion of the body and extend in a direction perpendicular to an axial direction of the body; and a plurality of gaps respectively formed between each of the elastic beams and the upper portion of the body and between each of the elastic beams and the lower portion of the body. 
         [0027]    A first end portion of each of the elastic beams may be connected to the upper portion of the body, and a second end portion of each of the elastic beams disposed opposite the first end portion may be connected to the lower portion of the body. 
         [0028]    The body may further include at least three stoppers respectively formed in spaces between each two adjacent elastic beams. 
         [0029]    The stopper may include: a first protrusion that protrudes and extends from the lower portion of the body toward the upper portion of the body in the axial direction of the body; and a second protrusion that protrudes and extends from the upper portion of the body toward the lower portion of the body in the axis direction of the body to surround the first protrusion. 
         [0030]    The first protrusion may include an intermediate portion having a relatively small width and an end portion having a relatively large width, and the second protrusion may include an intermediate portion having a relatively small width and an end portion having a relatively large width, wherein the first protrusion and the second protrusion are engaged with each other in a complementary form. 
         [0031]    The FBGs may be attached to the body across the stoppers. 
         [0032]    The FBGs may be attached to the body across the elastic beam. 
         [0033]    The robot arm may further include an adhesive to attach the FBGs to the body, wherein the adhesive is coated on the body to correspond only to two end portions of the FBGs. 
         [0034]    According to an aspect of the present disclosure, a system includes a robot arm, including a force sensing apparatus; and an instrument connected to the robot arm; wherein the robot arm comprises: a body that is elastically deformable and extends in an axial direction; and a cable that is connected to the instrument and controls an operation of the instrument, wherein the force sensing apparatus is attached to the body and senses a force exerted upon the body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0036]      FIG. 1  is a conceptual diagram illustrating an operational principle of a robot arm, according to an example embodiment; 
           [0037]      FIG. 2  is a schematic view illustrating a configuration and operation of direction conversion units illustrated in  FIG. 1 ; 
           [0038]      FIG. 3  is a schematic perspective of the robot arm illustrated in  FIGS. 1 and 2 , according to an example embodiment; 
           [0039]      FIG. 4  is a schematic perspective view illustrating a force sensing apparatus included in the robot arm illustrated in  FIG. 3 , according to an example embodiment; 
           [0040]      FIG. 5  is a schematic view illustrating a fiber Bragg grating (FBG) illustrated in  FIG. 4 , which is attached to a body of the force sensing apparatus of  FIG. 4 , according to an example embodiment; 
           [0041]      FIG. 6  is a schematic perspective view illustrating a force sensing apparatus, according to another example embodiment; and 
           [0042]      FIG. 7  is a schematic view illustrating a FBG illustrated in  FIG. 6 , which is attached to a body of the force sensing apparatus of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout and sizes of elements may be exaggerated for clarity and convenience of description. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
         [0044]      FIG. 1  is a conceptual diagram illustrating an operational principle of a robot arm  10 , according to an example embodiment. Referring to  FIG. 1 , the robot arm  10  includes a body  11  which has a pipe form and extends in an axial direction, a pair of grippers  25  and  26  that are pivotally installed at an operational end of the body  11 , and a pair of cables  23  and  24  respectively connected to terminal ends of the grippers  25  and  26  to control operations of the grippers  25  and  26 . In addition, a cover  30 , covering the body  11 , may be further disposed at the operational end of the body  11 . Thus, the grippers  25  and  26  may be pivotally installed on the cover  30  via a hinge  27 . Although  FIG. 1  exemplarily shows that the grippers  25  and  26  are installed at the operational end of the body  11 , other various instruments may also be connected to the operational end of the body  11 . For example, a surgical knife, scissors, a suction device, a compact camera, a cautery device, or the like, may be pivotally connected to the operational end of the body  11 . Thus, the grippers  25  and  26  are just an example presented for convenience of description, and the current embodiment is not limited thereto. In addition, although not illustrated in  FIG. 1 , a force sensing apparatus, which is to be described later may be installed at the body  11 . 
         [0045]    A torsion spring  28  which is coupled to the grippers  25  and  26  may be installed at the hinge  27 . Since the torsion spring  28  provides an elastic force in a direction in which the grippers  25  and  26  split apart, one pair of the grippers  25  and  26  may be in a normally opened state. Accordingly, by pulling one pair of cables  23  and  24 , the grippers  25  and  26  are closed to grip an object, such as, a surgical instrument, and by releasing the cables  23  and  24 , the grippers  25  and  26  are opened again. 
         [0046]    According to the current embodiment, in order to obtain a large grip force by pulling the cables  23  and  24  even with a small force, it is preferable that a distance between a pivotal axis of the grippers  25  and  26  and a point of action of the cables  23  and  24  is longer. That is, it may be preferable if a connection part between the cables  23  and  24  and the grippers  25  and  26  is farther from the hinge  27 . In addition, when the cables  23  and  24  pull the grippers  25  and  26 , a tension generated between the cables  23  and  24  and the grippers  25  and  26  acts on the body  11  and the body  11  may be deformed. In this case, the deformation of the body  11  may affect a sensing result of a force sensing apparatus that senses an external force acting on the body  11 . In particular, if a tension acts in an axial direction of the body  11 , a large distortion may be caused when sensing an external force that acts in the axial direction of the body  11 . 
         [0047]    According to the current embodiment, the cables  23  and  24  and the grippers  25  and  26  of the robot arm  10  are designed in consideration of the above-described details. For example, a base frame  22  is disposed in the body  11 , and direction conversion units  20  and  21  that convert movement directions of the cables  23  and  24  from parallel to the axial direction into perpendicular to the axial direction are arranged on the base frame  22 . The cables  23  and  24  are engaged with the direction conversion units  20  and  21 , and are respectively coupled to the terminal ends of the corresponding grippers  25  and  26 . For example, after being engaged with the first direction conversion unit  20 , the first cable  23  is coupled to the terminal end of the second grippers  26 ; also, after being engaged with the second direction conversion unit  21 , the second cable  24  is coupled to the terminal end of the first grippers  25 . The first cable  23  and the first direction conversion unit  20  may be arranged opposite to the terminal end of the second grippers  26  with respect to a center of the body  11 . Further, the second cable  24  and the second direction conversion unit  21  may be arranged opposite to the terminal end of the first grippers  25  with respect to the center of the body  11 . Accordingly, a connection portion between the first and second cables  23  and  24  and the first and second grippers  25  and  26  may be distanced apart from the hinge  27  as far as possible. 
         [0048]    In addition, the first and second cables  23  and  24  may move between the direction conversion units  20  and  21  and the first and second grippers  25  and  26  in a direction perpendicular to the axial direction of the body  11 . Accordingly, as the robot arm  10  operates, tension of the cables  23  and  24  works only in a direction perpendicular to the axial direction of the body  11 , and does not work in the axial direction. Accordingly, distortion is not generated in results of force sensing in the axial direction when the force sensing apparatus, which is to be described later senses a force acting on the body  11 . Moreover, since the first and second cables  23  and  24  move in opposite directions to each other at the same time between the direction conversion units  20  and  21  and the terminal ends of the first and second grippers  25  and  26 , tensions of the first and second cables  23  and  24  working in the direction perpendicular to the axis of the body  11  may offset each other. Accordingly, the tensions of the first and second cables  23  and  24  hardly work in the axial direction or in the direction perpendicular to the axial direction during an operation of the robot arm  10 , and thus, a force acting on the body  11  may be accurately sensed. 
         [0049]    The direction conversion units  20  and  21  may be each formed by at least one pulley.  FIG. 2  is a schematic view illustrating a configuration and operation of the direction conversion units  20  and  21  illustrated in  FIG. 1 . While  FIG. 1  illustrates that the two direction conversion units  20  and  21  and the first and second cables  23  and  24  are disposed on the same plane of the base frame  22 , both the direction conversion units  20  and  21  and the first and second cables  23  and  24  may be respectively arranged on two surfaces of the base frame  22  as illustrated in  FIG. 2 . For example, the first cable  23  and the first direction conversion unit  20  related to the second grippers  26  are illustrated in  FIG. 2 . In  FIG. 2 , the body  11  is not illustrated for convenience of illustration. 
         [0050]    Referring to  FIG. 2 , the first cable  23  is wound around first and second pulleys  20   a  and  20   b  and a movement direction of the first cable  23  may be changed by using the first and second pulleys  20   a  and  20   b.  For example, the first cable  23  is respectively wound around the first pulley  20   a  and the second pulley  20   b  in opposite directions, and a terminal end of the first cable  23  may be coupled to a terminal end  26   b  of the second grippers  26  via the second pulley  20   b.  While the second grippers  26  is opened by the torsion spring  28 , the terminal end  26   b  of the second grippers  26  and the second pulley  20   b  may be disposed at opposite ends to each other with respect to the center of the body  11 , and the first cable  23  may be arranged in a direction perpendicular to the axial direction between the terminal end  26   b  of the second grippers  26  and the second pulley  26   b.  In this configuration, by pulling the first cable  23  in the axial direction, the first pulley  20   a  and the second pulley  20   b  rotate in opposite directions, and the first cable  23  moves to the right in  FIG. 2  in the direction perpendicular to the axial direction between the terminal end  26   b  of the second grippers  26  and the second pulley  20   b.  Then, the grippers  26  are closed, and the terminal end  26   b  of the second grippers  26 , which is on the side of an object, may contact the object. Meanwhile, the second cable  24  disposed on the opposite side of the base frame  22 , illustrated in  FIG. 2 , may move to the left in  FIG. 2  in the direction perpendicular to the axial direction, according to the above-described principle. Accordingly, no tension is generated to the first and second cables  23  and  24  in the axial direction, and tensions generated in the first and second cables  23  and  24  in the direction perpendicular to the axial direction are in opposite directions, and thus, may offset each other. 
         [0051]      FIG. 3  is a schematic perspective view illustrating the robot arm  10  illustrated in  FIGS. 1 and 2 , according to an example embodiment. Referring to  FIG. 3 , for example, the robot arm  10  has the body  11  having a cylindrical form, and the cover  30  and the first and second grippers  25  and  26  may be installed at the operational end of the body  11 . Components, such as, the first and second cables  23  and  24  and the first and second direction conversion units  20  and  21  may be installed in the body  11 . Although not illustrated in the drawings, a lower portion of the robot arm  10  may be coupled to other joint portion of a robot. Further, a force sensing apparatus which includes a fiber Bragg grating (FBG)  13  and will be described later may be further installed at the body  11 . The force sensing apparatus may measure, for example, a force acting between a surgical instrument picked up by the first and second grippers  25  and  26  and a living tissue, and may return a value of the measured force to an operator of a surgery robot. 
         [0052]      FIG. 4  is a schematic perspective view illustrating a force sensing apparatus included in the robot arm  10  illustrated in  FIG. 3 , according to an example embodiment. Referring to  FIG. 4 , the force sensing apparatus may comprise, for example, at least three FBGs  13  that are attached on a surface of the body  11  and extend in the axial direction of the body  11  (i.e., a z-axis direction), a light source  32  supplying light to each of the FBGs  13 , and a light detector  33  that detects light reflected by each of the FBGs  13  or light that has passed through each of the FBGs  13 . The FBGs  13  may be formed of thin optical fibers. Accordingly, to easily attach the FBGs  13  to the body  11 , a groove  12  in which the FBGs  13  are safely mounted may be further formed in the surface of the body  11  in the axial direction. The fact that the FBGs  13  extend in the axial direction of the body  11  does not necessarily mean that the FBGs  13  are parallel to the axial direction of the body  11 , but may indicate that an arrangement direction of the FBGs  13  contains an axial directional component of the body  11 . 
         [0053]    The body  11  may be formed of a material which is capable of sensitively generating elastic deformation in response to even a small force. For example, the body  11  may be formed of a plastic material having a large degree of elastic deformation, such as, polypropylene (PP). In addition, although the body  11  illustrated in  FIG. 4  has a cylindrical shape, this is just an example, and is not limited thereto. For example, the body  11  may also have a polygonal cylinder shape. In addition, bolt holes  14  may be formed in upper and lower portions of the body  11  in order to fix the body  11  to other components of the robot arm  10 . 
         [0054]    The FBGs  13  may be attached to the body  11  by using an adhesive  40  (refer to  FIG. 5 ). According to the embodiment of  FIG. 4 , when a force acts on the body  11 , the entire body  11  may be elastically deformed. Accordingly, in order for the FBGs  13  to deform in the same way as the body  11 , the body  11  may be coated with the adhesive  40 , such that the FBGs  13  are covered by the adhesive  40  overall, as illustrated in  FIG. 5 . 
         [0055]    In this configuration, the force sensing apparatus may measure degrees of tension and compression of the FBGs  13  to calculate an intensity and direction of forces acting upon the body  11 . The FBGs  13  are formed by arranging a grating  13   a  (see  FIG. 5 ) whose refractive index periodically changes inside an optical fiber. For example, the FBGs  13  may be formed by alternately and repeatedly arranging two different materials having different refractive indices in a core of an optical fiber. Due to the grating  13   a,  light of a predetermined wavelength among light that travels inside the FBGs  13  is reflected. A wavelength of the reflected light may vary according to an arrangement period of the grating  13   a.  When the FBGs  13  expand or shrink, the arrangement period of the grating  13   a  also expands or shrinks, and thus, the wavelength of reflected light also varies. Accordingly, by measuring the wavelength of light reflected by the grating  13   a  at an input end of the FBGs  13  or by measuring a wavelength of light that passes through the FBGs  13  at an output end of the FBGs  13 , a degree by which the FBGs  13  expand or shrink may be accurately measured. In addition, when the FBGs  13  are attached along the axial direction of the body  11 , the FBGs  13  also expand or shrink according to a degree of tension or compression of the body  11 . Thus, by measuring light reflected by or light that passed through the FBGs  13 , the degree of tension or compression of the body  11  may be accurately calculated. 
         [0056]    The force sensing apparatus may further include a light transfer member  31  that transfers light emitted from the light source  32  to the FBG  13  and transfers light output from the FBG  13  to the light detector  33 . The light transfer member  31  may be, for example, an optical fiber. The light source  32  and the light detector  33  may be not directly attached to the body  11 , but may be connected to a computer (not shown) of a user or an exclusive calculation circuit (not shown) via the light transfer member  31 . Although the light detector  33  is illustrated in  FIG. 4  as being disposed both at the input end and the output end of the FBG  13 , the light detector  33  may be also disposed only at one of the input end and the output end of the FBG  13 . 
         [0057]    To measure degrees of tension and compression of the body  11  in at least three directions, at least three FBGs  13  may be attached at at least three different positions on the surface of the body  11 . In particular, in order to compensate for an error caused by temperature variation or the like, at least four FBGs  13  may be attached on the surface of the body  11 . Although the light source  32  and the light detector  33  are illustrated as being connected only to the single FBG  13  in  FIG. 4  for convenience of illustration, the current embodiment is not limited thereto. For example, the light source  32  and the light detector  33  may also be arranged at each of the plurality of FBGs  13 . The plurality of FBGs  13  may be arranged at predetermined intervals in an azimuth angle direction. For example, when four FBGs  13  are used, the FBGs  13  may be arranged at intervals of  90  degrees along the azimuth angle direction. 
         [0058]    Although the FBGs  13  are illustrated in  FIG. 4  as being attached to an external surface of the body  11 , the current embodiment is not limited thereto. The FBGs  13  may also be disposed on an inner surface of the body  11 . In this case, the light transfer member  31 , the light source  32 , and the light detector  33  may also be arranged in an inner space of the body  11  with other components. 
         [0059]    In order to increase the performance of the force sensing apparatus (for example, resolution, measurement range, or error rate), the body  11  may have a large elastic deformation ratio and a small sensitivity difference according to a direction in which a force acts upon the body  11 . For example, it is better when a difference between a sensitivity with respect to a force acting in the axial direction (z-axis direction) and a sensitivity with respect to a force acting in a direction perpendicular to the axial direction (x-axis or y-axis direction) is smaller. If the difference between sensitivities is large in each direction, an error in calculating the force may increase. In consideration of the above point, as illustrated in  FIG. 4 , a plurality of openings  15  may be formed in the body  11  between each two adjacent FBGs  13 . For example, assuming that a force acts upon an upper end of the body  11 , the FBGs  13  may be arranged in a portion where the most deformation of the body  11  occurs, and the openings  15  may be formed in a portion where the least deformation of the body  11  occurs. As the openings  15  are formed, deformation occurring in other portions of the body  11  may be further increased. In addition, the openings  15  may be arranged, so as to minimize a difference between deformation in the axis direction (for example, sensitivity in a z-axis direction) and deformation (for example, sensitivity in the x-axis direction and the y-axis direction) in the direction perpendicular to the axis direction. 
         [0060]    For example, the openings  15  may be arranged at an angle of  45  degrees from the bolt holes  14 , respectively arranged in upper and lower portions of the body  11 . That is, an extension line between a center of each of the bolt holes  14  and a center of each of the openings  15  may be inclined at  45  degrees from a horizontal plane. In this case, among spaces between two respectively adjacent openings  15 , largest deformation may occur in an upper portion of the body  11 , upon which a force works. Accordingly, the FBGs  13  may be arranged at upper portions of the body  11  between the two respective openings  15  where the largest deformation occurs. Further, to describe the position of the openings  15  with respect to each of the FBGs  13 , the center of the openings  15  may be arranged at a position lower than a center of each of the FBGs  13  between each two adjacent FBGs  13 . The lower position is defined by assuming the operational end of the force sensing apparatus as the upper end of the body  11 . The openings  15  may be arranged in respective spaces between the FBGs  13 . For example, when four FBGs  13  are used, four openings  15  may be formed in the body  11 . Thus, it is possible to increase the sensitivity of the force sensing apparatus, and also a difference in sensitivities in each direction may be minimized at the same time. 
         [0061]      FIG. 6  is a schematic perspective view illustrating a structure of a force sensing apparatus, according to another example embodiment. According to the embodiment of  FIG. 6 , a plurality of thin elastic beams  17  that connect upper and lower portions of the body  11  are formed instead of the openings  15  in order to improve the elastic deformation of the body  11 . Referring to  FIG. 6 , the body  11  is divided into an upper portion  11   a  and a lower portion  11   b  that are separated from each other by the thin elastic beams  17  formed in a direction perpendicular to an axial direction. The upper portion  11   a  and the lower portion  11   b  are connected to each other by using the elastic beams  17 . Referring to  FIG. 6 , a right end of the elastic beam  17  is connected to the upper portion  11   a,  and a left end of the elastic beam  17  is connected to the lower portion  11   b.  However, this is just an example, and thus, the current embodiment is not limited thereto. For example, the right end of the elastic beam  17  may be connected to the lower portion  11   b,  and the left end of the elastic beam  17  may be connected to the upper portion  11   a.  Spaces between the elastic beam  17  and the lower portion  11   b  and between the elastic beam  17  and the upper portion  11   a  may be cut to form a gap  16  in each space. For example, the elastic beam  17 , the upper portion  11   a,  and the lower portion  11   b  may be formed by partially cutting the single body  11  by using a wire electrical discharge machining (wire EDM) method. 
         [0062]    Each of the elastic beams  17  may be arranged between each two adjacent FBGs  13 . For example, if four FBGs  13  are used, four elastic beams  17  may be formed in the body  11 . Accordingly, when a force acts upon the body  11 , the gap  16  between the elastic beam  17  and the upper portion  11   a  and the gap  16  between the elastic beam  17  and the lower portion  11   b  may easily enlarge, and thus, an elastic deformation of the body  11  may further increase. According to the current embodiment, to prevent the elastic beams  17  from being easily damaged, the body  11  may be formed of a metal having a good rigidity such as titanium (Ti); however, the material forming the body is not limited thereto. 
         [0063]    In addition, the force sensing apparatus illustrated in  FIG. 6  may further include a stopper  18  formed between two adjacent elastic beams  17  to prevent an excessive deformation of the elastic beams  17 . The stopper  18  may be formed of the upper portion  11  a and the lower portion  11   b  of the body  11  that is divided by the gap  16 . For example, the stopper  18  may be formed of a first protrusion  18   a  that protrudes and extended from the lower portion  11   b  toward the upper portion  11   a  in the axis direction and a second protrusion  18   b  that is protruded and extended from the upper portion  11   a  toward the lower portion  11   b  in the axis direction so as to surround the first protrusion  18   a  of the lower portion  11   b.  The first protrusion  18   a  of the lower portion  11   b  may have an intermediate portion having a relatively small width and an end portion having a relatively large width, and the second protrusion  18   b  of the upper portion  11   a  may also have an intermediate portion having a relatively small width and an end portion having a relatively large width. That is, the first protrusion  18   a  of the lower portion  11   b  and the second protrusion  18   b  of the upper portion  11   a  may be formed in a complementary engagement. 
         [0064]    For example, the body  11  may be divided into the upper portion  11   a  and the lower portion  11   b  by using the gap  16  that is bent in the form of ‘Ω’ to form the second protrusion  18   b  and the first protrusion  18   a  in a complementary form. The first protrusion  18   a  and the second protrusion  18   b  are formed to be engaged with each other, thereby functioning as the stopper  18  that may prevent an excessive deformation of the elastic beams  17 . For example, when the upper portion  11   a  of the body  11  is acted upon by a large force in a +z direction, the first protrusion  18   a  and the second protrusion  18   b  contact each other in a portion B marked in  FIG. 6 . Then, movement of the upper portion  11   a  in the +z direction is restricted. Additionally, when the upper portion  11   a  of the body  11  is acted upon by a large force in a −z direction, the first protrusion  18   a  and the second protrusion  18   b  contact each other in a portion A marked in  FIG. 6 . Then, movement of the upper portion  11   a  in the −z direction is restricted. Accordingly, even when a large force above the measurement range of the force sensing apparatus acts upon the body  11 , the stopper  18  may prevent that the elastic beams  17  from being deformed outside a range in which they may be elastically restored. 
         [0065]    The elastic beams  17  and the stoppers  18  may be formed in a monolithic manner by forming a plurality of gaps  16  in the single body  11  by using, for example, a wire EDM method. Each of the gaps  16  may have two side portions formed to be in a direction perpendicular to the axial direction and a center portion curved in the form of ‘Ω’ so as to form the stopper  18 . Two adjacent gaps  16  are separated from each other in the axial direction and partially overlap each other in an azimuth angle direction. Moreover, the two side portions of the gap  16  extend in a direction perpendicular to the axial of the body  11  are arranged parallel to a side portion of another gap  16 , and thus, the elastic beam  17  may be formed between the two parallel gaps  16 . That is, the gap  16  may start between the lower portion  11   b  of the body  11  and the elastic beam  17  and pass through the stopper  18  to be extended up to a portion between the upper portion  11   a  of the body  11  and another elastic beam  17 . For example, when four elastic beams  17  and four stoppers  18  are to be formed, four gaps  16  may be formed in the body  11 . Further, in order to prevent the body  11  from wear due to fatigue caused repeated elastic deformations, openings  19   a  and  19   b  may be formed at two end portions of the gap  16 , respectively. 
         [0066]    Meanwhile, referring to  FIG. 6 , the FBGs  13  are attached to the body  11  across the stopper  18 . However, this is just an example, and the FBGs  13  may also be attached to the body  11  across the elastic beams  17 . In addition, as illustrated in  FIG. 7 , the FBGs  13  may be attached to the body  11  by using adhesives  40   a  and  40   b.  As in the embodiment of  FIG. 6 , when a force acts upon the body  11 , the entire body  11  is not elastically deformed, but rather peripheral portions of the gaps  16  and the elastic beams  17  are mainly deformed. Accordingly, when the FBGs  13  are attached to the body  11  overall, an excessive tension is applied to the FBGs  13  in peripheral portions of the gaps  16  and the elastic beams  17  so that the FBGs  13  may be cut. Thus, as illustrated in  FIG. 7 , the adhesives  40   a  and  40   b  may be coated on the body  11  so as to cover only the two end portions of the FBGs  13 . Further, the FBGs  13  may be attached to an inner surface of the body  11 . 
         [0067]    In addition, a tension of the cable for operating the instrument does not act in the axial direction of the robot arm, and thus, does not affect sensing of a force in the axial direction. 
         [0068]    As described above, according to the one or more of the above embodiments, the robot arm  10  including the above-described force sensing apparatuses may accurately measure the intensity and direction of a force acting upon the operational end of the robot arm  10 . Accordingly, the robot arm  10  according to the example embodiments may be used in medical surgery robots for performing, for example, surgical cautery. 
         [0069]    It should be understood that the exemplary embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.