Abstract:
Fingers for holding a target project from shift members, and shift in accordance with the movement of the shift members shifted in predetermined axial directions by a shift mechanism to hold the target. Sliding members for shifting the shift members in the predetermined axial directions project from the shift members, and guide the shift members by sliding. According to this structure, a robot hand becomes smaller in size by decreasing the clearances between the fingers (moving the shift members closer to each other). Thus, the robot hand can hold a small target object within a small work space even though the robot hand conducts a parallel shift of the fingers.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a robot hand, a robot, and a holding mechanism. 
     2. Related Art 
     There are two known robot hand holding systems for holding a target object using a plurality of fingers: holding the target by rotation of the roots of the fingers (for example, JP-A-2010-201538), and holding the target by a parallel shift of the fingers (for example, JP-A-5-220687). 
     According to the method which rotates the roots of the fingers, the contact angle of the fingers for contact with the target change according to the size of the target to be held, in which case the shape and force of the fingers need to change for each of the targets. Accordingly, this method requires complicated structure and control of the robot hand. Concerning this point, however, the method which conducts a parallel shift of the fingers does not vary the contact angle of the fingers for contact with the target, and thus can simplify the structure and control of the robot hand. 
     Despite of this advantage, the method of conducting a parallel shift of the fingers has a limitation in that the method is difficult to use for holding a small target. This limitation is imposed for the following reasons. In general, holding a small target takes place in a narrow working space. For example, in the case of picking up small parts and assembling the parts to predetermined positions, the parts to be assembled are often arranged within a small space with only small clearances between one another. In this case, there is generally only a limited space therebetween for receiving the robot hand for picking up the parts. Moreover, for the attachment of the picked-up parts, the parts often need to be assembled in a small space in accordance with the small size of the parts. On the other hand, the robot hand of the type conducting a parallel shift of the fingers has large components for supporting the movable fingers, therefore the robot hand has an increased size and thus is difficult to use for working in a narrow working space. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a robot hand, a robot, and a holding mechanism, which are constructed to hold a target by a parallel shift of fingers but are compact and suited for a work performed in a narrow working space. 
     A robot hand according to an aspect of the invention includes: a plurality of fingers which hold a target by changing the clearances between the fingers; a center member having a driving mechanism for shifting the plural fingers; a plurality of first peripheral members spaced apart from the surfaces of the center member in a first direction; first driving shafts projecting in the first direction from the first peripheral members and connecting with the driving mechanism; a plurality of second peripheral members spaced apart from the surfaces of the center member in a second direction crossing the first direction; second driving shafts projecting in the second direction from the second peripheral member and connecting with the driving mechanism; shift members disposed in the second direction with respect to the first peripheral members and in the first direction with respect to the second peripheral members, and carrying or supporting the fingers; first sliding shafts projecting in the first direction from the shift members and inserted into first sliding bores formed in the second peripheral members so as to slide in the first sliding bores; second sliding shafts projecting in the second direction from the shift members and inserted into second sliding bores formed in the first peripheral members so as to slide in the second sliding bores; and a second center sliding shaft projecting in the second direction from at least one of the second peripheral members and inserted into a second center sliding bore formed in the center member so as to slide in the second center sliding bore. 
     According to the robot hand of the aspect of the invention having this structure, the distance between the first peripheral members disposed in the first direction with the center member interposed therebetween can be changed by driving the first driving shafts using the driving mechanism housed within the center member. On the other hand, the distance between the second peripheral members disposed in the second direction with the center member interposed therebetween can be changed by driving the second driving shafts. The shift members are disposed in four positions located in the second direction with respect to the first peripheral members and in the first direction with respect to the second peripheral members. The fingers are attached to the shift members. The first sliding shafts are extended in the first direction from the shift members and inserted into the first sliding bores formed in the second peripheral members so as to slide in the first sliding bores. The second sliding shafts are extended in the second direction and inserted into the second sliding bores formed in the first peripheral members so as to slide in the second sliding bores. Furthermore, the second center sliding shaft is projected in the second direction from the second peripheral member and inserted into the second center sliding bore of the center member so as to slide in the second center sliding bore. According to this structure, the robot hand becomes larger when holding a large target, and becomes smaller when holding a small target. Thus, the robot hand can hold a small target within a narrow working space even though the robot hand conducts a parallel shift of the fingers (and the shift members) and holds a target. Moreover, the robot hand which holds a target by conducting a parallel shift of the fingers (and the shift members) need not change the contact angle of the fingers for contact with the target in accordance with the size of the target. 
     In the robot hand of the aspect of the invention described above, the cross-sectional shape of the second center sliding shaft in the direction perpendicular to the insertion direction may be polygonal, and the shape of the second center sliding bore in the direction perpendicular to the insertion direction may be polygonal (such as quadrangular, triangular, and pentagonal shapes). 
     According to this structure, the second center sliding shaft inserted into the second center sliding bore does not rotate within the second center sliding bore. Thus, rotation of the second peripheral members connecting with the second center sliding shaft can be avoided. The shift members and the fingers connect with the second peripheral members via the first sliding shafts. In this case, the fingers receive a reaction force in the first direction from the target when the robot hand holds the target in the first direction. This reaction force is transmitted through the first sliding shafts to the second peripheral members, and acts in a direction so as to rotate the second peripheral members around the second center sliding shaft. However, according to this aspect of the invention, the cross-sectional shape of the second center sliding shaft in the direction perpendicular to the insertion direction is polygonal, and the shape of the second center sliding bore in the direction perpendicular to the insertion direction is polygonal. These configurations can prevent the rotation of the second peripheral members. Accordingly, the rigidity of the robot hand when holding the target in the first direction can increase. 
     In the robot hand of the aspect of the invention described above, plural second center sliding shafts and plural second center sliding bores may be provided. 
     When plural second center sliding shafts and plural second center sliding bores are provided, rotation of the second peripheral members can be further avoided. Accordingly, the rigidity of the robot hand can improve. 
     In the robot hand of the aspect of the invention described above, the second center sliding shaft may project from each of the second peripheral members spaced apart from the surfaces of the center member in the second direction. 
     According to this structure, generation of a force twisting the target (shearing force) can be prevented when the target is held by a large force in the first direction. The details of this mechanism will be explained below. 
     In the robot hand of the aspect of the invention described above may further include a first center sliding shaft projecting in the first direction from at least one of the first peripheral members and inserted into a first center sliding bore formed in the center member so as to slide in the first center sliding bore. 
     According to this structure, generation of a force twisting the target (shearing force) can be prevented when the target is held by a large force in the second direction. 
     In the robot hand of the aspect of the invention described above, the cross-sectional shape of the first center sliding shaft in the direction perpendicular to the insertion direction may be polygonal, and the shape of the first center sliding bore in the direction perpendicular to the insertion direction may be polygonal. 
     According to this structure, the first peripheral member connected with the first center sliding shaft does not rotate around the first center sliding shaft. Therefore, this structure can prevent rotation of the first peripheral members around the first center sliding shaft caused by a reaction force applied in the second direction from the target to the fingers when the robot hand holds the target in the second direction. Accordingly, the rigidity of the robot hand holding the target in the second direction can improve. 
     In the robot hand of the aspect of the invention described above, plural first center sliding shafts and plural first center sliding bores may be provided. 
     When plural first center sliding shafts and plural first center sliding bores are provided, rotation of the first peripheral members can be further avoided. Accordingly, the rigidity of the robot hand can further improve. 
     The invention can also be practiced in the form of a robot having the robot hand described above. That is, a robot according to another aspect of the invention includes: a plurality of fingers which hold a target by changing the clearances between the fingers; a center member having a driving mechanism for shifting the plural fingers; a plurality of first peripheral members spaced apart from the surfaces of the center member in a first direction; first driving shafts projecting in the first direction from the first peripheral members and connecting with the driving mechanism; a plurality of second peripheral members spaced apart from the surfaces of the center member in a second direction crossing the first direction; second driving shafts projecting in the second direction from the second peripheral member and connecting with the driving mechanism of the center member; shift members disposed in the second direction with respect to the first peripheral members and in the first direction with respect to the second peripheral members, and carrying or supporting the fingers; first sliding shafts projecting in the first direction from the shift members and inserted into first sliding bores formed in the second peripheral members so as to slide in the first sliding bores; second sliding shafts projecting in the second direction from the shift members and inserted into second sliding bores formed in the first peripheral members so as to slide in the second sliding bores; and a second center sliding shaft projecting in the second direction from at least one of the second peripheral members and inserted into a second center sliding bore formed in the center member so as to slide in the second center sliding bore. 
     According to this aspect of the invention, the robot can hold a small target within a narrow working space. Moreover, the robot which holds a target by conducting a parallel shift of the fingers (and the shift members) need not change the contact angle of the fingers for contact with the target in accordance with the size of the target. 
     In the robot of the aspect of the invention described above, the cross-sectional shape of the second center sliding shaft in the direction perpendicular to the insertion direction may be polygonal, and the shape of the second center sliding bore in the direction perpendicular to the insertion direction may be polygonal (such as quadrangular, triangular, and pentagonal shapes) similarly to the robot hand of the above aspect of the invention. 
     According to this structure, the second peripheral member connecting with the second center sliding shaft does not rotate in the direction perpendicular to the insertion direction. Accordingly, the rigidity of the robot hand when holding the target in the first direction can increase. 
     In the robot of the above aspect of the invention described above, plural second center sliding shafts and plural second center sliding bores may be provided similarly to the robot hand of the above aspect of the invention. 
     When plural second center sliding shafts and plural second center sliding bores are provided, rotation of the second peripheral members can be further avoided. Accordingly, the rigidity of the robot hand mounted on the robot can improve. 
     In the robot of the aspect of the invention described above, the second center sliding shaft may project from each of the second peripheral members spaced apart from the surfaces of the center member in the second direction similarly to the robot hand of the above aspect of the invention. 
     According to this structure, generation of a force twisting the target (shearing force) can be prevented when the target is held by a large force in the first direction. The details of this mechanism will be explained below. 
     The robot of the aspect of the invention described above further may include a first center sliding shaft projecting in the first direction from at least one of the first peripheral members and inserted into a first center sliding bore formed in the center member so as to slide in the first center sliding bore. 
     According to this structure, generation of a force twisting the target (shearing force) can be prevented when the target is held by a large force in the second direction. 
     In the robot of the above aspect of the invention, the cross-sectional shape of the first center sliding shaft in the direction perpendicular to the insertion direction may be polygonal, and the shape of the first center sliding bore in the direction perpendicular to the insertion direction may be polygonal similarly to the robot hand of the above aspect of the invention. 
     According to this structure, the first peripheral member connected with the first center sliding shaft does not rotate around the first center sliding shaft. Therefore, this structure can prevent rotation of the first peripheral members around the first center sliding shaft caused by a reaction force applied in the second direction from the target to the fingers when the robot holds the target in the second direction using the robot hand. Accordingly, the rigidity of the robot holding the target in the second direction can improve. 
     In the robot of the above aspect of the invention described above, plural first center sliding shafts and plural first center sliding bores may be provided. 
     When plural first center sliding shafts and plural first center sliding bores are provided, rotation of the first peripheral members can be further avoided. Accordingly, the rigidity of the robot hand when the robot holding the target can further improve. 
     The invention can also be practiced in the form of a mechanism (or holding mechanism) which allows the robot hand described above to hold a target. That is, a holding mechanism according to still another aspect of the invention includes: a plurality of contact members which hold a target by changing the clearances between the contact members; a center member having a driving mechanism for shifting the plural fingers; a plurality of first peripheral members spaced apart from the surfaces of the center member in a first direction; first driving shafts projecting in the first direction from the first peripheral members and connecting with the driving mechanism; a plurality of second peripheral members spaced apart from the surfaces of the center member in a second direction crossing the first direction; second driving shafts projecting in the second direction from the second peripheral member and connecting with the driving mechanism of the center member; shift members disposed in the second direction with respect to the first peripheral members and in the first direction with respect to the second peripheral members, and carrying or supporting the contact members; first sliding shafts projecting in the first direction from the shift members and inserted into first sliding bores formed in the second peripheral members so as to slide in the first sliding bores; second sliding shafts projecting in the second direction from the shift members and inserted into second sliding bores formed in the first peripheral members so as to slide in the second sliding bores; and a second center sliding shaft projecting in the second direction from at least one of the second peripheral members and inserted into a second center sliding bore formed in the center member so as to slide in the second center sliding bore. 
     According to the above aspect of the invention, the holding mechanism can hold a small target within a narrow working space. Moreover, the holding mechanism which holds a target by conducting a parallel shift of the fingers (and the shift members) need not change the contact angle for contact with the fingers in accordance with the size of the target. Thus, the holding mechanism is applicable to various types of structures other than the robot hand as long as they have the function of holding a target. 
     In the holding mechanism of the aspect of the invention described above, the cross-sectional shape of the second center sliding shaft in the direction perpendicular to the insertion direction may be polygonal, and the shape of the second center sliding bore in the direction perpendicular to the insertion direction may be polygonal (quadrangular, triangular, pentagonal, or other shapes). 
     According to this structure, rotation of the second peripheral member connected with the second center sliding shaft can be avoided. Accordingly, the rigidity when the target is held in the first direction can increase. 
     In the holding mechanism of the aspect of the invention described above, plural second center sliding shafts and plural second center sliding bores may be provided. 
     When plural second center sliding shafts and plural second center sliding bores are provided, rotation of the second peripheral members can be further avoided. Accordingly, the rigidity of the holding mechanism can improve. 
     In the holding mechanism of the aspect of the invention described above, the second center sliding shaft may project from each of the second peripheral members spaced apart from the surfaces of the center member in the second direction. 
     According to this structure, generation of a force twisting the target (shearing force) can be prevented when the target is held by a large force in the first direction. The details of this mechanism will be described below. 
     The holding mechanism of the aspect of the invention may further include a first center sliding shaft projecting in the first direction from at least one of the first peripheral members and inserted into a first center sliding bore formed in the center member so as to slide in the first center sliding bore. 
     According to this structure, generation of a force twisting the target (shearing force) can be prevented when the target is held by a large force in the second direction. 
     In the holding mechanism of the aspect of the invention described above, plural first center sliding shafts and plural first center sliding bores may be provided. 
     When plural first center sliding shafts and plural first center sliding bores are provided, rotation of the first peripheral members can be further avoided. Accordingly, the rigidity of the holding mechanism holding the target can further improve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a perspective view illustrating the structure of a robot hand according to an embodiment. 
         FIG. 2  is a perspective view illustrating the structure of the robot hand according to the embodiment. 
         FIGS. 3A and 3B  are side views illustrating the structure of the robot hand according to the embodiment. 
         FIGS. 4A through 4C  illustrate the operation of the robot hand according to the embodiment. 
         FIG. 5  illustrates the structure of a driving mechanism of the robot hand according to the embodiment. 
         FIGS. 6A through 6C  illustrate the operation of the driving mechanism according to the embodiment. 
         FIGS. 7A and 7B  illustrate the function of second center sliding shafts. 
         FIG. 8  is a top view of the robot hand, illustrating the function of the second center sliding shafts. 
         FIGS. 9A and 9B  show advantages of the presence of the second center sliding shafts in conjunction with results of an experiment. 
         FIGS. 10A and 10B  are views for explaining the reasons why a robot hand  10  of the embodiment can hold a thin-plate-shaped small target W by a large force. 
         FIGS. 11A and 11B  illustrate an example of a robot hand according to a first modified example. 
         FIG. 12  illustrates an example of a robot hand according to a second modified example. 
         FIG. 13  illustrates an example of a robot hand according to a third modified example. 
         FIG. 14  illustrates an example of a single-arm robot provided with the robot hand. 
         FIG. 15  illustrates an example of a plural-arm robot provided with the robot hand. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     For clarifying the details of the invention, an embodiment according to the invention is hereinafter described in the following order. 
     A. Structure of Robot Hand of Embodiment 
     B. Holding Operation of Robot Hand of Embodiment 
     C. Function of Center Sliding Shafts 
     D. Configuration of End Member 
     E. Modified Examples
         E-1. First Modified Example   E-2. Second Modified Example   E-3. Third Modified Example       

     F. Application Examples 
     A. STRUCTURE OF ROBOT HAND OF EMBODIMENT 
       FIGS. 1 and 2  illustrate the structure of a robot hand  10  according to this embodiment. As can be seen from  FIG. 1 , the robot hand  10  in this embodiment includes a plurality of members, racks and sliding parts for connecting the members, and others. Initially, the structures of the respective members are explained. In  FIG. 1 , the members correspond to hatched parts. The robot hand  10  in this embodiment includes two first peripheral members  110 A and  110 B between which a center member  100  provided on a base case  160  is sandwiched in a first direction, two peripheral members  120 A and  120 B between which the center member  100  is sandwiched in a second direction, and four shift members  130 A though  130 D. 
     The shift member  130 A is disposed in a position so as to lie in the first direction with respect to the second peripheral member  120 B, and in the second direction with respect to the first peripheral member  110 A. The shift member  130 B is disposed in a position so as to lie in the first direction with respect to the second peripheral member  120 A, and in the second direction with respect to the first peripheral member  110 A. The shift member  130 C is disposed in a position so as to lie in the first direction with respect to the second peripheral member  120 A, and in the second direction with respect to the first peripheral member  110 A. The shift member  130 D is disposed in a position so as to lie in the first direction with respect to the second peripheral member  120 B, and in the second direction with respect to the first peripheral member  110 A. Fingers  140 A through  140 D are attached to the shift members  130 A through  130 D, respectively. 
     Each of the shift members  130 A through  130 D has a sliding shaft extending in the first direction, and a sliding shaft extending in the second direction. More specifically, the shift member  130 A has a first sliding shaft  131 A projecting in the first direction, and a second sliding shaft  132 A projecting in the second direction. The first sliding shaft  131 A is inserted into a first sliding bore  123 B penetrating the second peripheral member  120 B in the first direction so as to slide in the bore  123 B, while the second sliding shaft  132 A is inserted into a second sliding bore  114 A penetrating the first peripheral member  110 A in the second direction so as to slide in the bore  114 A. The shift member  130 B has a first sliding shaft  131 B projecting in the first direction, and a second sliding shaft  132 B projecting in the second direction. The first sliding shaft  131 B is inserted into a first sliding bore  123 A penetrating the second peripheral member  120 A in the first direction so as to slide in the bore  123 A, while the second sliding shaft  132 B is inserted into the second sliding bore  114 A penetrating the first peripheral member  110 A in the second direction so as to slide in the bore  114 A. Similarly, the shift member  130 C has a first sliding shaft  131 C projecting in the first direction, and a second sliding shaft  132 C projecting in the second direction. The first sliding shaft  131 C is inserted into the first sliding bore  123 A penetrating the second peripheral member  120 A in the first direction so as to slide in the bore  123 A, while the second sliding shaft  132 C is inserted into a second sliding bore  114 B penetrating the first peripheral member  110 B in the second direction so as to slide in the bore  114 B. The shift member  130 D has a first sliding shaft  131 D projecting in the first direction, and a second sliding shaft  132 D projecting in the second direction. The first sliding shaft  131 D is inserted into the first sliding bore  123 B penetrating the second peripheral member  120 B in the first direction so as to slide in the bore  123 B, while the second sliding shaft  132 D is inserted into the second sliding bore  114 B penetrating the first peripheral member  110 B in the second direction so as to slide in the bore  114 B. 
     As illustrated in  FIG. 2 , a first driving shaft  111 B projects in the first direction from the first peripheral member  110 B. The first driving shaft  111 B has gear teeth in the side surface thereof, and constitutes a rack and pinion mechanism in combination with a pinion gear provided within the center member  100 . Similarly, a first driving shaft  111 A having gear teeth in the side surface thereof projects in the first direction from the first peripheral member  110 A. The first driving shaft  111 A also constitutes a rack and pinion mechanism in combination with the pinion gear provided within the center member  100 . The internal structure of the center member  100  will be described below. 
     A second driving shaft  122 A projects in the second direction from the second peripheral member  120 A. The second driving shaft  122 A also has gear teeth in the side surface thereof, and constitutes a rack and pinion mechanism in combination with the pinion gear provided within the center member  100 . Similarly, a second driving shaft  122 B having gear teeth in the side surface thereof projects in the second direction from the second peripheral member  120 B. The second driving shaft  122 B also constitutes a rack and pinion mechanism in combination with the pinion gear provided within the center member  100 . 
     A second center sliding shaft  126 B projects in the second direction from the second peripheral member  120 B. The second center sliding shaft  126 B is inserted into a second center sliding bore  104  penetrating the center member  100  in the second direction so as to slide in the bore  104 . Similarly, a second center sliding shaft  126 A projects in the second direction from the second peripheral member  120 A. The second center sliding shaft  126 A is inserted into the second center sliding bore  104  penetrating the center member  100  in the second direction so as to slide in the bore  104 . 
     A screw shaft  150   a  having a screw in the outer circumferential surface thereof projects from the center of the top surface of the center member  100 . An end member  150  having a flat plate shape is attached to the tip of the screw shaft  150   a . The screw shaft  150   a  is connected with a driving mechanism (not-shown but described below) equipped within the center member  100 . Sliding shafts  150   b  project from the top surface of the center member  100  on both sides of the screw shaft  150   a  so as to slide in the center member  100 . The tips of the sliding shafts  150   b  are attached to the end member  150 . The end member  150  has a width that is smaller in the second direction than in the first direction. The base case  160  is attached to link units  312  of a robot arm. 
       FIGS. 3A and 3B  illustrate the positional relationship between the first driving shafts  111 A and  111 B, the second driving shafts  122 A and  122 B, the second center sliding shafts  126 A and  126 B, the first sliding shafts  131 A through  131 D, and the second sliding shafts  132 A through  132 D in the height direction as viewed from the side of the robot hand  10  in this embodiment.  FIG. 3A  is a side view of the robot hand  10  as viewed in the first direction from the side of the first peripheral member  110 B.  FIG. 3B  is a side view of the robot hand  10  as viewed in the second direction from the side of the second peripheral member  120 B. 
     As illustrated in the figures, the first driving shafts  111 A and  111 B, the second driving shafts  122 A and  122 B, the second center sliding shafts  126 A and  126 B, the first sliding shafts  131 A through  131 D, and the second sliding shafts  132 A through  132 D are arranged in four layers. The first driving shafts  111 A and  111 B and the first sliding shafts  131 A and  131 B are disposed in the layer closest to the base case  160  (hereinafter referred to as a first layer). The second driving shafts  122 A and  122 B and the second sliding shafts  132 A and  132 C are disposed on a layer immediately above the first layer (hereinafter referred to as a second layer). The first sliding shafts  131 C and  131 D are disposed on a layer immediately above the second layer (hereinafter referred to as a third layer). The second center sliding shafts  126 A and  126 B and the second sliding shafts  132 B and  132 D are disposed on the uppermost layer (hereinafter referred to as a fourth layer). 
     B. HOLDING OPERATION OF ROBOT HAND OF EMBODIMENT 
       FIGS. 4A through 4C  illustrate the operation of the robot hand  10  according to this embodiment when holding a target. For holding the target, the size of the robot hand  10  in the width direction is varied in accordance with the size of the target to be held. It is assumed herein that the width direction corresponds to the second direction. However, the width direction may be defined as the first direction.  FIG. 4A  shows a process for changing of the size of the robot hand  10  in the width direction. As noted above, the second sliding shafts  132 A through  132 D extending in the second direction from the shift members  130 A through  130 D are inserted into the second sliding bores  114 A and  114 B formed in the first peripheral members  110 A and  110 B so as to slide in the bores  114 A and  114 B (see  FIG. 1 ). According to this structure, the clearance between the shift member  130 A and the shift member  130 B, and the clearance between the shift member  130 D and the shift member  130 C can be simultaneously varied by changing the clearance between the second peripheral member  120 A and the second peripheral member  120 B. The fingers  140 A through  140 D are attached to the top surfaces of the shift members  130 A through  130 D. Thus, the fingers  140 A through  140 D attached to the shift members  130 A through  130 D can be moved by shifting the shift members  130 A through  130 D. The driving mechanism for changing the clearance between the second peripheral member  120 B and the second peripheral member  120 A will be described below.  FIG. 4A  shows a process for decreasing the clearance. 
     After adjustment of the size of the robot hand  10  in the width direction, the next process decreases the size of the robot hand  10  in the holding direction in accordance with the size of the target. It is assumed herein that the holding direction corresponds to the first direction. However, the holding direction may be defined as the second direction.  FIG. 4B  shows a process for decreasing the size of the robot hand  10  in the holding direction. As noted above, the first sliding shafts  131 A through  131 D extending in the first direction from the shift members  130 A through  130 D are inserted into the first sliding bores  123 A and  123 B formed in the second peripheral members  120 A and  120 B so as to slide in the bores  123 A and  123 B (see  FIG. 1 ). According to this structure, the clearance between the shift member  130 A and the shift member  130 D, and the clearance between the shift member  130 B and the shift member  130 C can be varied by changing the clearance between the first peripheral member  110 A and the first peripheral member  110 B. Accordingly, the clearance between the finger  140 A and the finger  140 D, and the clearance between the finger  140 B and the finger  140 C can be simultaneously decreased to hold the target. The mechanism for reducing the clearance between the first peripheral member  110 A and the first peripheral member  110 B will be described below. 
     According to the robot hand  10  in this embodiment, the top surface of the end member  150  can be brought into contact with the target by shifting the end member  150  in the up-down direction. In this case, the four fingers  140 A through  140 D and the end member  150  can hold the target, and the target can be maintained in a stable condition even when the target is a small object.  FIG. 4C  illustrates the end member  150  shifted upward. 
     The driving mechanism which varies the size of the robot hand  10  in the first direction or the second direction, and shifts the end member  150  in the up-down direction is now explained. 
       FIG. 5  illustrates a driving mechanism  200  positioned on the robot hand  10  according to this embodiment.  FIG. 5  shows only the driving mechanism  200  and a piezoelectric motor driving the driving mechanism  200  by solid lines, and shows the other components by broken lines representing only the outlines of the external shapes thereof for avoiding complication of the figure. 
     The driving mechanism  200  in this embodiment has a triple-pipe-shaped structure which includes three transmission shafts assembled coaxially with each other. A hollow and round-pipe-shaped second transmission shaft  212  is provided as an outermost transmission shaft of the driving mechanism  200 . A second pinion gear  206  is provided on the outer circumference of the upper end of the second transmission shaft  212 . A driving gear  212 G is attached to the lower end of the second transmission shaft  212 . 
     A hollow (not-shown) and round-pipe-shaped first transmission shaft  210  (see  FIG. 6B ) is housed within the second transmission shaft  212  so as to be rotatable relative to the second transmission shaft  212 . The first transmission shaft  210  is longer than the second transmission shaft  212 . A first pinion gear  204  is provided on the outer circumference of the upper end of the first transmission shaft  210 , while a driving gear  210 G is attached to the lower end of the first transmission shaft  210 . The second pinion gear  206  and the first pinion gear  204  have the same outside diameter. 
     A hollow (not-shown) and round-pipe-shaped third transmission shaft  208  (see  FIG. 6C ) is housed within the first transmission shaft  210  so as to be rotatable relative to the first transmission shaft  210 . The third transmission shaft  208  is further longer than the first transmission shaft  210 . A screw portion  202  having a screw in the inside surface thereof is provided on the outer circumference of the upper end of the third transmission shaft  208 . A driving gear  208 G is provided at the lower end of the third transmission shaft  208 . The screw portion  202  engages with the screw shaft  150   a  connected with the end member  150 . 
     Substantially the upper half of the driving mechanism  200  thus constructed is housed within the center member  100 , while substantially the lower half of the driving mechanism  200  is housed within the base case  160 . The base case  160  further accommodates a driving motor (not-shown) for driving the driving gear  212 G of the second transmission shaft  212 , a driving motor (not-shown) for driving the driving gear  210 G of the first transmission shaft  210 , a driving motor (not-shown) for driving the driving gear  208 G of the third transmission shaft  208 , and others. 
       FIGS. 6A through 6C  illustrate the operation for varying the size of the robot hand  10  in the first direction and the second direction, and the operation for shifting the end member  150  in the up-down direction according to this embodiment.  FIG. 6A  shows a process for driving the driving gear  212 G of the second transmission shaft  212 .  FIG. 6A  shows the components associated with the operation by bold solid lines, and shows the other parts by fine broken lines for avoiding complication of the drawing. 
     As illustrated in the figure, the second transmission shaft  212  rotates in response to driving of the driving gear  212 G, whereby the second pinion gear  206  at the upper end of the second transmission shaft  212  rotates. The second pinion gear  206  engages with the first driving shafts  111 A and  111 B. The first driving shaft  111 A connects with the first peripheral member  110 A, while the first driving shaft  111 B connects with the first peripheral member  110 B. According to this structure, the clearance between the first peripheral member  110 A and the first peripheral member  110 B (clearance in the first direction, i.e., the holding direction in this embodiment) changes by the rotation of the second pinion gear  206 . For example, when the second pinion gear  206  rotates clockwise in  FIG. 6A , the first peripheral member  110 A and the first peripheral member  110 B shift in directions so as to decrease the clearance therebetween. On the other hand, when the second pinion gear  206  rotates anticlockwise, the first peripheral member  110 A and the first peripheral member  110 B shift in directions so as to increase the clearance therebetween. 
       FIG. 6B  shows a process for driving the driving gear  210 G of the first transmission shaft  210 . Similarly to  FIG. 6A ,  FIG. 6B  shows the components associated with the operations by bold solid lines, and shows the other parts by fine broken lines for avoiding complication of the drawing. The first transmission shaft  210  rotates in response to driving of the driving gear  210 G, whereby the first pinion gear  204  at the upper end of the first transmission shaft  210  rotates. The first pinion gear  204  engages with the second driving shafts  122 A and  122 B. The second driving shaft  122 A connects with the second peripheral member  120 A, while the second driving shaft  122 B connects with the second peripheral member  120 B. According to this structure, the clearance between the second peripheral member  120 A and the second peripheral member  120 B (clearance in the second direction, i.e., the width direction in this embodiment) changes by the rotation of the first pinion gear  204 . For example, when the first pinion gear  204  rotates clockwise in  FIG. 6B , the second peripheral member  120 A and the second peripheral member  120 B shift in directions so as to increase the clearance therebetween. On the other hand, when the first pinion gear  204  rotates anticlockwise, the second peripheral member  120 A and the second peripheral member  120 B shift in directions so as to decrease the clearance therebetween. 
       FIG. 6C  shows a process for driving the driving gear  208 G of the third transmission shaft  208 . Similarly to  FIGS. 6A and 6B ,  FIG. 6C  shows the components associated with the operations by bold solid lines, and shows the other parts by fine broken lines for avoiding complication of the drawing. The third transmission shaft  208  rotates in response to driving of the driving gear  208 G, whereby the screw portion  202  at the upper end of the third transmission shaft  208  rotates. The screw portion  202  engages with the screw shaft  150   a . The upper end of the screw shaft  150   a  connects with the end member  150 . Moreover, the sliding shafts  150   b  extended from the upper surface of the center member  100  are attached to the end member  150 . According to this structure, the end member  150  can shift in the up-down direction relative to the center member  100 , but cannot rotate relative thereto. Therefore, with rotation of the screw portion  202 , the screw shaft  150   a  engaging with the screw portion  202  shifts in the up-down direction, and the end member  150  shifts in the up-down direction accordingly. In the case of the example shown in  FIG. 6C , the end member  150  shifts downward when the screw portion  202  rotates clockwise, and shifts upward when the screw portion  202  rotates anticlockwise. 
     C. FUNCTION OF CENTER SLIDING SHAFTS 
     According to the robot hand  10  in this embodiment described herein, the second center shafts  126 A and  126 B are extended in the second direction from the second peripheral members  120 A and  120 B, and inserted into the second center sliding bores  104  of the center member  100  so as to slide in the bores  104 . The second center sliding shafts  126 A and  126 B have the function of preventing the target from receiving a force so as to twist the target (shearing force) when the target is held by a large force. The details of this point are now explained. 
       FIGS. 7A and 7B  illustrate the function of the second center sliding shafts  126 A and  126 B.  FIG. 7A  is a side view of the robot hand  10  holding a target W as viewed in the second direction from the side of the second peripheral member  120 B. For focusing the explanation on the fingers  140 A and  140 D, the shift members  130 A and  130 D, and the second peripheral member  120 B,  FIG. 7A  shows the parts associated with these components by bold solid lines, and shows the other parts by fine broken lines. 
     Initially, concerning the finger  140 A and the shift member  130 A, the finger  140 A receives a reaction force F from the target W. In this case, the finger  140 A and the shift member  130 A try to rotate around the second sliding shaft  132 A (anticlockwise rotation in  FIG. 7A ). As a result, a force R 1  is generated at the position of contact between the first sliding shaft  131 A and the second peripheral member  120 B and tries to rotate the second peripheral member  120 B (clockwise rotation in  FIG. 7A ). The force R 1  is calculated as F×LB 1 /LB 2  (see  FIG. 7A  for LB 1  and LB 2 ) based on the balance of moments. 
     As for the finger  140 D and the shift member  130 D, the finger  140 D receives the reaction force F from the target W. In this case, the finger  140 D and the shift member  130 D try to rotate around the second sliding shaft  132 D (clockwise rotation in  FIG. 7A ). As a result, a force R 2  is generated at the position of contact between the first sliding shaft  131 D and the second peripheral member  120 B and tries to rotate the second peripheral member  120 B (anticlockwise rotation in  FIG. 7A ). The force R 2  is calculated as F×LB 3 /LB 4  (see  FIG. 7A  for LB 3  and LB 4 ) based on the balance of moments. 
     As apparent from  FIG. 7A , a distance LB 5  between the point of application of the force R 1  and the second driving shaft  122 B is longer than a distance LB 6  between the point of application of the force R 2  and the second driving shaft  122 B when compared with one another. Thus, the second peripheral member  120 B tries to rotate clockwise in  FIG. 7A  around the second driving shaft  122 B. The moment of this rotation is calculated as R 1 ×LB 5 −R 2 ×LB 6 . 
     The operation of the fingers  140 B and  140 C on the second peripheral member  120 A side is substantially similar to the operation of the fingers  140 A and  140 D on the second peripheral member  120 B side described above.  FIG. 7B  shows the side view of the robot hand  10  holding the target W as viewed in the second direction from the side of the second peripheral member  120 A. For focusing the explanation on the fingers  140 B and  140 C, the shift members  130 B and  130 C, and the second peripheral member  120 A,  FIG. 7B  shows the parts associated with these components by bold solid lines, and shows the other parts by fine broken lines. 
     As illustrated in  FIG. 7B , the finger  140 C also receives the reaction force F from the target W. In this case, the finger  140 C and the shift member  130 C try to rotate around the second sliding shaft  132 C (anticlockwise rotation in  FIG. 7B ). As a result, a force R 3  is generated at the position of contact between the first sliding shaft  131 C and the second peripheral member  120 A and tries to rotate the second peripheral member  120 A (clockwise rotation in  FIG. 7B ). The force R 3  is calculated as F×LA 1 /LA 2  (see  FIG. 7B  for LA 1  and LA 2 ) based on the balance of moments. 
     Concerning the finger  140 B and the shift member  130 B, the finger  140 B receives the reaction force F from the target W. In this case, the finger  140 B and the shift member  130 B try to rotate clockwise around the second sliding shaft  132 B. As a result, a force R 4  is generated at the position of contact between the first sliding shaft  131 B and the second peripheral member  120 A and tries to rotate the second peripheral member  120 A anticlockwise. The force R 4  is calculated as F×LA 3 /LA 4  (see  FIG. 7B  for LA 3  and LA 4 ) based on the balance of moments. 
     As apparent from  FIG. 7B , a distance LA 5  between the point of application of the force R 3  and the second driving shaft  122 A is longer than a distance LA 6  between the point of application of the force R 4  and the second driving shaft  122 A when compared with one another. Thus, the second peripheral member  120 A tries to rotate clockwise in  FIG. 7B  around the second driving shaft  122 A. The moment of this rotation is calculated as R 3 ×LA 5 −R 4 ×LA 6 . 
       FIG. 8  is a top view of the robot hand  10  holding the target W. As described with reference to  FIGS. 7A and 7B , the second peripheral member  120 B tries to rotate downward in  FIG. 8 , while the second peripheral member  120 A tries to rotate upward in  FIG. 8 . Accordingly, when the target W is held by a large force, a force twisting the target W (shearing force) may be generated. According to the robot hand  10  in this embodiment, however, the second center sliding shaft  126 A extends from the second peripheral member  120 A, and supports the second peripheral member  120 A trying to rotate around the second driving shaft  122 A by the generated force. Similarly, the second center sliding shaft  126 B extends from the second peripheral member  120 B, and supports the second peripheral member  120 B trying to rotate around the second driving shaft  122 B by the generated force. This structure prevents rotations of the second peripheral members  120 A and  120 B in the opposite directions (see  FIG. 8 ), thereby avoiding generation of the force twisting the target W (shearing force) even when the target W is held by a large force. 
       FIGS. 9A and 9B  illustrate the advantages produced by the presence of the second center sliding shafts  126 A and  126 B based on the result of an experiment. According to this experiment, the finger  140 A and the finger  140 D are brought into contact with each other, and the finger  140 B and the finger  140 C are brought into contact with each other in the first step (see  FIG. 9A ). In this condition, the finger  140 A and the finger  140 D are pressed against each other, and simultaneously the finger  140 B and the finger  140 C are pressed against each other, whereby a reaction force acts on each of the fingers  140 A through  140 D. As a result, the second peripheral member  120 A tries to rotate clockwise in  FIG. 9A , and the second peripheral member  120 B rotates anticlockwise in  FIG. 9A  by the mechanism explained with reference to  FIGS. 7A and 7B . Accordingly, the position of contact between the finger  140 A and the finger  140 D and the position of contact between the finger  140 B and the finger  140 C shift in the first direction. The amount of this shift increases as the pressing force between the finger  140 A and the finger  140 D (corresponding to the holding force F of the target W) and the pressing force between the finger  140 B and the finger  140 C (corresponding to the holding force F) become larger. 
       FIG. 9B  shows the amount of the shift measured by the actual measurement while varying the pressing force (holding force F) for the structure provided with the second center sliding shafts  126 A and  126 B and the structure not provided with the second center sliding shafts  126 A and  126 B. As apparent from the result in the figure, the amount of the shift considerably decreases by the presence of the second center sliding shafts  126 A and  126 B. It is therefore concluded that the robot hand  10  having the second center sliding shafts  126 A and  126 B in this embodiment can prevent generation of the force twisting the target W (shearing force) even when the target W is held by a large force. 
     D. CONFIGURATION OF END MEMBER 
     As illustrated in  FIGS. 1 and 2 , the robot hand  10  in this embodiment has the end member  150  configured to be shorter in the second direction than in the first direction. Accordingly, the target W can be securely held even when the target W is a small and thin-plate-shaped object. The reasons for this advantage are herein explained. 
     When the thin-plate-shaped target W has a sufficient width, the thin-plate-shaped target W can be held by decreasing the clearances between the fingers  140 A through  140 D in the first direction with the clearances between the fingers  140 A through  140 D in the second direction widened. Since the clearances of the fingers  140 A through  140 D in the second direction are sufficiently long, the fingers  140 A through  140 D do not interfere with the end member  150  even at the time of rise of the end member  150  for supporting the target W. Thus, the end member  150  can rise until contact with the target W. 
     On the other hand, when the target W having a thin-plate shape and a small width (small size) is held, the clearances between the fingers  140 A through  140 D in the first direction are decreased with the clearances between the fingers  140 A through  140 D in the second direction also decreased for a certain amount. Since the clearances between the fingers  140 A through  140 D in the second direction are small, there is a possibility of interference between the end member  150  and the fingers  140 A through  140 D at the time of rise of the end member  150  for supporting the target W. When the end member  150  is not raised up to contact with the target W due to the interference, the target W cannot be sufficiently held. 
     According to the robot hand  10  in this embodiment, however, the end member  150  has a small width in the second direction. In this case, even when the clearances between the fingers  140 A through  140 D in the second direction are small, the end member  150  does not easily interfere with the fingers  140 A through  140 D. Therefore, the target W can be securely held by the support of the end member  150  even when the target W is a thin-plate-shaped small object. 
     Moreover, according to the robot hand  10  in this embodiment, the direction where the shorter sides of the end member  150  extend (second direction) agrees with the direction where the second center sliding shafts  126 A and  126 B project. In this case, the target W is not broken by a shearing force even when the target is a thin-plate-shaped small object and held by a large force. 
       FIGS. 10A and 10B  are views for explaining the reasons why the robot hand  10  in this embodiment can hold the thin-plate-shaped small target W by a large force.  FIG. 10A  shows a condition where the robot hand  10  in this embodiment holds the thin-plate-shaped target W. As can be seen from the figure, the end member  150  has shorter sides in the second direction. In this case, when the clearance between the finger  140 A and the finger  140 D (clearance in the first direction) and the clearance between the finger  140 B and the finger  140 C (clearance in the first direction) are considerably decreased, the end member  150  can pass through the space between the fingers  140 A and the finger  140 B and the space between the finger  140 D and the finger  140 C. Accordingly, the end member  150  does not easily interfere with the fingers  140 A through  140 D even at the time of rise. 
       FIG. 10B  is a top view of the robot hand  10  holding the thin-plate-shaped small target W. When the second center sliding shafts  126 A and  126 B are not provided, a force twisting the target W (shearing force) is generated as discussed with reference to  FIG. 8 . In this case, the target W may be broken by the shearing force produced when the thin-plate-shaped target W is held by a large force as illustrated in  FIG. 10B . According to the robot hand  10  in this embodiment provided with the second center sliding shafts  126 A and  126 B, however, there is no possibility of damage to the thin-plate-shaped target W through avoidance of generation of the force twisting the target W (shearing force) even when the target W is held by a large force. 
     E. MODIFIED EXAMPLES 
     E-1. First Modified Example 
     According to the robot hand  10  in this embodiment described herein, the two second center sliding shafts  126 A and  126 B are provided in the second direction. However, either one of the two second center sliding shafts  126 A and  126 B may be eliminated. For example, the robot hand  10  is allowed to have only the second center sliding shaft  126 A (or only the second center sliding shaft  126 B) as illustrated in  FIG. 11A . When at least the second center sliding shaft  126 A (or the second center sliding shaft  126 B) is equipped as in this structure, a force twisting the target W (shearing force W) is not applied similarly to the embodiment. 
     According to the robot hand  10  in this embodiment, components corresponding to the second center sliding shafts  126 A and  126 B are not provided in the first direction. However, shafts similar to the second center sliding shafts  126 A and  126 B (hereinafter referred to as first center sliding shafts) may also be equipped in the first direction as shown in  FIG. 11B . More specifically, a first center sliding shaft  116 A extending in the first direction from the first peripheral member  110 A is inserted into a first center sliding bore  103  formed in the center member  100  so as to slide in the bore  103 . Similarly, a first center sliding shaft  116 B extending in the first direction from the first peripheral member  110 B is inserted into the first center sliding bore  103  formed in the center member  100  so as to slide in the bore  103 . According to this structure, generation of a force twisting the target (shearing force) is avoided when the target is held in the second direction. 
     E-2. Second Modified Example 
     According to the embodiment described herein, each of the second center sliding shafts  126 A and  126 B and the center sliding bores  104  receiving the second center sliding shafts  126 A and  126 B has a round shape in the direction perpendicular to the insertion direction. However, these shapes are not limited to round shapes but may be polygonal shapes. For example, as illustrated in  FIG. 12 , the cross-sectional shapes of the second center sliding shafts  126 A and  126 B in the direction perpendicular to the insertion direction, and the shapes of the second center sliding bores  104  in the direction perpendicular to the insertion direction may be quadrangular shapes. 
     According to this structure, rotation of the second center sliding shafts  126 A and  126 B within the second center sliding bores  104  is regulated, and the rigidity of the robot hand  10  at the time of hold of the target in the first direction increases. When the first center sliding shafts  116 A and  116 B are equipped, the cross-sectional shapes of the first center sliding shafts  116 A and  116 B in the direction perpendicular to the insertion direction, and the shapes of the first center sliding bores  103  in the insertion direction may be polygonal shapes. In this case, the rigidity of the robot hand  10  when holding the target in the second direction can increase similarly to above. 
     E-3. Third Modified Example 
     According to this embodiment described herein, only the one second center sliding shaft  126 A projects from the second peripheral member  120 A, while only the one second center sliding shaft  126 B projects from the second peripheral member  120 B. However, the number of the second center sliding shafts  126 A and  126 B projecting from the second peripheral members  120 A and  120 B is not limited one for each, but may be plural for each. For example, as illustrated in  FIG. 13 , the two second center sliding shafts  126 A and the two second sliding shafts  126 B may project from the second peripheral members  120 A and  120 B, respectively. 
     According to this structure, rotation of the second peripheral members  120 A and  120 B around the second center sliding shafts  126 A and  126 B, respectively, can be regulated. Accordingly, the rigidity of the robot hand  10  when holding the target in the first direction can increase. 
     A plural number of the first center sliding shafts  116 A and a plural number of the first center sliding shafts  116 B may be provided and extended from the first peripheral members  110 A and  110 B, respectively. According to this structure, rotation of the first peripheral members  110 A and  110 B around the first center sliding shaft  116 A and the first center sliding shaft  116 B can be regulated. Accordingly, the rigidity of the robot hand  10  when holding the target in the second direction can increase. 
     F. APPLICATION EXAMPLES 
     The robot hand  10  according to this embodiment and the modified examples is applicable to the following robots. 
       FIG. 14  illustrates an example of a single-arm robot  300  provided with the robot hand  10 . As illustrated in the figure, the robot  300  includes an arm  310  having a plurality of the link units  312 , and joints  320  which connect the link units  312  in a condition so that the link units  312  can bend. The robot hand  10  is connected with the tip of the arm  310 . According to this structure, the robot hand  10  can approach the position of the target by driving the arm  310  to hold the target. 
       FIG. 15  illustrates a plural-arm robot  350  provided with the robot hand  10  as an example. As illustrated in the figure, the robot  350  includes a plurality of (two in the example shown in the figure) the arms  310  each of which has a plurality of the link units  312 , and the joints  320  connecting the link units  312  in a manner so that the link units  312  can bend. The robot hand  10  and a tool  301  are connected with each tip of the arms  310 . A plurality of cameras  353  are mounted on a head  352 . A control unit  356  for controlling the overall operation is provided within a main body  354 . Casters  358  are further equipped on the bottom surface of the main body  354  for transfer. According to the structure of the robot  350 , the robot hands  10  can similarly approach the position of the target by driving the arms  310  to hold the target. 
     It is intended that the invention is not limited to the robot hands and robots described in the embodiment and modified examples herein, but may be practiced in various other forms without departing from the scope and spirit of the invention. 
     The entire disclosure of Japanese Patent Application No. 2012-138480 filed Jun. 20, 2012 is expressly incorporated by reference herein.