Patent Publication Number: US-2020298424-A1

Title: Knee structure of robot

Description:
TECHNICAL FIELD 
     The present disclosure relates to a knee structure of a robot in which auxiliary torque is applied to knees when a robot resumes to an upright state from a state where the knees are bent. 
     BACKGROUND ART 
     Conventionally, biped walking or two-legged robots have been developed. Since such a robot is capable of performing works similar to those which are performed by a human, it is expected to work at places into which a human is difficult to enter, such as removal of rubbles and lifesaving when a disaster occurs. 
     When resuming such a robot from a state where knees are bent to an upright state where the knees are straight, large torques are needed in the knee joints. Therefore, a configuration is proposed in which elastic bodies are disposed in the knee joints, and restoring forces of the elastic bodies are used when resuming the upright state from the knee-bent state. 
     Patent Document 1 discloses a legged robot in which elastic bodies are disposed in knee joints. In the legged robot of Patent Document 1, springs are attached to the knee joints, and when resuming the upright state from the knee-bent state, the restoring forces of the springs are used. Patent Document 1 also discloses a configuration in which, when a large restoring force is needed, the spring attached to the knee is pulled to increase the restoring force. 
     REFERENCE DOCUMENT OF CONVENTIONAL ART 
     Patent Document 
     
         
         [Patent Document 1] JP2001-287177A 
       
    
     DESCRIPTION OF THE DISCLOSURE 
     Problem to be Solved by the Disclosure 
     In the legged robot disclosed in Patent Document 1, the spring is attached to the knee joint so that it is pulled according to bending of the knee. Thus, when the robot bends the knees, the springs are pulled according to bending of the knees. However, since the spring attached to the knee is pulled each time the knee is bent, both a torque for bending the knee and a torque for pulling the spring are needed even when the knee is to be bent small. Therefore, even when bending the knee deeply or bending the knee shallowly, the torque needed for bending the knee each time the knee is bent becomes larger and, thus, electric power consumed for bending the knee increases. Since the power consumption of a humanoid robot becomes larger, the operation cost increases accordingly. 
     Therefore, the present disclosure is made in view of the above situation, and one purpose thereof is to provide a knee structure of a robot in which a torque needed for bending a knee is kept small, when bending the knee shallowly. 
     SUMMARY OF THE DISCLOSURE 
     A knee structure of a robot according to the present disclosure is provided. The robot has bendable legs, and each of the legs includes a thigh, a lower leg disposed below the thigh, a knee joint bendably connecting the thigh with the lower leg, and a restoring force applying member attached to the thigh and the lower leg so as to be across the thigh and the lower leg, with a slack when the leg stands straight. A restoring force caused by the restoring force applying member is given to the thigh and the lower leg, when the leg is bent deeply so that the restoring force applying member is stretched. 
     According to the knee structure of the robot with this configuration, since the restoring force applying member is configured to be attached to the thigh and the lower leg so as to be across the thigh and the lower leg with the slack, the restoring force applying member is pulled only when the leg is bent deeply somewhat. Therefore, a tensile force is not applied to the restoring force applying member when the leg is bent shallowly and, thus, a torque needed for bending the leg can be small. Further, when the leg is in a state where it is bent deeply so that the restoring force applying member is stretched, the restoring force is given to the thigh and the lower leg by the restoring force applying member and, thus, the torque is given auxiliary to the thigh and the lower leg. 
     Moreover, the restoring force applying member may be made of rubber and is formed in a belt-like shape. 
     Since the restoring force applying member is made of the belt-like rubber, the restoring force applying member easily extends and contracts so that the restoring force can suitably be given to the thigh and the lower leg. 
     Moreover, the restoring force applying member may be attached so as to close a gap between the thigh and the lower leg, the gap being produced when the leg is bent. 
     Since the restoring force applying member is attached so as to close a gap between the thigh and the lower leg which is produced when the leg is bent, it can prevent that an object enters into the gap between the thigh and the lower leg. 
     Moreover, the restoring force applying member may be attached so as to pass through a position on a projecting side of the knee joint when the leg is bent from a state where the leg stands straight. 
     Since the restoring force applying member is attached so as to pass through the position on the projecting side when the leg is bent from the state where the leg stands straight, the restoring force applying member is stretched when the leg is bent deeply, whereas the restoring force applying member contracts or shrinks when the leg makes the degree of bending shallower. Therefore, when the leg makes the degree of bending shallower from the state where the leg is bent deeply, the restoring force applying member can efficiently give the restoring force to the thigh and the lower leg. 
     Moreover, the knee structure may include a part to be compressed, attached to either one of the thigh and the lower leg, at a position on the opposite side of the projecting side of the knee joint when the leg is bent from a state where the leg stands straight, and configured to generate a restoring force by being pinched and compressed between the thigh and the lower leg, when the leg reaches a bending angle more than a given angle. 
     Since the restoring force by the part to be compressed is given to the thigh and the lower leg, the restoring force can be given to the thigh and the lower leg at the position separated from the restoring force applying member. Therefore, the restoring force can be given to the thigh and the lower leg complexly from the restoring force applying member and the part to be compressed, that is, a large restoring force can be given to the thigh and the lower leg. Further, since the restoring force by the part to be compressed is generated when the leg reaches the bending angle more than the given angle, the part to be compressed is not compressed in a case where the leg is bent shallowly, and it can avoid that the torque for compression of the part to be compressed is to be used. 
     Effect of the Disclosure 
     According to the present disclosure, only when the leg is bent deeply which needs comparatively large torque, the restoring force applying member gives the restoring force to the thigh and the lower leg. That is, the restoring force applying member can give the restoring force to the thigh and the lower leg only when necessary. When the leg is bent shallowly, the tensile force is not applied to the restoring force applying member so that the leg can be bent with small torque. Therefore, consumed electric power can be kept small and the operation cost of the robot can be kept low. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1( a )  is a front view of a humanoid robot according to a first embodiment of the present disclosure, and  FIG. 1( b )  is a side view of the humanoid robot of  FIG. 1( a ) . 
         FIG. 2  is a block diagram illustrating a configuration of a control system of the humanoid robot of  FIGS. 1( a ) and ( b ) . 
         FIG. 3  is a partial cross-sectional view illustrating the inside of a knee structure in which a cover of a knee joint in the humanoid robot of  FIGS. 1( a ) and ( b )  and a cover of a lower leg are partially cut off. 
         FIG. 4  is a perspective view of the knee structure in the humanoid robot of  FIGS. 1( a ) and ( b )  in a state where the cover of the knee joint is removed and the knee is bent. 
         FIG. 5( a ) to ( e )  are side views illustrating a leg at respective processes when bending the leg in the humanoid robot of  FIGS. 1( a ) and ( b )  from an upright state, and then resuming the upright state. 
         FIG. 6( a ) to ( c )  are side views illustrating the leg at respective processes when bending the leg in the humanoid robot of  FIGS. 1( a ) and ( b )  from the upright state, where a cover is removed to illustrate an internal structure. 
         FIG. 7( a )  is a perspective view of a knee structure in a humanoid robot according to a second embodiment of the present disclosure in a state where a cover of a knee joint is removed and the knee is bent, and  FIG. 7( b )  is a plan view of the knee structure of  FIG. 7( a ) . 
     
    
    
     MODES FOR CARRYING OUT THE DISCLOSURE 
     Hereinafter, knee structures of robots according to embodiments of the present disclosure are described with reference to the accompanying drawings. 
     First Embodiment 
     First, a knee structure of a robot according to a first embodiment of the present disclosure is described.  FIG. 1( a )  is a front view of a humanoid robot according to the first embodiment of the present disclosure, and  FIG. 1( b )  is a side view of the humanoid robot. 
     The humanoid robot  1  as the robot of this embodiment has an imitated human shape, and includes a head  2 , a body (trunk or torso)  3 , arms  4 , and legs  5 . The humanoid robot  1  controls driving of the legs  5  to perform two-legged walking. Note that, by driving the arms  4  and the legs  5  to move the hands and feet, works similar to those which are performed by a human can be performed. 
     The arm  4 , the leg  5 , etc. are comprised by connecting a plurality of links through joints. Between the links, it is configured to be bendable at the joints. A drive unit, such as a servomotor or an actuator, is disposed at each joint. By controlling the driving of the drive unit, a degree of bending between the links is controlled and the driving of the arms  4 , the legs  5 , etc. is controlled. A plurality of drive units are provided corresponding to the plurality of bendable joints provided to the humanoid robot  1 . 
     A cable  7  is connected to a rear surface part  6  of the body  3  in the humanoid robot  1 . The cable  7  is made of a flexible material, and therefore, it is configured to be bendable. Wiring is disposed inside the cable  7 , and current from a power supply is supplied to the humanoid robot  1  through the wiring disposed inside the cable  7 . In this embodiment, the wiring from the cable  7  branches and they are connected to the plurality of servo motors. The current from the cable  7  is supplied to the servo motors through the wiring. 
     An ankle  8  is provided to a tip-end part of each leg  5 . A foot  9  is attached to the outside of the ankle  8 . The foot  9  is detachably attached to the ankle  8 . 
     Each leg  5  has a thigh  10  and a lower thigh or lower leg  11  disposed below the thigh  10 . A knee joint  12  is provided between the thigh  10  and the lower leg  11 . The knee joint  12  bendably connects the thigh  10  with the lower leg  11 . Here, the knee joint  12  refers to a part configured to be bendable by a hinge etc. between the thigh  10  and the lower leg  11 . 
     In this embodiment, the knee joint  12  is provided as a rotation axis which is rotatable between the thigh  10  and the lower leg  11 , a part of the leg  5  above the knee joint  12  as the rotation axis is referred to as the thigh  10 , and a part of the leg  5  below the knee joint  12  as the rotation axis is referred to as the lower leg  11 . 
     A compression spring (part to be compressed)  16  is attached to the rear side of the lower leg  11  in the leg  5 . That is, the compression spring  16  is attached to a position of the lower leg  11  on the opposite side (rear side) from a projecting side (front side) than the knee joint  12  when bending the leg  5  from an upright state. 
     The compression spring  16  is attached to a position at which it is pinched and compressed between the thigh  10  and the lower leg  11  when the leg  5  is bent. Moreover, the compression spring  16  is attached to a position at which it is not compressed when the leg  5  stands straight and it is compressed when the leg  5  is bent. In this embodiment, particularly, the compression spring  16  is attached to a position at which it is pinched and compressed between the thigh  10  and the lower leg  11  only when the leg  5  becomes a bending angle more than a given angle. Here, the bending angle more than the given angle refers to a bending angle at which the compression spring  16  contacts the thigh  10  or the lower leg  11  as a result of bending the leg  5  deeply. When the bending angle becomes more than the given angle, the compression spring  16  is pinched between the thigh  10  and the lower leg  11  to generate a restoring force thereat. 
     Note that, in this embodiment, although the compression spring  16  is attached to the lower leg  11 , the present disclosure is not limited to this configuration. The compression spring  16  may be attached to the thigh  10  as long as it is attached to either the thigh  10  or the lower leg  11 . 
     Next, a control configuration of the humanoid robot  1  is described.  FIG. 2  illustrates a block diagram of the control configuration in the humanoid robot  1 . 
     As illustrated in  FIG. 2 , a control part  14  in the humanoid robot  1  includes a processor  14   a , a memory  14   b , and a servo controller  14   c.    
     For example, the control part  14  is a robot controller provided with a computer, such as a microcontroller. Note that the control part  14  may be comprised of a sole control part  14  which carries out a centralized control, or may be comprised of a plurality of control parts  14  which collaboratively carry out a distributed control. 
     The memory  14   b  stores information on a basic program as a robot controller, various fixed data, etc. The processor  14   a  controls various operations of a body part  100  by reading and executing software, such as the basic program stored in the memory  14   b . That is, the processor  14   a  generates a control command of the body part  100 , and outputs it to the servo controller  14   c . For example, the processor  14   a  is comprised of a processor unit. 
     The servo controller  14   c  is configured to control the driving of the drive units corresponding to the respective joints of the humanoid robot  1  based on the control commands generated by the processor  14   a.    
     Next, the knee structure of the humanoid robot  1  is described. 
       FIG. 3  illustrates a partial cross-sectional view illustrating the inside of the knee structure in which a cover  12   a  of the knee joint  12  and a cover  11   a  of the lower leg  11  are partially cut off.  FIG. 3  illustrates a state where the legs  5  stand straight. 
     As illustrated in  FIG. 3 , a rubber member  15  is provided to the knee joint  12  as a restoring force applying member between a part of the thigh  10  in the leg  5  and a part of the lower leg  11  in the leg  5 . An elastic body is used as the restoring force applying member, and in this embodiment, the rubber member  15  is used. The rubber member  15  is made of rubber, and in this embodiment, the rubber member  15  is comprised of a bundle of rubber tubes. As illustrated in  FIG. 3 , the rubber member  15  may be constituted as a bundle of rubber member  15  in which a plurality of stacked rubber tubes are attached, or may be constituted as a bundle of rubber member  15  in which a single rubber tube is folded and stacked a plurality of times. 
     The rubber member  15  is attached to an upper rubber member supporting part  10   a  which extends downwardly from the thigh  10 , at a position above the knee joint  12 . Moreover, the rubber member  15  is attached to a lower rubber member supporting part  11   b  which is provided to an upper end of the lower leg  11 , at a position below the knee joint  12 . That is, the rubber member  15  is provided so as to bridge between the upper rubber member supporting part  10   a  of the thigh  10  and the lower rubber member supporting part  11   b  of the lower leg  11 . 
     In this embodiment, the upper rubber member supporting part  10   a  of the thigh  10  and the lower rubber member supporting part  11   b  of the lower leg  11  have a pipe shape. The rubber member  15  constituted by the rubber tube is wound around the pipe-like upper rubber member supporting part  10   a  and lower rubber member supporting part  11   b  at respective positions so that it is provided to bridge between the upper rubber member supporting part  10   a  and the lower rubber member supporting part  11   b.    
     The rubber member  15  is attached across the thigh  10  and the lower leg  11  in a state where it is slack when the leg  5  stands straight. A support  5   a  of the leg  5  extends in the vertical direction and is disposed rearward of the rubber member  15 . In this embodiment, the knee joint  12  is a bendable part of the support  5   a.    
     Next, the knee structure in the humanoid robot  1  when bending the knee is described. 
       FIG. 4  illustrates the knee structure in a state where the knee of the leg  5  is bent.  FIG. 4  is a perspective view of the knee structure in a state where the cover  12   a  of the knee joint  12  is removed and the knee is bent. 
     As illustrated in  FIG. 4 , the rubber member  15  passes through a position forward of the knee joint  12  and is provided between the thigh  10  and the lower leg  11 . Thus, the rubber member  15  is attached so as to pass through a position forward of the knee joint  12  which projects from the upright state when the leg  5  is bent. 
     When the knee of the leg  5  is bent, a distance between the upper rubber member supporting part  10   a  of the thigh  10  and the lower rubber member supporting part  11   b  of the lower leg  11  becomes longer. Thus, the rubber member  15  provided so as to bridge between the upper rubber member supporting part  10   a  and the lower rubber member supporting part  11   b  is pulled in the vertical direction. 
     Since the rubber member  15  is attached in the state where it is slack when the leg  5  stands straight, the rubber member  15  itself is not stretched in a state where the knee is bent shallowly, but only the slack part is stretched or tightened (eliminating the slackness). 
     When the knee is bent somewhat deeply, the rubber member  15  is further pulled from the state where the slack part of the rubber member  15  is stretched or tightened, and therefore, the rubber member  15  itself is then stretched. When the rubber member  15  is stretched, it produces a restoring force in a direction of contracting or shrinking the rubber member  15 . 
       FIG. 5( a ) to ( e )  illustrate side views of the leg  5  for respective states from the humanoid robot bending the knee until the robot resuming the upright state of the knee. 
       FIG. 5( a )  illustrates a side view of the leg  5  in a state where it stands straight. Since the leg  5  is in the state where it stands straight (upright state), the rubber member  15  is disposed in the slack state so as to bridge between the upper rubber member supporting part  10   a  and the lower rubber member supporting part  11   b.    
       FIG. 5( b )  illustrates a side view of the leg  5  in a state where it is bent shallowly. When the leg  5  is bent shallowly, the rubber member  15  becomes in the state where the slack part is stretched or tightened. In the state of  FIG. 5( b ) , the rubber member  15  itself is not stretched, but only the slack part of the rubber member  15  is stretched or tightened. That is, the length of the rubber member  15  illustrated in  FIG. 5( b )  is a natural length of the rubber member  15 . 
       FIG. 5( c )  illustrates a side view of the leg  5  in a state where it is bent deeply. When the knee is further bent deeply from the state illustrated in  FIG. 5( b ) , a tensile force F 1  further acts on the stretched rubber member  15 . Thus, the rubber member  15  is further pulled, and therefore, the rubber member  15  itself is stretched. 
     Further, when the leg  5  is bent deeply, the compression spring  16  is compressed. The compression spring  16  is attached to a position at which it is not compressed in the upright state of the leg  5 , but it is compressed when the leg  5  is bent. In this embodiment, particularly, the compression spring  16  is attached to a position at which it is compressed for the first time when the leg  5  is bent somewhat deeply and the bending angle of the leg  5  becomes more than a certain angle. When the leg  5  is bent deeply, the compression spring  16  is compressed by being pinched between the thigh  10  and the lower leg  11 . Therefore, a compressive force F 2  acts on the compression spring  16 . 
       FIG. 5( d )  illustrates a side view of the leg  5  in a state where it again stands straight from the deeply-bent state of the leg  5 . In the state illustrated in  FIG. 5( d ) , since the leg  5  is bent deeply, the rubber member  15  is in the state where it is pulled. Since the rubber member  15  is in the already-pulled state, the rubber member  15  has a restoring force F 3  for contracting or shrinking from the state. That is, when the leg  5  is in the state where it is bent deeply so that the rubber member  15  is stretched, the restoring force F 3  of the rubber member  15  is given to the thigh  10  and the lower leg  11 . When the leg  5  stands straight, this restoring force F 3  of the rubber member  15  is used auxiliary. By the rubber member  15  causing the restoring force F 3  to act on the thigh  10  and the lower leg  11 , torque which rotates the thigh  10  in a D 1  direction centering on the knee joint  12  occurs. Moreover, torque which rotates the lower leg  11  in a D 2  direction centering on the knee joint  12  occurs. Thus, when the leg  5  resumes the upright state from the state where it is bent deeply, the restoring force F 3  caused by the rubber member  15  is given to the thigh  10  and the lower leg  11 . 
     Note that although in this embodiment the restoring force F 3  caused by the rubber member  15  is given to the thigh  10  and the lower leg  11  when the leg  5  resumes the upright state from the state where it is bent deeply, the present disclosure is not limited to this embodiment. When the leg  5  makes the degree of bending shallower from the state where the leg  5  is bent deeply and the rubber member  15  is stretched, the restoring force F 3  may be given to the thigh  10  and the lower leg  11 . Even if the leg  5  does not resumes the upright state, but it just makes the bending angle shallower, the restoring force F 3  caused by the rubber member  15  may also be given to the thigh  10  and the lower leg  11 . 
     In the state illustrated in  FIG. 5( d ) , since the leg  5  is bent deeply, the compression spring  16  is in the compressed state. In  FIG. 5( d ) , since the compression spring  16  is in the already-compressed state, the compression spring  16  has a restoring force F 4  for extending from this state. When the leg  5  resumes the upright state, this restoring force F 4  of the compression spring  16  is used auxiliary. By the compression spring  16  causing the restoring force F 4  to act on the thigh  10  and the lower leg  11 , the torque which rotates the thigh  10  in the D 1  direction centering on the knee joint  12  and the torque which rotates the lower leg  11  in the D 2  direction centering on the knee joint  12  occur. That is, when resuming the upright state from the state where the leg  5  is bent, the restoring force F 4  caused by the compression spring  16  is given to between the thigh  10  and the lower leg  11 . 
     Note that although in this embodiment the restoring force F 4  caused by the compression spring  16  is given to the thigh  10  and the lower leg  11  when the leg  5  resumes the upright state from the state where it is bent deeply, the present disclosure is not limited to this embodiment. When the leg  5  makes the degree of bending shallower from the deeply-bent state where the compression spring  16  is compressed, the restoring force F 4  caused by the compression spring  16  may be given to the thigh  10  and the lower leg  11 . Even if the leg  5  does not resume the upright state, but it just makes the bending angle shallower, the restoring force F 4  caused by the compression spring  16  may also be given to the thigh  10  and the lower leg  11 . 
     By making the bending angle of the leg  5  gradually shallower, while auxiliary using the restoring force F 3  caused by the rubber member  15  and the restoring force F 4  caused by the compression spring  16 , the leg  5  resumes the upright state. 
       FIG. 5( e )  illustrates a side view of the leg  5  in a state where the leg  5  again stands straight. From the state illustrated in  FIG. 5( d ) , the thigh  10  rotates in the D 1  direction centering on the knee joint  12  and the lower leg  11  rotates in the D 2  direction centering on the knee joint  12 , the leg  5  approaches the upright state. When the leg  5  becomes straight, the leg  5  stands straight. 
     The torque needed for driving the knee joint  12  when resuming the knee joint  12  to the upright state from the bent state becomes larger as the bent angle increases. 
       FIG. 6( a ) to ( c )  illustrate a structure inside the cover of the leg  5 .  FIG. 6( a ) to ( c )  are side views of the leg  5  in a state where the cover is removed. 
       FIG. 6( a )  illustrates an internal structure of the leg  5  in the state where it stands straight. As illustrated in  FIG. 6( a ) , the leg  5  is provided with a plurality of actuators  17 . In this embodiment, three actuators  17  are attached to each leg  5 . 
     In  FIG. 6( a ) to ( c ) , the actuator attached to the front side of the thigh  10  is an actuator  17   a , the actuator attached to the rear side of the thigh  10  is an actuator  17   b , and the actuator attached to the lower leg  11  is an actuator  17   c.    
     By activating the actuators  17  to change lengths of respective links  18  connected to the corresponding actuators  17 , the bending angle of the knee joint  12  can be changed. 
     In this embodiment, the link connected to the actuator  17   a  is a link  18   a , the link connected to the actuator  17   b  is a link  18   b , and the link connected to the actuator  17   c  is a link  18   c.    
     In the state where the leg  5  stands straight, a load L 1  is applied to the support  5   a  from the body. Therefore, the support  5   a  in the upright state supports the load L 1  from the body. 
       FIG. 6( b )  illustrates the internal structure of the leg  5  in the state where the leg  5  bends the knee joint  12  shallowly. 
     In the state where the leg  5  is bent, a load L 2  from the body is supported by the support  5   a , the link  18   a , and the link  18   b . In order to support the load L 2  from the body, a load L 4  acts on the actuator  17   a , and a load L 5  acts on the actuator  17   b . Moreover, in order to support a load L 3  from the body, a load L 6  acts on the actuator  17   c . In order for the leg  5  to maintain the posture and to maintain the bending angle of the knee joint  12 , a load corresponding to the load L 4  is applied to the actuator  17   a , a load corresponding to the load L 5  is applied to the actuator  17   b , and a load corresponding to the load L 6  is applied to the actuator  17   c.    
       FIG. 6( c )  illustrates the internal structure of the leg  5  in the state where the leg  5  bends the knee joint  12  deeply. 
     In the deeply-bent state, the leg  5  supports a load L 7  from the body by the support  5   a , the link  18   a , and the link  18   b . In order to support the load L 7  from the body, a load L 9  acts on the actuator  17   a , and a load L 10  acts on the actuator  17   b . Moreover, in order to support a load L 8  from the body, a load L 11  acts on the actuator  17   c.    
     In order to support the load L 7  from the body, the load L 9  acts on the actuator  17   a , and the load L 10  acts on the actuator  17   b . Moreover, in order to support the load L 8  from the body, the load L 11  acts on the actuator  17   c . In order for the leg  5  to maintain the posture and to maintain the bending angle of the knee joint  12 , a load corresponding to the load L 9  is applied to the actuator  17   a , a load corresponding to the load L 10  is applied to the actuator  17   b , and a load corresponding to the load L 11  is applied to the actuator  17   c.    
     At this time, the support  5   a  cannot support a load component which acts in the front-and-rear direction of the humanoid robot  1 . The load L 9  of the actuator  17   a  in  FIG. 6( c )  becomes larger than the load L 4  of the actuator  17   a  in  FIG. 6( b ) . Moreover, the load L 10  of the actuator  17   b  in  FIG. 6( c )  becomes larger than the load L 5  of the actuator  17   b  in  FIG. 6( b ) . Moreover, the load L 11  of the actuator  17   c  in  FIG. 6( c )  becomes larger than the load L 6  of the actuator  17   c  in  FIG. 6( b ) . The load of the actuator  17  becomes larger as the bending angle of the leg  5  becomes deeper. Therefore, in order to maintain the bending angle of the leg  5  and to maintain the posture of the leg  5 , the load which acts on the actuator  17  becomes larger as the bending angle of the leg  5  becomes deeper. 
     Moreover, when driving the leg  5  in order to resume the leg  5  to the upright state from the bent posture, it is necessary to make the torque exceeding the load for maintaining the posture of the leg  5  act on each actuator  17 . Therefore, in the deeply-bent state of the leg  5 , a large torque is needed for each actuator  17  to resume the leg  5  to the upright state from the bent posture. 
     As illustrated in  FIG. 5( d ) , in this embodiment, when resuming the leg  5  to the upright state from the deeply-bent state, the knee joint  12  of the leg  5  can be driven by driving the actuator  17  while auxiliary using the restoring force F 3  caused by the rubber member  15  and the restoring force F 4  caused by the compression spring  16 . Therefore, the required driving force of each actuator  17  can be reduced. 
     Moreover, in this embodiment, since the rubber member  15  is attached so as to be slack, the tensile force acts on the rubber member  15  for the first time when the leg  5  is bent deeply as illustrated in  FIG. 5( c ) , and from this state, the restoring force F 3  acts on the rubber member  15  when the leg  5  resumes the upright state, as illustrated in  FIG. 5( d ) . Therefore, only when the leg  5  resumes the upright state from the deeply-bent state, the restoring force F 3  caused by the rubber member  15  can be used auxiliary. That is, the restoring force F 3  caused by the rubber member  15  can be used auxiliary, only when the large load is applied and the large driving force caused by the actuator  17  is needed. 
     On the other hand, as illustrated in  FIG. 5( b ) , in the state where the leg  5  is bent shallowly, the rubber member  15  itself is not stretched, but only the slack part of the rubber member  15  is stretched or tightened. Therefore, the rubber member  15  itself is not stretched only by bending the leg  5  shallowly. That is, when bending the leg  5  shallowly, torque for generating the tensile force in the rubber member  15  is not needed. Therefore, when bending the leg  5  shallowly, the driving force of the actuator  17  needed for bending the leg  5  can be kept small. Since the driving force caused by the actuator  17  can be kept small, the electric power consumed by the humanoid robot  1  can be kept small. Therefore, the operation cost of the humanoid robot  1  can be kept low. 
     Moreover, by thus attaching the rubber member  15  with the slack, the restoring force F 3  caused by the rubber member  15  is used auxiliary, only when resuming the leg  5  to the upright state from the deeply-bent state. Therefore, the humanoid robot  1  in which the restoring force F 3  caused by the rubber member  15  is used only when resuming the leg  5  to the upright state from the deeply-bent state can be provided with the simple configuration. Moreover, the configuration for causing the rubber member  15  to selectively generate the restoring force F 3  only when needed can be provided, without using a special drive unit. Thus, since the configuration for causing the rubber member  15  to selectively generate the restoring force can be provided with the simple configuration, the manufacture cost of the humanoid robot  1  can be kept low. 
     Moreover, in this embodiment, the compression spring  16  is attached to the position at which it is pinched and compressed between the thigh  10  and the lower leg  11  for the first time when the leg  5  is bent deeply. Therefore, as illustrated in  FIG. 5( c ) , the compression spring  16  is compressed for the first time when the leg  5  is bent deeply, and, as illustrated in  FIG. 5( d ) , from this state, the compression spring  16  generates the restoring force F 4  when resuming the uptight state of the leg  5 . Therefore, the restoring force F 4  caused by the compression spring  16  can be used auxiliary, only when resuming the upright state from the deeply-bent state of the leg  5 . That is, the restoring force F 4  caused by the compression spring  16  can be used auxiliary, only when the large load is applied to the leg  5  and the large driving force caused by the actuator  17  is needed. 
     On the other hand, in the state where the leg  5  is bent shallowly as illustrated in  FIG. 5( b ) , the compression spring  16  is neither pinched nor compressed. Therefore, the compression spring  16  is not compressed only by bending the leg  5  shallowly. Therefore, when bending the leg  5  shallowly, the torque for causing the compressive force to act on the compression spring  16  is not needed. Therefore, when bending the leg  5  shallowly, the driving force of the actuator  17  needed for bending the leg  5  can be kept small. Since the driving force caused by the actuator  17  can be kept small, the electric power consumed by the humanoid robot  1  can be kept small. Therefore, the operation cost of the humanoid robot  1  can be kept low. 
     Moreover, in this embodiment, the compression spring  16  is attached rearward of the knee joint  12 . That is, the compression spring  16  is attached to the position on the opposite side (rear side) from the projecting side (front side) of the knee joint  12  when bending the leg  5  from the upright state. Therefore, the compression spring  16  can be attached to the position on the opposite side of the rubber member  15 . Therefore, a space inside the knee structure can be efficiently used and the leg  5  can be downsized. 
     Moreover, by the rubber member  15  and the compression spring  16  which are attached to the separated positions, the restoring forces can be exerted complexly on the thigh  10  and the lower leg  11 . Therefore, since the different types of restoring forces can be exerted from the separated positions onto the thigh  10  and the lower leg  11 , the total restoring force can be increased. Thus, the driving force of the actuator  17  needed for causing the leg  5  to stand straight can be kept small. 
     Note that although in this embodiment the rubber member  15  made of rubber is constituted as the restoring force applying member, the present disclosure is not limited to this embodiment. The restoring force applying member may be made of materials other than rubber. The restoring force applying member may be made of other materials, as long as it can give the restoring force to the thigh  10  and the lower leg  11  when the leg  5  resumes the upright state from the deeply-bent state. 
     Second Embodiment 
     Next, a knee structure of the robot according to a second embodiment of the present disclosure is described. Note that description of parts constituted similarly to the first embodiment is omitted, and only different parts are described. 
     In the first embodiment, the rubber member  15  which is comprised of a plurality of stacked rubber tubes is used. On the other hand, a configuration of the second embodiment differs from that of the first embodiment in that a rubber member  15   a  has a belt-like shape. 
       FIG. 7( a )  illustrates a perspective view of the knee structure of the humanoid robot  1  according to the second embodiment, and  FIG. 7( b )  illustrates a plan view of the knee structure of the humanoid robot  1 .  FIGS. 7( a ) and ( b )  illustrate the knee structure in a state where the cover  12   a  of the knee joint  12  is removed. 
     In the second embodiment, the rubber member  15   a  which has the belt-like shape and is formed in the band-like shape is attached to the leg  5 . The rubber member  15   a  is provided so as to bridge between the upper rubber member supporting part  10   a  of the thigh  10  and the lower rubber member supporting part  11   b  of the lower leg  11 . Also in the second embodiment, the rubber member  15   a  is attached across the thigh  10  and the lower leg  11  so as to be slack when the leg  5  stands straight. 
     In the second embodiment, since the rubber member  15   a  is formed in the belt-like shape, the rubber member  15   a  easily extends and contracts in the extending and contracting direction of the rubber member  15   a , when the leg  5  bends and stands straight. Therefore, it is easy to adjust the restoring force caused by the rubber member  15   a . Therefore, the restoring force caused by the rubber member  15   a  can be suitably and easily given to the thigh  10  and the lower leg  11 . 
     Moreover, since the rubber member  15   a  has belt-like shape, the rubber member  15   a  can close or cover a gap between the thigh  10  and the lower leg  11  which is produced when the leg  5  is bent. 
     As illustrated in the plan view of  FIG. 7( b ) , in this embodiment, the rubber member  15   a  is attached so as to close or cover almost entirely the gap between the thigh  10  and the lower leg  11  which is produced when the leg  5  is bent, as the knee structure is seen in the plan view. Since the rubber member  15   a  is attached so as to close or cover the gap between the thigh  10  and the lower leg  11  which is produced when the leg  5  is bent, it can prevent that an object enters into the gap between the thigh  10  and the lower leg  11 . 
     Moreover, the rubber member  15   a  is disposed so as to close or cover a gap inside the cover  11   a  of the lower leg  11 . Since particularly in this embodiment the gap inside the cover  11   a  of the lower leg  11  can be closed or covered, it can be prevented that an object enters inside the cover  11   a.    
     As illustrated in  FIG. 7( b ) , when the lower leg  11  is seen in the plan view, the rubber member  15   a  is attached so as to close or cover almost entirely the gap inside the cover  11   a  of the lower leg  11 . 
     When the knee structure is seen in the plan view, since an upper surface of the cover  11   a  of the lower leg  11  is closed or covered almost entirely by the rubber member  15   a , and a gap C 1  inside the cover  11   a  is closed or covered almost entirely, it can be prevented that an object falls and enters inside the gap C 1 . Therefore, it can be prevented that a tool or a part or component falls and enters inside the gap C 1 . If the humanoid robot  1  is driven while a tool or a part has fallen inside the gap C 1 , the tool or part which fell inside the gap C 1  may contact the component(s) of the humanoid robot  1  to cause a failure. In this embodiment, since the fall of the tool or part inside the gap C 1  can be prevented, the reliability of the humanoid robot  1  can be improved. 
     Moreover, it can be prevented that a person puts his/her finger etc. into the gap between the thigh  10  and the lower leg  11 . Particularly, in this embodiment, since the gap C 1  inside the cover  11   a  of the lower leg  11  is closed or covered, it can be prevented that the person puts the finger etc. into the gap C 1 . Therefore, the safety of the humanoid robot  1  can be improved. 
     Moreover, the rubber member  15   a  has both the function as the restoring force applying member which gives the restoring force and the function as a protection member which closes or covers the gap between the thigh  10  and the lower leg  11 . By using the rubber member  15   a , since the knee structure has the function as the restoring force applying member and the function as the protection member, the space can be efficiently used, compared with a case where members having these functions are disposed separately. Therefore, the knee structure can be downsized. Moreover, since the knee structure can be downsized, the manufacture cost of the knee structure can be kept low. 
     Note that the rubber member  15   a  may be a member which closes or covers at least a part of the gap between the thigh  10  and the lower leg  11  which is produced when the leg  5  is bent. The rubber member  15   a  does not need to be configured to entirely cover the gap formed between the thigh  10  and the lower leg  11 . 
     Moreover, although in this embodiment the rubber member  15   a  closes or covers the upper surface of the cover  11   a  of the lower leg  11  almost entirely and closes or covers the gap C 1  inside the cover  11   a  almost entirely, when the knee structure is seen in the plan view, the present disclosure is not limited to this embodiment. The rubber member  15   a  may not be configured to close or cover the gap C 1  inside the cover  11   a , as long as it can close or cover at least a part of the gap between the thigh  10  and the lower leg  11 . 
     Moreover, although in this embodiment the rubber member  15   a  is comprised of a sheet, the present disclosure is not limited to this embodiment. The rubber member  15   a  may be comprised of a plurality of sheets. In such a case, the plurality of rubber members  15   a  may be wound around the knee, just like a person winds a supporter around his/her knee. Alternatively, the rubber member  15   a  of a sheet may be elongated, and the rubber member  15   a  may be wound around the knee a plurality of times. 
     Note that although in the first embodiment the rubber member  15  is comprised of the rubber tubes and in the second embodiment the rubber member  15   a  has the belt-like shape, the present disclosure is not limited to these embodiments. The rubber member as the restoring force applying member may have other shapes. The rubber member may have other shapes, as long as the rubber is pulled and its restoring force is used auxiliary as the torque for the leg standing straight. 
     Moreover, although in the above embodiments the compression spring  16  is attached to the position at which it is pinched and compressed between the thigh  10  and the lower leg  11  when the leg  5  is bent, the present disclosure is not limited to these embodiments. It may not be a spring but may be other things which are pinched and compressed between the thigh  10  and the lower leg  11  when the leg  5  is bent. Other things may be used as long as that, upon the leg  5  is bent deeply and other things are compressed and the leg  5  then stands straight, the restoring force is auxiliary used as the torque when the leg stands straight. For example, a member made of rubber may be disposed. 
     Moreover, although in the above embodiments, the configuration in which the compression spring  16  is attached is described, the present disclosure is not limited to these embodiments. The compression spring  16  may not be attached. When resuming the leg to the upright state from the deeply-bent state, only the restoring force of the rubber member may be used. 
     Moreover, although in the above embodiments the humanoid robot  1  is used as the robot, the present disclosure is not limited to these embodiments. The robot may not be a humanoid type as long as it has the knee structure of the present disclosure. The present disclosure may be applied to a robot having only a body and legs, without having arm(s). 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1  Humanoid Robot 
           10  Thigh 
           11  Lower Leg 
           12  Knee Joint 
           15  Rubber Member