Patent Publication Number: US-2022219321-A1

Title: Robot apparatus, method for controlling robot apparatus, and load compensation apparatus

Description:
TECHNICAL FIELD 
     A technique disclosed herein relates to a robot apparatus including a movable portion such as a leg or an arm, a method for controlling the robot apparatus, and a load compensation apparatus. 
     BACKGROUND ART 
     In connection with the recent spread of automation techniques, robot apparatuses that include a movable portion such as a leg or an arm and that can be automatically operated have been utilized. The movable portion typically includes a multilink structure including a plurality of links connected together by a joint, and the movable portion is moved to any posture by driving the joint with use of an actuator such as a motor. 
     Additionally, a load due to weights of the links constantly acts on the movable portion of the robot apparatus, and thus even in a case where the movable portion is to be kept stationary in a particular posture such as an upright posture, the actuator for driving the joint needs to be continuously operated, leading to power consumption. Thus, a robot apparatus with a deadweight compensation function has been developed, the apparatus including an elastic body such as a spring assembled with the joint to reduce the load due to a weight of the robot apparatus during the stationary state. 
     For example, there has been proposed a deadweight compensation apparatus that is applied to a link mechanism such as an arm to change a spring constant according to a load (see PTL 1) Additionally, there has been proposed a load compensation apparatus that extends or contracts a string according to a weight of an object placed on a platform to change elasticity for compensation (see PTL 2). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     JP 2018-140475A 
     [PTL 2] 
     JP 2015-229539A 
     [PTL 3] 
     JP 2014-140300A 
     SUMMARY 
     Technical Problem 
     An object of a technique disclosed herein is to provide a robot apparatus including a load compensation function for a movable portion, a method for controlling the robot apparatus, and the load compensation apparatus. 
     Solution to Problem 
     A technique disclosed herein is developed in view of the above-described object, and a first aspect of the technique is a robot apparatus including one or more movable portions, a load compensation section utilizing an elastic body to compensate for a load acting on the movable portion, and an initial displacement amount setting section applying, to the elastic body, an initial displacement amount corresponding to a desired position or posture of the movable portion. 
     The initial displacement setting section includes an actuator displacing the elastic body by an initial displacement amount and locks the actuator with the elastic body remaining displaced by the initial displacement amount. 
     Additionally, the movable portion is a leg including a joint portion having at least a degree of rotational freedom around a pitch axis. Further, the initial displacement amount setting section calculates an initial displacement amount to be applied to the elastic body on the basis of a toe force of the leg calculated from a weight of luggage placed on the robot apparatus and a weight of the robot apparatus main body and a position of a center of gravity. 
     A second aspect of the technique described herein is a method for controlling a robot apparatus including one or more movable legs and utilizing an elastic body to compensate for a load acting on the movable leg. The method includes the steps of calculating an initial displacement amount to be applied to the elastic body on the basis of a toe force of the leg calculated from a weight of luggage placed on the robot apparatus and a weight of the robot apparatus main body and a position of a center of gravity, and using an actuator to displace the elastic body by the initial displacement amount and locking the actuator with the elastic body remaining displaced by the initial displacement amount. 
     A third aspect of the technique disclosed herein is a load compensation apparatus for a robot apparatus including one or more movable portions. The load compensation apparatus includes an elastic body compensating, by a restoring force, for a load acting on the movable portion, and an initial displacement amount setting section applying, to the elastic body, an initial displacement amount corresponding to a desired position or posture of the movable portion. 
     Advantageous Effect of Invention 
     The technique disclosed herein can provide a robot apparatus including a load compensation function dealing with a variation in load, a method for controlling the robot apparatus, and a load compensation apparatus. 
     Effects described herein are only illustrative, and effects produced by the technique disclosed herein are not limited to the effect described herein. Additionally, in addition to the above-described effect, the technique disclosed herein may further produce additional effects. 
     Further other objects, features, and advantages of the technique disclosed herein will be clarified by more detailed description based on an embodiment described below and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration example of a degree of freedom of a robot apparatus  100  including a movable portion. 
         FIG. 2  is a diagram illustrating a configuration example of an electric system of the robot apparatus  100 . 
         FIG. 3  is a diagram illustrating a weight of the robot apparatus  100  acting on the robot apparatus  100 . 
         FIG. 4  is a diagram illustrating movable legs of the robot apparatus  100 , the movable legs being assembled with springs for deadweight compensation. 
         FIG. 5  is a diagram illustrating luggage being placed on the robot apparatus  100 . 
         FIG. 6  is a diagram illustrating an initial displacement amount to be set for a spring for load compensation. 
         FIG. 7  is a diagram illustrating the initial displacement amount to be set for the spring for load compensation. 
         FIG. 8  is a diagram illustrating the initial displacement amount to be set for the spring for load compensation. 
         FIG. 9  is a diagram illustrating the initial displacement amount to be set for the spring for load compensation. 
         FIG. 10  is a diagram illustrating the initial displacement amount to be set for the spring for load compensation. 
         FIG. 11  is a diagram illustrating a configuration example of a movable leg  110  to which a load compensation mechanism enabling the initial displacement amount to be adjusted is applied. 
         FIG. 12  is a diagram illustrating an operation example of a load compensation mechanism  1100  illustrated in  FIG. 11 . 
         FIG. 13  is a diagram schematically illustrating a configuration of a series elastic actuator. 
         FIG. 14  is a diagram illustrating a configuration example of the movable leg  110  including a combination of the load compensation mechanism and the series elastic actuator. 
         FIG. 15  is a diagram illustrating luggage being placed on the robot apparatus  100 . 
         FIG. 16  is a diagram illustrating the robot apparatus  100  standing hallway up a hill. 
         FIG. 17  is a flowchart illustrating a processing procedure for performing load compensation in the robot apparatus  100 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     An embodiment of a technique disclosed herein will be described below in detail with reference to the drawings. 
     A. Apparatus Configuration 
       FIG. 1  schematically illustrates a configuration example of the degree of freedom of a robot apparatus  100  including a movable portion. The illustrated robot apparatus  100  includes a loading portion  101  on which luggage can be loaded, and four movable legs  110 ,  120 ,  130 , and  140  coupled to respective four corners of the loading portion  101 . The robot apparatus  100  is a walking robot that walks by synchronously operating the movable legs  110 ,  120 ,  130 , and  140 . Additionally, the robot apparatus  100  is assumed to be a luggage carriage robot carrying the luggage placed on the loading portion  101 . 
     The movable leg  110  includes two links  111  and  112  and a joint portion  113  connecting the link  111  and the link  112 . The other end (lower end) of the link  111  corresponds to a sole and is installed on a floor surface. Additionally, an upper end of the link  112  is attached to the loading portion  101  via a joint portion  114 . The joint portion  113  has a degree of rotational freedom around a pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  111  around the pitch axis with respect to the link  112 . Additionally, the joint portion  114  has at least a degree of rotational freedom around the pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  112  around the pitch axis with respect to the loading portion  101 . 
     Additionally, the movable leg  120  includes two links  121  and  122  and a joint portion  123  connecting the link  121  and the link  122 . The other end (lower end) of the link  121  corresponds to a sole and is installed on the floor surface. Additionally, an upper end of the link  122  is attached to the loading portion  101  via a joint portion  124 . The joint portion  123  has a degree of rotational freedom around the pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  121  around the pitch axis with respect to the link  122 . Additionally, the joint portion  124  has at least a degree of rotational freedom around the pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  122  around the pitch axis with respect to the loading portion  101 . 
     Additionally, the movable leg  130  includes two links  131  and  132  and a joint portion  133  connecting the link  131  and the link  132 . The other end (lower end) of the link  131  corresponds to a sole and is installed on the floor surface. Additionally, an upper end of the link  132  is attached to the loading portion  101  via a joint portion  134 . The joint portion  133  has a degree of rotational freedom around the pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  131  around the pitch axis with respect to the link  132 . Additionally, the joint portion  134  has at least a degree of rotational freedom around the pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  132  around the pitch axis with respect to the loading portion  101 . 
     Additionally, the movable leg  140  includes two links  141  and  142  and a joint portion  143  connecting the link  141  and the link  142 . The other end (lower end) of the link  141  corresponds to a sole and is installed on the floor surface. Additionally, an upper end of the link  142  is attached to the loading portion  101  via a joint portion  144 . The joint portion  143  has a degree of rotational freedom around the pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  141  around the pitch axis with respect to the link  142 . Additionally, the joint portion  144  has at least a degree of rotational freedom around the pitch axis and can be caused by an actuator such as a pitch axis rotation motor (not illustrated) to drive the link  142  around the pitch axis with respect to the loading portion  101 . 
     Luggage (not illustrated) with a weight equal to or smaller than a rated weight can be placed on the loading portion  101 . Additionally, the robot apparatus  100  can move on foot by synchronously driving the four movable legs  110 ,  120 ,  130 , and  140 , thus allowing carriage of the luggage placed on the loading portion  101 . Additionally, the placement surface of the loading portion  101  is desirably kept substantially level to prevent the luggage from slipping down from the loading portion  101  during conveyance. 
     Note that a “reference posture” of the robot apparatus  100  illustrated in  FIG. 1  refers to a position and a posture often used for walking. More specifically, the “reference posture” of the robot apparatus  100  refers to the position and the posture in which the placement surface of the loading portion  101  is kept substantially level to prevent the luggage from slipping down from the loading portion  101 . 
       FIG. 2  illustrates a configuration example of an electric system of the robot apparatus  100 . 
     The robot apparatus  100  includes, as an external sensor section  210 , cameras  211 L and  211 R functioning as a left and a right “eyes” of the robot apparatus  100 , a microphone  212  functioning as an “ear,” a touch sensor  213 , and the like. The cameras  211 L and  211 R, the microphone  212 , the touch sensor  213 , and the like are disposed at predetermined positions. As the cameras  211 L and  211 R, for example, cameras that include an imaging element such as a CMOS (Complementary Metal Oxide Semiconductor) or a CCD (Charge Couple Device) are used. 
     Note that, although not illustrated, the external sensor section  210  may further include another sensor. For example, the external sensor section  210  includes sole sensors that measure a floor reaction force acting on the sole of each of the movable legs  110 ,  120 ,  130 , and  140 , or the like. Each of the sole sensors includes, for example, a 6DOF (Degree Of Freedom) force sensor or the like. 
     Additionally, the external sensor section  210  may also include a LIDAR (Laser Imaging Detection and Ranging) sensor, a TOF (Time Of Flight) sensor, or a laser range sensor that can measure or estimate the direction of a predetermined target and the distance to the predetermined target. Additionally, the external sensor section  210  may include a GPS (Global Positioning System) sensor, an infrared sensor, a temperature sensor, a humidity sensor, an illuminance sensor, or the like. 
     Additionally, the robot apparatus  100  includes, as an output section, a speaker  221 , a display section  222 , and the like disposed at predetermined positions. The speaker  221  functions to output voice and provides, for example, voice guidance. Additionally, the display section  222  displays the state of the robot apparatus  100  and a response to a user. 
     A main control section  231 , a battery  232 , an internal sensor section  233 , an external memory  234 , and a communication section  235  are disposed in a control unit  230 . The internal sensor section  233  includes a battery sensor  233 A and an acceleration sensor  233 B. 
     The cameras  211 L and  211 R of the external sensor section  210  image a surrounding situation and transmit an image signal S 1 A obtained to the main control section  231 . The microphone  212  collects voice input from the user and transmits a voice signal S 1 B obtained to the main control section  231 . The input voice provided to the robot apparatus  100  by the user include an activation word, various instruction voices (voice commands) such as “walk,” “turn to the right,” “hurry,” and “stop,”. Note that  FIG. 2  illustrates only one microphone  82  but that two or more microphones may be provided for the left and right ears, for example, to estimate the direction of a voice source. 
     Additionally, the touch sensor  213  of the external sensor section  210  is, for example, laid on the placement surface of the loading portion  101  and detects a pressure received in a place where the luggage is placed on the loading portion  101 . The touch sensor  213  transmits a detection result to the main control section  231  as a pressure detection signal S 1 C. 
     The battery sensor  233 A of the internal sensor section  233  detects the amount of energy remaining in the battery  232  every predetermined periods and transmits a detection result to the main control section  231  as a battery remaining amount detection signal S 2 A. 
     The acceleration sensor  233 B detects, for movement of the robot apparatus  100 , accelerations in the directions of three axes (an x (roll) axis, a y (pitch) axis, and a z (yaw) axis) every predetermined periods and transmits a detection result to the main control section  231  as an acceleration detection signal S 2 B. For example, the acceleration sensor  233 B may be an IMU (Inertia Measurement Unit) equipped with a three-axis gyroscope, an acceleration sensor for three directions, and the like. The IMU can be used to measure the angle and the acceleration of the robot apparatus  100  main body or the loading portion  101 . 
     The external memory  234  stores programs, data, control parameters, and the like and provides any of the programs and data to a memory  231 A built in the main control section  231  as needed. Additionally, the external memory  234  receives the data or the like from the memory  231 A and stores it. Note that the external memory  234  may be configured as, for example, a cartridge type memory card such as an SD card and may be attachable to and removable from the robot apparatus  100  main body (or the control unit  230 ). 
     The communication section  235  performs data communication with the outside on the basis of, for example, a communication scheme such as Wi-Fi (registered trademark) or LTE (Long Term Evolution). For example, a program such as an application executed by the main control section  231  or data needed to execute the program can be acquired from the outside via the communication section  235 . 
     The main control section  231  incorporates the memory  231 A. The memory  231 A stores programs and data, and the main control section  231  executes any of the programs stored in the memory  231 A to perform various steps of processing. In other words, the main control section  231  determines the surrounding or internal situation of the robot apparatus  100 , the presence or absence of an instruction from the user or approach from the user, or the like on the basis of the image signal S 1 A, the voice signal S 1 B, and the pressure detection signal S 1 C (hereinafter collectively referred to as an external sensor signal S 1 ) respectively provided by the cameras  211 L and  211 R, the microphone  212 , and the touch sensor  213  of the external sensor section  210  and on the basis of the battery remaining amount detection signal S 2 A and the acceleration detection signal S 2 B (hereinafter collectively referred to as an internal sensor signal S 2 ) respectively provided by the battery sensor  233 A, the acceleration sensor  233 B, and the like of the internal sensor section  233 . Note that the memory  231 A may store, in advance, the weight of the robot apparatus  100  main body and the position of the center of gravity of the robot apparatus  100  main body (in this case, no luggage is placed on the loading portion  101 ). 
     Then, on the basis of a determination result for the surrounding or internal situation of the robot apparatus  100  or the presence or absence of an instruction from the user or approach from the user, on the basis of the control programs stored in the internal memory  231 A in advance, or on the basis of various control parameters stored in the external memory  234  installed at that time, the main control section  231  determines action of the robot apparatus  100  and an exhibited operation to be performed for the user, generates a control command based on a determination result, and transmits the control command to sub-control sections  241 ,  242 , . . . . 
     The sub-control sections  241 ,  242 , . . . are responsible for controlling operations of subsystems in the robot apparatus  100  and drives the subsystems on the basis of the control command provided by the main control section  231 . The above-described movable legs  110 ,  120 ,  130 , and  140  correspond to the subsystems and are driven and controlled by the sub-control sections  241 ,  242 ,  243 , and  244 , respectively. Specifically, the sub-control sections  241 ,  242 ,  243 , and  244  drive and control the joint portions  113 ,  123 ,  133 , and  143  and perform control such as setting of an initial displacement amount for each of load compensation mechanisms (described below). 
     B. Load Compensation 
       FIG. 3  schematically illustrates the weight of the robot apparatus  100  acting on the robot apparatus  100 . Strictly speaking, the weight of the robot apparatus  100  includes the weights of the movable legs  110 ,  120 ,  130 , and  140 , but for simplification of description,  FIG. 3  illustrates a weight mg of the robot apparatus  100  with a mass m acting on the loading portion  101 . 
     The weight mg acts distributively on the movable legs  110 ,  120 ,  130 , and  140 . Then, floor reaction forces F 1 , F 2 , F 3 , and F 4  act on the soles of the movable legs  110 ,  120 ,  130 , and  140 , respectively. In a case where the weight mg acts evenly distributively on the movable legs  110 ,  120 ,  130 , and  140 , then F 1 =F 2 =F 3 =F 4  (=F). 
     In short, the weight mg constantly acts on each of the movable legs  110 ,  120 ,  130 , and  140 . Thus, to make the robot apparatus  100  stationary in a posture as illustrated in  FIG. 3  (for example, the “reference posture” in which a height H from the floor surface to the loading portion  101  is constant and in which the placement surface of the loading portion  101  lies level), the actuators for pitch axis driving for the joint portions  113 ,  123 ,  133 , and  143  need to be operated, leading to continued power consumption even in the stationary state. 
       FIG. 4  illustrates the movable legs  110 ,  120 ,  130 , and  140  being assembled with springs  401 ,  402 ,  403 , and  404  for deadweight compensation. When link structures of the movable legs  110 ,  120 ,  130 , and  140  act to close due to the weight of the robot apparatus  100 , the springs  401 ,  402 ,  403 , and  404  contract to generate restoring forces. Consequently, by using the restoring forces of the springs  401 ,  402 ,  403 , and  404  to reduce loads imposed on the joint portions  113 ,  123 ,  133 , and  143  due to the weight of the robot apparatus  100 , power consumption for operating the actuators for pitch axis driving for the joint portions  113 ,  123 ,  133 , and  143  can be decreased. 
     Note that, as the springs used for load compensation such as deadweight compensation, tension springs applying twisting moment around the pitch axes of the joint portions  113 ,  123 ,  133 , and  143  may be utilized besides coil springs as illustrated in  FIG. 4  (compression coil springs or tension coil springs). 
     The weight mg is assumed to be evenly distributed over the movable legs  110 ,  120 ,  130 , and  140 , leading to action of the same floor reaction force F. Additionally, all of the springs  401 ,  402 ,  403 , and  404  are assumed to each have a spring constant K. In this case, if the movable legs  110 ,  120 ,  130 , and  140  are assembled with the springs  401 ,  402 ,  403 , and  404  with an initial displacement amount Δx applied to the springs  401 ,  402 ,  403 , and  404  such that a restoring force K·Δx that counters the floor reaction force F (=mg/4) acts on each of the springs  401 ,  402 ,  403 , and  404 , the loads imposed on the joint portions  113 ,  123 ,  133 , and  143  can be reduced to make the robot apparatus  100  stationary in the “reference posture” in which the height H from the floor surface to the loading portion  101  is constant and in which the placement surface of the loading portion  101  lies level. At this time, it is substantially unnecessary to operate the actuators for driving the pitch axes of the joint portions  113 ,  123 ,  133 , and  143 . 
     Additionally, the robot apparatus  100  is a luggage carriage robot that conveys the luggage placed on the loading portion  101  (as described above).  FIG. 5  schematically illustrating luggage  501  with a mass m luggage  being placed on the loading portion  101  of the robot apparatus  100 . In this case, a load Mg acts, the load Mg corresponding to the weight of the robot apparatus  100  main body plus the weight of the luggage  501  (in this regard, M=m+m luggage ). 
     The load Mg is assumed to be evenly distributed over the movable legs  110 ,  120 ,  130 , and  140 , leading to action of the same floor reaction force F. Additionally, all of the springs  401 ,  402 ,  403 , and  404  are assumed to each have the spring constant K. In this case, if the movable legs  110 ,  120 ,  130 , and  140  are assembled with the springs  401 ,  402 ,  403 , and  404  with the initial displacement amount Δx applied to the springs such that the restoring force K·Δx that counters the floor reaction force F (=Mg/4) acts on each of the springs  401 ,  402 ,  403 , and  404 , the loads imposed on the joint portions  113 ,  123 ,  133 , and  143  can be reduced to make the robot apparatus  100  stationary in the “reference posture” in which the height H from the floor surface to the loading portion  101  is constant and in which the placement surface of the loading portion  101  lies level. At this time, it is substantially unnecessary to operate the actuators for driving the pitch axes of the joint portions  113 ,  123 ,  133 , and  143 . 
     However, the weight of the luggage  501  loaded on the robot apparatus  100  is not constant. Thus, when light luggage is loaded, the restoring force K·Δx for load compensation acting on each of the springs  401 ,  402 ,  403 , and  404  is excessive, making the height from the floor surface to the loading portion  101  larger than the desired height H. In contrast, when heavy luggage is loaded, the restoring force K·Δx for load compensation acting on each of the springs  401 ,  402 ,  403 , and  404  is small, making the height from the floor surface to the loading portion  101  smaller than the desired height H. 
     Additionally, the load Mg is not necessarily evenly distributed over the movable legs  110 ,  120 ,  130 , and  140 . When the movable legs  110 ,  120 ,  130 , and  140  are assembled with the springs  401 ,  402 ,  403 , and  404  such that the restoring force K·Δx acts equally on all of the springs  401 ,  402 ,  403 , and  404 , those of the movable legs  110 ,  120 ,  130 , and  140  over which a load larger than the restoring force K·Δx is distributed are contracted, and in contrast, those of the movable legs  110 ,  120 ,  130 , and  140  over which a load smaller than the restoring force K·Δx is distributed are extended. In other words, the placement surface of the loading portion  101  is tilted to cause deviation from the “reference posture,” leading to the risk of slippage of the luggage. 
     The luggage placed on the loading portion  101  does not necessarily always have the same weight, and the load is not necessarily distributed evenly. Accordingly, the restoring force for load compensation is assumed to be different for each of the movable legs  110 ,  120 ,  130 , and  140  and to vary. Thus, the present embodiment introduces a mechanism for enabling any initial displacement amount to be set independently for each of the springs  401 ,  402 ,  403 , and  404  and causing an individual restoring force for load compensation to act on each of the springs  401 ,  402 ,  403 , and  404  to allow independent load compensation to be performed on each of the movable legs. 
     First, with reference to  FIGS. 6 to 8 , an initial displacement amount x adj  set for the springs will be described. A spring  600  as used herein is assumed to be, for example, at least any one of the springs  401 ,  402 ,  403 , and  404  applied to load compensation at the joint pitch axes of the movable legs of the robot apparatus  100 . 
       FIG. 6  illustrates the spring  600  with a natural length L.  FIG. 7  illustrates the spring  600  contracted in response to application of the initial displacement amount x adj . For example, by driving an actuator not illustrated to displace a lower end of the spring  600  upward by the initial displacement amount x adj  and at that position, locking the actuator, the spring  600  can be held with low power consumption, with the initial displacement amount x adj  as illustrated in  FIG. 7  applied to the spring  600 . 
     Note that locking of the actuator can be achieved by using, for example, a fixation mechanism such as a brake or a plunger. Alternatively, in a case where an ultrasonic motor or a brushless motor is utilized as the actuator, when power is cut off, the actuator itself exerts a high holding force, thus eliminating a need for a fixation mechanism such as a brake. 
       FIG. 8  illustrates a load of luggage  800  being applied to an upper end of the spring  600  contracted in response to application of the initial displacement amount x adj , the spring  600  being further contracted by x s . A load  800  as used herein corresponds to a component force resulting from distribution, to the movable leg, of the total Mg of the weight m of the robot apparatus  100  main body and the load of the loaded luggage m luggage . 
     In a state illustrated in  FIG. 8 , the spring  600  is shortened by x adj +x, compared to the natural length L. Consequently, assuming that K is the spring constant of the spring  600 , a restoring force F indicated by Equation (1) below acts on the spring  600 . 
     
       
         
           
             
               
                 
                   
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     The restoring force F is balanced with the load  800  applied to the spring  600 . Additionally, a contraction amount x s  of the spring  600  contracted when the load  800  is applied can be represented by Equation (2) below by using the initial displacement amount x adj   
     
       
         
           
             
               
                 
                   
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     In this regard, an upper end position x d  of the spring  600  that is desired when the load  800  is applied corresponds to the reference posture of the robot apparatus  100 . For convenience, the upper end position x d  of the spring  600  is hereinafter referred to as the “reference posture.” A relation represented by Equation (3) below is satisfied between the reference posture x d  and the contraction amount x s  of the spring  600 , as can be seen in  FIG. 8 . 
     
       
         
           
             
               
                 
                   
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     Then, when Equation (2) above is used to delete the contraction amount x s  of the spring  600  from Equation (3), the initial displacement amount x adj  to be applied to the spring  600  is represented by Equation (4) by using the reference posture x d  of the robot apparatus  100 . 
     
       
         
           
             
               
                 
                   
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     Consequently, according to Equation (4) above, the initial displacement amount x adj  of the spring  600  can be determined according to the load  800  applied to the spring  600  (balanced with the restoring force F). 
     As also can be seen in  FIG. 8 , applying the appropriate initial displacement amount x adj  to the spring  600  allows the upper end position of the spring  600  to align with the reference posture x d  of the robot apparatus  100 . 
     Additionally, the weight of the luggage  800  applied to the spring  600  varies according to the mass m luggage  of the luggage placed on the loading portion  101 . However, adaptive variation of the initial displacement amount x adj  allows the upper end position of the spring  600  to be kept in the reference posture x d  of the robot apparatus  100 . For example, as illustrated in  FIG. 9 , when luggage  900  with a larger weight is placed, applying a larger initial displacement amount x adj_long  to the spring  600  allows the upper end position of the spring  600  to be kept in the reference posture x d  of the robot apparatus  100 . In contrast, as illustrated in  FIG. 10 , when luggage  1000  with a small weight is placed, applying a small initial displacement amount x adj_short  to the spring  600  similarly allows the upper end position of the spring  600  to be kept in the reference posture x d  of the robot apparatus  100 . 
       FIG. 11  illustrates a configuration example in which a load compensation mechanism enabling the initial displacement amount to be adjusted is applied to the movable leg  110 . However, the load compensation mechanism is intended to compensate for a load acting on the joint portion  113  around the pitch axis, the joint portion  113  connecting the link  111  and the link  112 . Additionally, for simplification, in this case, the degree of freedom of the joint portion  114  connecting the upper end of the link  112  to the loading portion  101  is neglected. Note that the illustration and description of the other movable legs  120 ,  130 , and  140  are omitted. However, it should be appreciated that a load compensation mechanism similar to that for the movable leg  110  illustrated in  FIG. 11  is applied to each of the movable legs  120 ,  130 , and  140 . 
     The load compensation mechanism  1100  includes two springs  1101  and  1102  disposed in series. The load compensation mechanism  1100  is attached to the movable leg  110  so as to stride over the joint portion  113 , with one end of the load compensation mechanism  1100  fixed to the link  112  and the other end of the load compensation mechanism  1100  fixed to the link  111 . Consequently, the load compensation mechanism  1100  elastically supports the link  111  to the link  112 . 
     Additionally, when the load including the weight of the robot apparatus  100  and the weight of the luggage (not illustrated in  FIG. 11 ) placed on the loading portion  101  is applied to the movable leg  110 , a torque acts to rotate the joint portion  113  of the movable leg  110  around the pitch axis. When the link  111  rotates around the pitch axis via a joint portion  112  with respect to the link  112 , as represented by an arrow  1110 , the springs  1101  and  1102  are contracted and a restoring force that acts to extend the springs  1101  and  1102  to the original lengths is generated. Consequently, the load compensation mechanism  1100  can be said to elastically support the link  111  to the link  112  to compensate for at least a portion of the load acting on the joint portion  113  around the pitch axis. 
     The load compensation mechanism  1100  further includes an initial displacement amount setting section  1120  that can set any initial displacement amount for the springs  1101  and  1102 . In an example illustrated in  FIG. 11 , the initial displacement amount setting section  1120  is configured as a linear actuator driving, in the up and down direction in the sheet of  FIG. 11 , a midpoint position  1103  between the spring  1101  and the spring  1102  connected in series. When the linear actuator  1120  is driven to displace the midpoint position  1103  upward in the sheet of  FIG. 11  to apply the initial displacement amount x adj , the spring  1101  is contracted to generate a restoring force that acts to extend the spring  1101 , whereas the other spring  1102  is extended to generate a restoring force that acts to contract the spring  1102 . The load compensation mechanism  1100  as a whole applies, to between the link  111  and the link  112 , a restoring force (in other words, an assist force for resisting the load) acting downward in the sheet of  FIG. 11 , allowing compensation for at least a portion of the load acting on the joint portion  113  around the pitch axis. Then, the linear actuator  1120  locks the joint portion  113  at a position where the initial displacement amount x adj  is applied. 
     Incidentally, when the linear actuator  1120  is driven to displace the midpoint position  1103  downward in the sheet of  FIG. 11  (this displacement is not illustrated in  FIG. 11 ), the spring  1101  is extended to generate a restoring force that acts to contract the spring  1101 , whereas the other spring  1102  is contracted to generate a restoring force that acts to extend the spring  1102 . The load compensation mechanism  1100  as a whole can apply, to between the link  111  and the link  112 , a restoring force acting upward in the sheet of  FIG. 11 . 
       FIG. 12  illustrates the linear actuator  1120  being driven to displace the midpoint position  1103  upward in the sheet of  FIG. 12 , applying the initial displacement amount to the load compensation mechanism  1100 . 
     In a case where the midpoint position  1103  is displaced upward by Δx in the sheet of  FIG. 12 , the spring  1101  is contracted by Δx, whereas the spring  1102  is extended by Δx. Then, assuming that the spring  1101  and the spring  1102  both have a natural length of L/2 and a spring constant of K/2, the load compensation mechanism  1100  as a whole exerts a restoring force K·Δx/2+K·Δx/2=K·Δx downward in the sheet of  FIG. 12  to compensate for the load. 
     In this regard, the displacement amount Δx of the midpoint position  1103  is the total of the initial displacement amount x adj  of the midpoint position  1103  applied by a linear actuator  1102  used as the initial displacement amount setting section and the displacement amount x s  of the midpoint position  1103  attributed to the weight of the robot apparatus  100  and the load of the luggage placed on the loading portion  101  (in other words, Δx=x adj +x 5 ). Consequently, the load compensation mechanism  1100  applies, to between the link  111  and the link  112 , the restoring force F indicated by Equation (5), compensating for at least a portion of the load acting on the joint portion  113  around the pitch axis. 
     
       
         
           
             
               
                 
                   
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     The displacement amount of the midpoint position  1103  displaced when the weight of the robot apparatus  100  and the load of the luggage placed on the loading portion  101  are applied can be represented by Equation (6) below by using the initial displacement amount x adj  applied to the linear actuator  1120 . 
     
       
         
           
             
               
                 
                   
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     The upper end position of the link  112  that is desired when the weight of the robot apparatus  100  and the load of the luggage placed on the loading portion  101  are applied corresponds to the reference posture of the robot apparatus  100 . Then, the displacement amount x d  of the midpoint position  1103  from the natural length (=L/2) of the springs  1101  and  1102  is in a unique relation with the desired upper end position of the link  112 . For convenience, the displacement amount x d  of the midpoint position  1103  is hereinafter referred to as the “reference posture.” A relation represented by Equation (7) below is satisfied between the reference posture x d  and the contraction amount x s  of the spring  600 . 
     
       
         
           
             
               
                 
                   
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     Then, when Equation (6) above is used to delete the contraction amount x s  of the spring  600  from Equation (7), the initial displacement amount x adj  to be applied to the springs  1101  and  1102  by the linear actuator  1120  is represented by Equation (8) by using the reference posture x d  of the robot apparatus  100 . 
     
       
         
           
             
               
                 
                   
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     By locking the linear actuator  1120  at the position where the initial displacement amount x adj  is applied to the springs  1101  and  1102 , at least a portion of the load acting on the joint portion  113  around the pitch axis can be compensated for by the load compensation mechanism  1100  when the robot apparatus  100  with luggage with any weight placed on the loading portion  101  is held in the reference posture. 
     As illustrated in  FIGS. 11 and 12 , the two coil springs  1101  and  1102  may be provided, and a preload may be applied to between the springs  1101  and  1102 . This enables a restoring force to be generated even with the joint portion  113  bent around the pitch axis, with no additional driving source, thus allowing load compensation performance to be improved. 
     C. Combination of Load Compensation Mechanism and Series Elastic Actuator 
     As the actuator rotating and driving the joint portion  113  around the pitch axis, a rotation motor with a reduction gear may be applied, but for example, a series elastic actuator (SEA) may be applied. Needless to say, the series elastic actuator may also be applied similarly to the joint portions  123 ,  133 , and  143  of the other movable legs  120 ,  130 , and  140 . 
     The series elastic actuator is configured to transmit power output from a driving motor to an output side target via a spring used as an elastic member (see, for example, PTL 3) and has the feature that the series elastic actuator has a high compliance and mitigate shock applied to an apparatus including the series elastic actuator, compared to actuators of other types. Consequently, using the series elastic actuator for the joint portion  113  of the robot apparatus  100  has the advantage of mitigating shock given by the ground surface when a toe of the movable leg  110  comes into contact with the ground surface. Additionally, the series elastic actuator allows a load applied to the spring to be measured on the basis of the measured value of displacement amount of the spring and the spring constant of the spring. The load thus measured is related to the power actually transmitted to the output side target, and thus the measurement result for the load can be utilized for driving and controlling the series elastic actuator. 
       FIG. 13  schematically illustrates the configuration of the series elastic actuator. A series elastic actuator  1300  illustrated includes an actuator  1301  and a load  1302  connected in series by using an elastic body  1303  such as a spring. The elastic body  1303  having a higher compliance than the other mechanical elements intercepts transmission of a high-frequency wave between the actuator  1301  and the load  1302 , allowing prevention of transmission of frequent, slight displacement of the actuator  1301  and the like. Additionally, micro vibration and backlash are filtered by the elastic body  1303 , enabling more accurate force control. 
     Additionally, energy can be accumulated in (or released from) the elastic body  1303 , and thus the series elastic actuator  1300  allows efficient motion to be achieved. A force accumulated during the preceding operation can be released during the next operation, enabling an efficient operation as performed by animals using muscles. 
       FIG. 14  schematically illustrates a configuration example in which the load compensation mechanism  1100  illustrated in  FIG. 11  and the series elastic actuator  1300  as illustrated in  FIG. 13  are combined in an area around the joint portion  113  of the movable leg  110 . Both the load compensation mechanism  1100  and the series elastic actuator  1300  are components driven in a longitudinal direction and are compatible with each other in terms of mechanical design. For example, by the use of some common components or the like, the load compensation mechanism  1100  and the series elastic actuator  1300  having reduced sizes and weights can be mounted following the shape of the link  112 . 
     The series elastic actuator  1300  typically includes a force detection section (not illustrated). For example, even with a temporal change in the spring constant of the spring  1101  or  1102  in the load compensation mechanism  1100 , the change can be corrected on the basis of a detection result from the force detection section. Consequently, for the load compensation mechanism  1100 , a spring that includes a material such as carbon fiber reinforced plastic (CFRP) which is light but which changes over time can be utilized. Additionally, by using an ultrasonic motor or a brushless motor for an initial displacement amount setting section  1103 , the weights of the driving mechanism and the lock mechanism for the midpoint position  1103  can be reduced. 
     D. Load Compensation Control for Plurality of Movable Legs 
     When the load compensation mechanism enabling any initial displacement amount to be set as illustrated in  FIGS. 11 to 14  is disposed at each of the movable legs  110 ,  120 ,  130 , and  140  of the robot apparatus  100 , the load compensation operation can be achieved independently for each of the movable legs  110 ,  120 ,  130 , and  140 . 
     In the example illustrated in  FIGS. 3 to 5 , the load Mg is assumed to act distributively on the movable legs  110 ,  120 ,  130 , and  140 , the load Mg including the weight mg of the robot apparatus  100  main body and the load m luggage  of the luggage placed on the loading portion  101 . In such a case, to perform load compensation to set the movable legs  110 ,  120 ,  130 , and  140  in the same reference posture x d , it is sufficient if an equal initial displacement amount x adj  is set for each of the movable legs  110 ,  120 ,  130 , and  140  to cause the same restoring force F to act on each of the movable legs  110 ,  120 ,  130 , and  140 . 
     However, the weight mg of the robot apparatus  100  main body does not necessarily act distributively on the movable legs  110 ,  120 ,  130 , and  140 , or rather such design may be difficult. 
     Additionally, in the example illustrated in  FIG. 5 , the luggage  501  is placed approximately in the center of the placement surface of the loading portion  101 , and thus the load Mg is easily evenly distributed over the movable legs  110 ,  120 ,  130 , and  140 . On the other hand, as illustrated in  FIG. 15 , in a case where luggage  1501  is placed on the placement surface of the loading portion  101  at a far distance from the center of the placement surface, different floor reaction forces F 1 , F 2 , F 3 , and F 4  are assumed to act on the soles of the respective movable legs  110 ,  120 ,  130 , and  140 . In such a case, even with the same reference posture x d  for the movable legs  110 ,  120 ,  130 , and  140 , different initial displacement amounts x (1)   adj , x (2)   adj , x (3)   adj , and x (4)   adj  need to be set for the respective movable legs  110 ,  120 ,  130 , and  140  according to the respective floor reaction forces F 1 , F 2 , F 3 , and F 4 , as indicated by Equations (9) to (12). 
     
       
         
           
             
               
                 
                   
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     For example, a heavier load acts on a movable leg closer to the luggage placed, and thus a larger initial displacement amount needs to be set for this movable leg. However, a smaller load acts on a movable leg at a farther distance from the luggage, and thus it is sufficient if a smaller initial displacement amount is set for this movable leg. 
     Note that the position where the luggage is placed may be measured on the basis of detection results from the touch sensor  213  (described above) laid on the placement surface of the loading portion  101  and that the position of the center of gravity including the luggage may be calculated, to derive the floor reaction forces F 1 , F 2 , F 3 , and F 4  that act on the toes of the movable legs  110 ,  120 ,  130 , and  140 . Alternatively, force sensors may be installed at the toes of the movable legs  110 ,  120 ,  130 , and  140  to directly measure the floor reaction forces F 1 , F 2 , F 3 , and F 4 . 
     Additionally, in a case where the robot apparatus  100  stands on a level floor surface, the same reference posture x d  may be used for each of the movable legs  110 ,  120 ,  130 , and  140  to keep the placement surface of the loading portion  101  level. However, in a case where the robot apparatus  100  stands on a hill or a bumpy, irregular ground, varying reference postures (or varying heights from the toe to the loading portion  101 ) are used for the respective movable legs  110 ,  120 ,  130 , and  140  to keep the placement surface of the loading portion  101  level.  FIG. 16  illustrates the robot apparatus  100  standing halfway up a hill. In the example of the hill illustrated in  FIG. 16 , it will be appreciated that the reference postures x (1)   d  and x (2)   d  of the movable legs  110  and  120  used as front legs are short, whereas the reference postures x (3)   d  and x (4)   d  of the movable legs  130  and  140  used as rear legs are long. 
     In such a case, as indicated by Equations (13) to (16), different initial displacement amounts x (1)   adj , x (2)   adj , x (3)   adj , and x (4)   adj  need to be set for the respective movable legs  110 ,  120 ,  130 , and  140  according to the respective floor reaction forces F 1 , F 2 , F 3 , and F 4  such that the movable legs  110 ,  120 ,  130 , and  140  correspond to the different reference postures x (1)   d , x (2)   d , x (3)   d , and x (4)   d . 
     
       
         
           
             
               
                 
                   
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     Note that the sensor included in the external sensor section  210 , such as the LIDAR, the TOF sensor, or the laser range sensor (described above), may be used to measure or estimate the distance from the placement surface of the loading portion  101  to the toe of each of the movable legs  110 ,  120 ,  130 , and  140  and that, based on measurement or estimation results, reference postures x (1)   d , x (2)   d , x (3)   d , and x (4)   d  of the respective movable legs  110 ,  120 ,  130 , and  140  may be acquired. 
       FIG. 17  illustrates, in a flowchart form, a processing procedure for performing load compensation on the movable legs  110 ,  120 ,  130 , and  140  in the robot apparatus  100 . The illustrate processing procedure is basically executed by the main control section  231  used a subject. 
     Before execution of the processing procedure, information related to the reference posture of the robot apparatus  100  is set. The information related to the reference posture is specifically set in the format of the displacement amount x d  with respect to the natural length of the load compensation spring for each of the movable legs  1110 ,  120 ,  130 , and  140 . 
     Then, on the basis of information read out from the memory  231 A, detection results from the external sensor section  210 , and the like, the main control section  231  measures the weights of the robot apparatus  100  main body and the luggage placed on the loading portion  101  and the position of the center of gravity of the robot apparatus  100  (step S 1701 ). 
     Then, the main control section  231  calculates the toe forces F 1 , F 2 , F 3 , and F 4  of the respective movable legs  110 ,  120 ,  130 , and  140  required to stabilize the position and the posture of the robot apparatus  100  main body or the loading portion  101  (step S 1702 ). 
     Then, on the basis of the toe forces F 1 , F 2 , F 3 , and F 4 , the main control section  231  calculates the initial displacement amounts x (1)   adj , x (2)   adj , x (3)   adj , and x (4)   adj  to be set for the load compensation mechanisms for the movable legs  110 ,  120 ,  130 , and  140 , according to Equations (9) to (12) described above or Equations (13) to (16) described above (step S 1703 ). 
     Then, the main control section  231  calculates actuator driving amounts for the initial displacement amount setting sections  1120  for the movable legs  110 ,  120 ,  130 , and  140 , the actuator driving amounts being used to achieve the initial displacement amounts calculated in step S 1703  (step S 1704 ), and provides control commands to the sub-control sections  241 ,  242 ,  243 , and  244 . 
     The sub-control sections  241 ,  242 ,  243 , and  244  drive the actuators according to control commands from the main control section  231  to lock the actuators with the initial displacement amounts x (1)   adj , x (2)   adj , x (3)   adj , and x (4)   adj  applied to the load compensation springs for the movable legs  110 ,  120 ,  130 , and  140  (step S 1705 ). 
     By applying the initial displacement amounts x (1)   adj , x (2)   adj , x (3)   adj , and x (4)   adj  to the load compensation springs for the movable legs  110 ,  120 ,  130 , and  140  according to steps S 1701  to S 1705 , the robot apparatus  100  is set to allow the reference posture to be maintained using reduced power consumption. 
     Then, in a case where the luggage placed on the loading portion  101  is varied (Yes in step S 1706 ) after the robot apparatus  100  starts any operation and before the robot apparatus  100  ends the operation, the robot apparatus  100  returns to step S 1701  to re-set the initial displacement amounts x (1)   adj , x (2)   adj , x (3)   adj , and x (4)   adj  for the load compensation springs for movable legs  110 ,  120 ,  130 , and  140 . 
     The “variation” of the luggage as used herein is assumed to include an increase or a decrease in the amount of luggage and a variation in the position of the luggage placed on the placement surface of the loading portion  101 . In addition, the following case may be considered to correspond to the “variation” as used herein. That is, during the operation of a bot apparatus  100 , one or some of the movable legs become unusable due to failure or destruction and are thus, and increased burdens are placed on the remaining normal movable legs. 
     Additionally, in a case where the robot apparatus  100  moves to a hill, an irregular ground, or the like, and the reference posture x d  needs to be changed in order to keep the placement surface of the loading portion  101  level (Yes in step S 1707 ), the robot apparatus  100  returns to step S 1701  to re-set the initial displacement amounts x (1)   adj , x (2)   adj , x (3)   adj , and x (4)   adj  for the load compensation springs for movable legs  110 ,  120 ,  130 , and  140 . 
     Note that whether the robot apparatus  100  has entered a hill, an irregular ground, or the like, is detected by detecting, by the IMU (described above), tilt in the robot apparatus  100  main body or the loading portion  101 , or by detecting, by the sole sensor or the like, a change in toe force in at least one of the movable legs  110 ,  120 ,  130 , and  140 . 
     In addition, in step S 1707 , a change in the state of the hill or the irregular ground is also detected, before processing returns to step S 1701 . For example, when the inclination of the hill changes or the robot apparatus  100  passes through the hill or the irregular ground and returns to a flat road surface, the reference posture x d  needs to be changed in order to keep the placement surface of the loading portion  101  level. 
     Subsequently, when the robot apparatus  100  ends the operation, the processing also ends. 
     E. Effects 
     Finally, effects produced by the robot apparatus  100  according to the present embodiment will be described. 
     The robot apparatus  100  according to the present embodiment includes the load compensation mechanisms that are each provided in each of the movable legs  110 ,  120 ,  130 , and  140  and that each utilize an elastic body such as a spring, and the initial displacement amount is set for the elastic body according to the weight of the robot apparatus  100  main body and the load of the luggage. This enables a reduction in power consummation of the joint driving actuators for keeping the robot apparatus  100  in the desired reference posture. 
     Additionally, for example, following the processing procedure as depicted in  FIG. 17 , according to each toe force calculated based on the weight of the robot apparatus  100  main body and the load of the luggage carried, the robot apparatus  100  according to the present embodiment performs an operation for applying the appropriate initial displacement amount to the load compensation elastic body to execute load compensation to allow the robot apparatus  100  to maintain the reference posture. Additionally, when, during the operation of the robot apparatus  100 , the amount of the luggage is increased or reduced, the position of the luggage is varied, or the floor surface on which the robot apparatus  100  is installed is tilted, the initial displacement amount appropriate to the elastic body for load compensation is re-calculated and set again to enable load compensation in real time for allowing the robot apparatus  100  to maintain the reference posture. 
     Note that, in the above-described embodiment, the load compensation mechanisms are provided in all of the movable legs of the robot apparatus  100  but that, even when the load compensation mechanism is provided in only one movable leg or the load compensation mechanism are provided in some of the movable legs, the effect of reducing the power consumption of the joint driving actuators can be expected, the joint driving actuators being used to keep the robot apparatus  100  in the desired reference posture. Additionally, in a case where, during the operation of the robot apparatus  100 , one or some of the movable legs become unusable due to failure, malfunction, or destruction, then by using the remaining normal movable legs to perform load compensation, the effect of reducing the power consumption of the joint driving actuators can be expected, the joint driving actuators being used to keep the robot apparatus  100  in the desired reference posture. 
     The robot apparatus  100  according to the present embodiment compensates for the load corresponding to the combination of the weight of the robot apparatus  100  main body and the weight of the luggage placed on the loading portion  101 , allowing energy efficiency to be improved. As a result, the life of the battery  232  is extended, enabling the robot apparatus  100  to be operated for a long time. 
     By adjusting the initial displacement amount applied to the spring for load compensation provided in each of the movable legs  110 ,  120 ,  130 , and  140 , the robot apparatus  100  according to the present embodiment can maintain the reference posture without using an additional driving source, enabling a reduction in weight. Additionally, using two or more springs to impose a preload enables load compensation performance to be improved with no additional driving source. 
     In a case where series elastic actuators are used as actuators for pitch axis driving for the joint portions  113 ,  123 ,  133 , and  143 , a reduction in size and weight can be achieved by using some common components for the load compensation mechanisms and the series elastic actuators. 
     In the case of the robot apparatus  100  including a plurality of movable legs as in the present embodiment, the initial displacement amounts can be set independently for the respective movable legs to apply different assist forces to the respective movable legs for load compensation. Consequently, even in a case where varying loads are applied to the movable legs due to the position where the luggage is placed and the like, an appropriate assist force can be applied to each of the movable legs to suppress electric current consumption of the actuators for pitch axis driving, allowing the robot apparatus  100  to be operated for a long time. This also applies to a case where the robot apparatus  100  has entered a hill or an irregular ground or a case where some of the movable legs become inoperative due to failure, malfunction, destruction, or the like. 
     INDUSTRIAL APPLICABILITY 
     The technique disclosed herein has been described in detail with reference to the particular embodiment. However, obviously, a person skilled in the art may achieve modification or replacement of the embodiment without departing the spirits of the technique disclosed herein. 
     The embodiment in which the technique disclosed herein is applied to a four legged robot have been mainly described. However, the spirits of the technique disclosed herein are not limited to this. The technique disclosed herein can be similarly applied to a two legged robot, an arm robot, and various types of robot apparatuses including a multilink structure. 
     In short, the technique disclosed herein have been described in a form of illustration, and the details of the specification should not be interpreted in a limited manner. For determination of the spirits of the technique disclosed herein, claims should be taken into account. 
     Note that the technique disclosed herein can also take the following configurations. 
     (1) 
     A robot apparatus including: 
     one or more movable portions, 
     a load compensation section utilizing an elastic body to compensate for a load acting on the movable portion, and 
     an initial displacement amount setting section applying, to the elastic body, an initial displacement amount corresponding to a desired position or posture of the movable portion. 
     (2) 
     The robot apparatus according to (1) described above, in which 
     the initial displacement setting section includes an actuator displacing the elastic body by an initial displacement amount and locks the actuator with the elastic body remaining displaced by the initial displacement amount. 
     (3) 
     The robot apparatus according to (2) described above, in which 
     the movable portion is a leg including a joint portion having at least a degree of rotational freedom around a pitch axis, and 
     the load compensation section supports the joint portion. 
     (4) 
     The robot apparatus according to (3) described above, in which 
     the initial displacement amount setting section sets, on the basis of a toe force of the leg, an initial displacement amount for causing the elastic body to generate a restoring force for maintaining the desired position or posture. 
     (5) 
     The robot apparatus according to (4) described above, in which 
     the initial displacement amount setting section calculates an initial displacement amount to be applied to the elastic body, on the basis of a toe force of the leg calculated from a weight of luggage placed on the robot apparatus and a weight of the robot apparatus main body and a position of a center of gravity. 
     (6) 
     The robot apparatus according to any one of (1) to (5) described above, in which 
     the elastic body includes a coil spring or a torsion spring. 
     (7) 
     The robot apparatus according to any one of (2) to (5) described above, in which 
     the locking is achieved by a brake or a plunger, or fixation of the actuator itself (in a case where the actuator is an ultrasonic motor or a brushless motor). 
     (8) 
     The robot apparatus according to (5) described above, further including: 
     a plurality of legs, in which 
     each of the legs includes the load compensation section and the initial displacement amount setting section. 
     (9) 
     The robot apparatus according to (8) described above, in which 
     the initial displacement amount setting section of each of the legs sets an initial displacement amount corresponding to a toe force of each of the legs such that the robot apparatus is in a predetermined reference posture. 
     (10) 
     The robot apparatus according to (9) described above, in which 
     the robot apparatus includes a loading portion on which luggage is placed, and 
     the reference posture is a position and a posture of the robot apparatus in which the loading portion lies level. 
     (11) 
     The robot apparatus according to any one of (8) to (10) described above, in which, 
     in a case where toe forces of the legs vary according to the weight of the luggage placed on the robot apparatus and the weight of the robot apparatus main body and the position of the center of gravity, the initial displacement amount setting sections for the respective legs set different initial displacement amounts. 
     (12) 
     The robot apparatus according to (9) or (10) described above, in which 
     the initial displacement amount setting sections for the respective legs set different initial displacement amounts such that the robot apparatus is in a predetermined reference posture on a hill or in an irregular ground. 
     (13) 
     The robot apparatus according to any one of (8) to (12) described above, in which 
     the initial displacement amount setting sections set initial displacement amounts each time a change in the weight of the luggage placed on the robot apparatus and the weight of the robot apparatus main body or in the position of the center of gravity is detected. 
     (14) 
     The robot apparatus according to any one of (8) to (13) described above, in which 
     the initial displacement amount setting sections set initial displacement amounts each time the robot apparatus enters a hill or an irregular ground or a condition of a road surface is detected. 
     (15) 
     The robot apparatus according to any one of (8) to (14) described above, in which, 
     when at least one leg becomes unusable due to failure or malfunction, the initial displacement amount setting sections for the other legs set the initial displacement amounts. 
     (16) 
     The robot apparatus according to any one of (1) to (15) described above, in which 
     the elastic body includes a first elastic body and a second elastic body that are connected in series, and 
     the elastic body includes a structure imposing a preload between the first elastic body and the second elastic body. 
     (17) 
     The robot apparatus according to any one of (1) to (16) described above, further including: 
     a series elastic actuator driving the movable portion, in which 
     at least some of components of the load compensation section or the initial displacement amount setting section are common to the series elastic actuator. 
     (18) 
     A method for controlling a robot apparatus including one or more movable legs and utilizing an elastic body to compensate for a load acting on the movable leg, the method including the steps of: 
     calculating an initial displacement amount to be applied to the elastic body on the basis of a toe force of the leg calculated from a weight of luggage placed on the robot apparatus and a weight of the robot apparatus main body and a position of a center of gravity; and 
     using an actuator to displace the elastic body by the initial displacement amount and locking the actuator with the elastic body remaining displaced by the initial displacement amount. 
     (19) 
     A load compensation apparatus for a robot apparatus including one or more movable portions, the load compensation apparatus including: 
     an elastic body compensating, by a restoring force, for a load acting on the movable portion; and 
     an initial displacement amount setting section applying, to the elastic body, an initial displacement amount corresponding to a desired position or posture of the movable portion. 
     (20) 
     The load compensation apparatus according to (19) described above, in which 
     the initial displacement setting section includes an actuator displacing the elastic body by an initial displacement amount and locks the actuator with the elastic body remaining displaced by the initial displacement amount. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100 : Robot apparatus 
               101 : Loading portion 
               110 : Movable leg 
               111 ,  112 : Link 
               113 ,  114 : Joint portion 
               120 : Movable leg 
               121 ,  122 : Link 
               123 ,  124 : Joint portion 
               130 : Movable leg 
               131 ,  132 : Link 
               133 ,  134 : Joint portion 
               140 : Movable leg 
               141 ,  142 : Link 
               143 ,  144 : Joint portion 
               210 : External sensor section 
               211 L,  211 R: Camera 
               212 : Microphone 
               213 : Touch sensor 
               221 : Speaker 
               222 : Display section 
               230 : Control unit 
               231 : Main control section 
               232 : Battery 
               233 : Internal sensor section 
               233 A: Battery sensor 
               233 B: Acceleration sensor 
               234 : External memory 
               235 : Communication section 
               401 ,  402 ,  403 ,  404 : Spring for deadweight compensation 
               501 : Luggage 
               600 : Spring 
               800 : Luggage 
               900 : Luggage (heavy weight) 
               1000 : Carrying (light weight) 
               1100 : Load compensation mechanism 
               1101 ,  1102 : Spring 
               1120 : Initial displacement amount setting section 
               1300 : series elastic actuator 
               1301 : Actuator 
               1302 : Load 
               1303 : Elastic body 
               1501 : Luggage