As examples of technologies concerning hip joint structures for legged mobile robots are known from Japanese Patent Publication No. 2592340 ('340) and Japanese Laid-Open Application Nos. 2001-62761 ('761) and 2001-150371 ('371). In '340 (especially at pages 4 and 5 and FIG. 2), there is taught a configuration in which the motors for driving the hip joints are disposed on the body side so as to reduce weight toward the distal ends of the legs and lower the inertial moment produced in the legs.
The teaching of '761 (especially at paragraphs 0053 to 0055 and FIG. 12) relates to a configuration in which the hip joints are provided with a parallel link mechanism with which the right and left legs are connected and the parallel link mechanism is operated at free leg footfall to move the leg upward so as to mitigate the ground impact force.
The teaching of '371 (especially at 0070 to 0086 and FIGS. 5 and 7) relates to a configuration in which each rotary shaft providing the degree of freedom to rotate about the yaw axis among the degrees of freedom of the hip joints is offset relative to the roll axis direction to avoid interference between the feet when the robot changes directions of movement.
Aside from the above, in the case where the body of the legged mobile robot is to be bent (i.e., bent forward or backwards), situations sometimes arise in which the desired amount of bending cannot be realized solely within the movable range of the respective shafts that rotate about the pitch axis and the roll axis of the hip joints.
It has therefore been proposed to increase the amount of bending of the body by dividing the body into an upper section and a lower section, connecting the two sections through a joint having a degree of freedom to rotate about the pitch axis and rotating the upper and lower sections relative to each other so as to realize an amount of body bending that is larger than that realized by the movable range of the hip joints (this can be seen by, for example, Kawada Industries, Inc., “‘Rise/Lie Actions’ Achieved by Humanoid Worker Robot,” [online], Sep. 19, 2002, Kawada Industries Homepage, topics, [retrieved May 2, 2003], Internet <URL: http://www.kawada.co.jp/general/topics/020919_hrp-2p.html>).
However, the structure described on the aforesaid website has a problem in that the division of the body into upper and lower sections degrades the ability of the body to accommodate equipment internally.
Moreover, the fact that the movable range of hip joints is deficient means not only that the amount of body bending is deficient but also simultaneously that the movable range of the legs cannot be increased. In the technology described on the website, the amount of bending of the body is increased by equipping the body with the joint, which does nothing to increase the moveable range of legs, so that no improvement is achieved in the degree of posture and gait freedom of the whole robot including its lower part.