Abstract
The concept of “passive dynamic walking robot” refers to the robot that can walk down a shallow slope stably without any actuation and control which shows a limit cycle during walking. By adding actuation at some joints, the passive dynamic walking robot can walk stably on level ground and exhibit more versatile gaits than fully passive robot, namely, the “limit cycle walker.” In this article, we present the mechanical structures and control system design for a passive dynamic walking robot with series elastic actuators at hip joint and ankle joints. We built a walking model that consisted of an upper body, knee joints, and flat feet and derived its walking dynamics that involve double stance phases in a walking cycle based on virtual power principle. The instant just before impact was chosen as the start of one step to reduce the number of independent state variables. A numerical simulation was implemented by using MATLAB, in which the proposed passive dynamic walking model could walk stably down a shallow slope, which proves that the derived walking dynamics are correct. A physical passive robot prototype was built finally, and the experiment results show that by only simple control scheme the passive dynamic robot could walk stably on level ground.
Highlights
Humans can walk so well on ground that they walk with natural gait, low energy consumption, high stability and great versatility
Because the main control schemes are based on the zero moment point (ZMP) method, by which all joints must be controlled at every instant to keep the ZMP within the convex hull of the feet to reach static equilibrium all the time, the walking energy consumption is high[3] and the walking gaits are quite unnatural
A detailed walking dynamics for the passive model that consisted of an upper body, two legs, two flat feet, a hip joint, two knee joints, and two ankle joints is derived based on virtual power principle
Summary
Humans can walk so well on ground that they walk with natural gait (inverted pendulum-like gait), low energy consumption, high stability and great versatility. When the trailing foot loses contact with the ground depending on the ground reaction force acting on the foot and the direction of the acceleration of the footground contact point,[20] in order to obtain the ground reaction force, at least two extra generalized coordinates xh and yh should be added to the system During this phase, the constraint of the system can be described as follows xh + l Á sin (u1) + lf 1 Á cos (u3) À lf e(q) yh xh yh. After the leading foot’s heel impact with the ground, the thigh and shank still rotate forward together for a while, which means that there is a double stance phase existing during this part of walking.
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