Online impedance regulation techniques for compliant humanoid balancing

This paper presents three distinct techniques, aimed at the online active impedance regulation of compliant humanoid robots, which endeavours to induce a state of balance to the system once it has been perturbed. The presence of passive elastic elements in the drives powering this class of robots leads to under-actuation, thereby rendering the control of compliant robots an intricate task. Consequently, the impedance regulation procedures proposed in this paper directly account for these elastic elements. In order to acquire an indication of the robot’s state of balance in an online fashion, an energy (Lyapunov) function is introduced, whose sign then allows one to ascertain whether the robot is converging to or diverging from, a desired equilibrium position. Computing this function’s time derivative unequivocally gives the energy-injecting nature of the active stiffness regulation, and reveals that active damping regulation has no bearing on the system’s state of stability. Furthermore, the velocity margin notion is interpreted as a velocity value beyond which the system’s balance might be jeopardized, or below which the robot will be guaranteed to remain stable. As a result, the unidirectional and bidirectional impedance optimization methods rely upon the use of bounds that have been defined based on the energy function’s derivative, in addition to the velocity margin. Contrarily, the third technique’s functionality revolves solely around the use of Lyapunov Stability Margins (LSMs). A series of experiments carried out using the COmpliant huMANoid (COMAN), demonstrates the superior balancing results acquired when using the bidirectional scheme, as compared to utilizing the two alternative techniques

Efficient recursive dynamics algorithms for operational-space control with application to legged locomotion

This paper presents new recursive dynamics algorithms that enable operational-space control of floating-base systems to be performed at faster rates. This type of control approach requires the computation of operational-space quantities and suffers from high computational order when these quantities are directly computed through the use of the mass matrix and Jacobian from the joint-space formulation. While many efforts have focused on efficient computation of the operational-space inertia matrix Λ, this paper provides a recursive algorithm to compute all quantities required for floating-base control of a tree-structure mechanism. This includes the first recursive algorithm to compute the dynamically consistent pseudoinverse of the Jacobian J¯ for a tree-structure system. This algorithm is extended to handle arbitrary contact constraints with the ground, which are often found in legged systems, and uses effective ground contact dynamics approximations to retain computational efficiency. The usefulness of the algorithm is demonstrated through application to control of a high-speed quadruped trot in simulation. Our contact-consistent algorithm demonstrates pitch and roll stabilization for a large dog-sized quadruped running at 3.6 m/s without any contact force sensing, and is shown to outperform a simpler Raibert-style posture controller. In addition, the operational-space control approach allows the dynamic effects of the swing legs to be effectively accounted for at this high speed. J ¯ for a tree-structure system. This algorithm is extended to handle arbitrary contact constraints with the ground, which are often found in legged systems, and uses effective ground contact dynamics approximations to retain computational efficiency. The usefulness of the algorithm is demonstrated through application to control of a high-speed quadruped trot in simulation. Our contact-consistent algorithm demonstrates pitch and roll stabilization for a large dog-sized quadruped running at 3.6 m/s without any contact force sensing, and is shown to outperform a simpler Raibert-style posture controller. In addition, the operational-space control approach allows the dynamic effects of the swing legs to be effectively accounted for at this high speed.National Science Foundation (U.S.) (Graduate Research Fellowship)National Science Foundation (U.S.) (Grant No. CNS-0960061, with subaward to Ohio State University