Reactive Balance Control for Legged Robots under Visco-Elastic Contacts

Contacts between robots and environment are often assumed to be rigid for control purposes. This assumption can lead to poor performance when contacts are soft and/or underdamped. However, the problem of balancing on soft contacts has not received much attention in the literature. This paper presents two novel approaches to control a legged robot balancing on visco-elastic contacts, and compares them to other two state-of-the-art methods. Our simulation results show that performance heavily depends on the contact stiffness and the noises/uncertainties introduced in the simulation. Briefly, the two novel controllers performed best for soft/medium contacts, whereas “inverse-dynamics control under rigid-contact assumptions” was the best one for stiff contacts. Admittance control was instead the most robust, but suffered in terms of performance. These results shed light on this challenging problem, while pointing out interesting directions for future investigation.

Fixed-Time Inverse Dynamics Control for Robot Manipulators

This paper concerns with global fixed-time trajectory tracking of robot manipulators. A simple nonlinear inverse dynamics control (IDC) is proposed by using bi-limit homogeneity technique. Lyapunov stability theory and geometric bi-limit homogeneity technique are employed to prove global fixed-time tracking stability. It is proved that there exists a convergence time that is uniformly bounded a priori and such a bound is independent of the initial states such that the tracking errors converge to zero globally. The appealing advantages of the proposed control are that it is fairly easy to construct and has the global fixed-time tracking stability featuring faster transient and higher steady-state precision. Numerical simulation comparisons are provided to demonstrate the improved performance of the proposed approach.