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

Stiffness evaluation of a novel ankle rehabilitation exoskeleton with a type-variable constraint

This paper presents a novel ankle rehabilitation exoskeleton with two rotational degrees of freedom, which is suitable for dynamical rehabilitation for patients with neurological impairments. Its stiffness performance is assessed in consideration that the interaction between the footplate and the ground may deflect the mechanism away from the desired/predefined motion patterns. The novel design employs a universal-prismatic-universal (U-P-U) joint link, whose constraint type changes between a couple and a line vector during manipulation of the exoskeleton. To conduct a stiffness analysis of such a mechanism with a type-variable constraint – for the first time – a modified screw-based method (SBM) is proposed. Comparisons with the results obtained from finite element analysis verified that, the modified SBM provides reliable estimates of the exoskeleton's stiffness within the complete workspace (covering the constraint-type transition configurations). The stiffness of the exoskeleton is further evaluated by acquiring the minimum/maximum stiffness values, after computing the distribution of the most crucial linear and angular stiffness parameters within the workspace. Moreover, the influence of the architectural parameters on the stiffness properties is considered for further design optimization

Progress in Classical and Quantum Variational Principles

We review the development and practical uses of a generalized Maupertuis least action principle in classical mechanics, in which the action is varied under the constraint of fixed mean energy for the trial trajectory. The original Maupertuis (Euler-Lagrange) principle constrains the energy at every point along the trajectory. The generalized Maupertuis principle is equivalent to Hamilton's principle. Reciprocal principles are also derived for both the generalized Maupertuis and the Hamilton principles. The Reciprocal Maupertuis Principle is the classical limit of Schr\"{o}dinger's variational principle of wave mechanics, and is also very useful to solve practical problems in both classical and semiclassical mechanics, in complete analogy with the quantum Rayleigh-Ritz method. Classical, semiclassical and quantum variational calculations are carried out for a number of systems, and the results are compared. Pedagogical as well as research problems are used as examples, which include nonconservative as well as relativistic systems

Tackling illegal, unreported, and unregulated fishing through port state measures

Alongside collective mismanagement, illegal, unreported, and unregulated (IUU) fishing practices pose a serious threat to marine species and ecosystems around the globe. Drastically reducing IUU fishing must form part of global efforts to promote more responsible and just exploitation of marine living resources. While attention on tackling IUU fishing has increased over the past two decades, progress toward its elimination remains slow, largely due to the inherently transboundary nature of IUU practices and the practical limitations of flag and coastal State jurisdiction. This article argues that port States can and should play a central role in international efforts to tackle IUU fishing. It considers the steps port States can lawfully take to remove IUU practices from global supply chains, and explores the conditions and limitations general international law, the law of the sea, and international trade law impose on various port State measures. While port State control raises significant issues of jurisdictional competence, substantive and procedural fairness, and multilateral coordination, it is shown that port State measures are both a feasible and defensible means of addressing IUU practices. By exploring the conditions that attach to the design, adoption, and implementation of port State measures, the article resolves key debates concerning their lawfulness, thus allowing policymakers, practitioners, and officials to renew their attention on developing the political will and technical capabilities necessary for such measures to play an effective and appropriate role in closing regulatory and enforcement gaps in conservation and management regimes

Selective-compliance based lagrange model and multilevel non-collocated feedback control of a humanoid robot

This paper presents unified control schemes for compliant humanoid robots that are aimed at ensuring successful execution of both balancing tasks and walking trajectories for this class of bipeds, given the complexity of under-actuation. A set of controllers corresponding to the single support (SS) and double support (DS) walking phases has been designed based on the flexible sagittal joint dynamics of the system, accounting for both the motor and link states. The first controller uses partial state feedback (PDD), whereas the second considers the full state of the robot (PPDD), whilst both are mathematically proven to stabilize the closed-loop systems for regulation and trajectory tracking tasks. It is demonstrated mathematically that the PDD controller possesses better stability properties than the PPDD scheme for regulation tasks, even though the latter has the advantage of allowing for its associated gain-set to be generated by means of standard techniques, such as Linear Quadratic Regulator (LQR) control. A switching condition relating the Centre-of-Pressure (CoP) to the energy functions corresponding to the DS and SS models, has also been established. The theoretical results are corroborated by means of balancing and walking experiments using the COmpliant huMANoid (COMAN), whilst a practical comparison between the designed controller and a classical PD controller for compliant robots, has also been performed. Overall, and a key conclusion of this paper, the PPDD scheme has produced superior trajectory tracking performance, with 9%, 15% and 20% lower joint space error for the hip, knee and ankle respectively