123 research outputs found
Robust Output Regulation for Autonomous Robots:self-learning mechanisms, task-space control and multi-agent systems
This thesis focuses on robust output regulation for autonomous robots. The control objective of output regulation is to design a feedback controller to achieve asymptotic tracking and/or disturbance rejection for a class of exogenous reference and/or disturbance while maintaining closed-loop stability. We investigate three research problems that pertain to the constructive design of robust output regulation for fully actuated Euler-Lagrange systems from centralized to distributed fashions. The first one is the global robust output regulation of second-order affine nonlinear systems with input disturbances that encompass the fully-actuated Euler-Lagrange systems. Based on a certainty equivalence principle method, we proposed a novel class of nonlinear internal models taking a cascade interconnection structure with strictly relaxed conditions than before. The second one is the output regulation for robot manipulators working in task-space. An internal model-based adaptive controller is designed to cope with uncertain manipulator kinematic and dynamic parameters, as well as unknown periodic reference trajectories generated by harmonic oscillators. The last one is the formation control of manipulators’ end-effector subject to external disturbances or parameter uncertainties. We present and analyze gradient descent-based distributed formation controllers for end-effectors. Internal models are used to reject external disturbances. Moreover, by introducing an extra integrator and an adaptive estimator for gravitational compensation and stabilization, respectively, we extend the proposed gradient-based design to the case where the plant parameters are not exactly known
Global Phase Diagram of Disordered Type-II Weyl Semimetals
With electron and hole pockets touching at the Weyl node, type-II Weyl
semimetal is a newly proposed topological state distinct from its type-I
cousin. We numerically study the localization effect for tilted type-I as well
as type-II Weyl semimetals and give the global phase diagram. For dis- ordered
type-I Weyl semimetal, an intermediate three-dimensional quantum anomalous Hall
phase is confirmed between Weyl semimetal phase and diffusive metal phase.
However, this intermediate phase is absent for disordered type-II Weyl
semimetal. Besides, near the Weyl nodes, comparing to its type-I cousin,
type-II Weyl semimetal possesses even larger ratio between the transport
lifetime along the direction of tilt and the quantum lifetime. Near the phase
boundary between the type-I and the type-II Weyl semimetals, infinitesimal
disorder will induce an insulating phase so that in this region, the concept of
Weyl semimetal is meaningless for real materials.Comment: 7 pages, 5 figure
Output regulation of Euler-Lagrange systems based on error and velocity feedback
Based on a certainty equivalence property, we propose an adaptive internal model control law that solves global robust output regulation of uncertain Euler-Lagrange (EL) systems based only on error (or relative position) and velocity feedback. The proposed controller does not require apriori knowledge of reference signal and its derivatives, which are commonly assumed in literature. It enables a self-learning mechanism of the closed-loop EL systems where the adaptive internal model-based controller is able to learn the desired trajectories and adapt itself to the uncertain plant parameters. Furthermore, the analysis offers insights to the design of internal model-based output regulation for multivariable nonlinear systems with uniform vector relative degree two
On self-learning mechanism for the output regulation of second-order affine nonlinear systems
This paper studies global robust output regulation of second-order nonlinear systems with input disturbances that encompass the fully-actuated Euler-Lagrange systems. We assume the availability of relative output (w.r.t. a family of reference signals) and output derivative measurements. Based on a specific separation principle and self learning mechanism, we develop an internal model-based controller that does not require apriori knowledge of reference and disturbance signals and it only assumes that the kernels of these signals are a family of exosystems with unknown parameters (e.g., amplitudes, frequencies or time periods). The proposed control framework has a self-learning mechanism that extricates itself from requiring absolute position measurement nor precise knowledge of the feedforward kernel signals. By requiring the high-level task/trajectory planner to use the same class of kernels in constraining the trajectories, the proposed low-level controller is able to learn the desired trajectories, to suppress the disturbance signals, and to adapt itself to the uncertain plant parameters. The framework enables a plug-and-play control mechanism in both levels of control
Scanning tunneling microscopy and spectroscopy of nanoscale twisted bilayer graphene
Nanoscale twisted bilayer graphene (TBG) is quite instable and will change
its structure to Bernal (or AB-stacking) bilayer with a much lower energy.
Therefore, the lack of nanoscale TBG makes its electronic properties not
accessible in experiment up to now. In this work, a special confined TBG is
obtained in the overlaid area of two continuous misoriented graphene sheets.
The width of the confined region of the TBG changes gradually from about 22 nm
to 0 nm. By using scanning tunnelling microscopy, we studied carefully the
structure and the electronic properties of the nanoscale TBG. Our results
indicate that the low-energy electronic properties, including twist-induced van
Hove singularities (VHSs) and spatial modulation of local density-of-state, are
strongly affected by the translational symmetry breaking of the nanoscale TBG.
Whereas, the electronic properties above the energy of the VHSs are almost not
influenced by the quantum confinement even when the width of the TBG is reduced
to only a single moire spot.Comment: 4 Figure
Distributed formation control for manipulator end-effectors
We present three classes of distributed formation controllers for achieving
and maintaining the 2D/3D formation shape of manipulator end-effectors to cope
with different scenarios due to availability of modeling parameters. We firstly
present a distributed formation controller for manipulators whose system
parameters are perfectly known. The formation control objective is achieved by
assigning virtual springs between end-effectors and by adding damping terms at
joints, which provides a clear physical interpretation of the proposed
solution. Subsequently, we extend it to the case where manipulator kinematic
and system parameters are not exactly known. An extra integrator and an
adaptive estimator are introduced for gravitational compensation and
stabilization, respectively. Simulation results with planar manipulators and
with seven degree-of-freedom humanoid manipulator arms are presented to
illustrate the effectiveness of the proposed approach.Comment: arXiv admin note: text overlap with arXiv:2103.1459
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