arXiv subjects

Michael Walker

Publications and source records attributed to Michael Walker.

9 recordsLinked to original sources

arXiv2026

Log-linear Dynamic Inversion for Thrusting Spacecraft on SE2(3)

Body-fixed spacecraft thrust couples attitude tracking error to translational motion because the applied inertial acceleration depends on spacecraft attitude. In this work, we formulate the dynamics of a continuously thrusting spacecraft on $\mathrm{SE}_2(3)$ to make this coupling explicit while retaining a structured form suitable for analysis. We represent attitude, velocity, and position within a single Lie-group state and derive the logarithmic dynamics of a left-invariant tracking error with position-dependent gravity. The resulting dynamics show explicitly how attitude error affects translational velocity through the applied acceleration and separate the structured linear dynamics from nonlinear contributions due to body-frame input mismatch and position-dependent gravity difference. We specialize the gravity contribution to the two-body model and derive an analytical bound in terms of the reference orbital radius, logarithmic position error, and attitude error. Numerical simulations of a continuous-thrust maneuver in a highly elliptical Molniya-like orbit show agreement between the derived logarithmic error dynamics and errors reconstructed from independent nonlinear state propagation.

eess.SY↗

arXiv2026

High-Temperature Hydrogen Sensors Based on Gallium Oxide Heterojunction Diodes

Long-term, high temperature operation of Ga2O3 devices is a crucial hurdle that must be overcome before widespread adoption of the technology can be achieved, but is largely absent from the overall body of work. Demonstrations up to this point show devices are either limited by material or dopant instability that leads to performance degradation with time. Herein, Ga2O3-based hydrogen sensors employing Pt Schottky and Cr2O3/Ga2O3 p-n diodes (Mg- and N-doped) were fabricated and evaluated for long-term stability at 600C for 800-1,800 hours, with cyclic exposure to N2 and low-concentration H2 (500-1,500 ppm). Transient current density (measured at -0.1 V) and periodic J-V characterization were used to track performance. Despite gradual declines in sensor signal and sensitivity, devices distinguished hydrogen concentrations throughout weeks of operation. Degradation was architecture-dependent: Cr2O3:Mg degraded gradually, consistent with known Mg migration; the Pt Schottky diode showed dramatic changes after 1,000 hours; and Cr2O3:N showed the lowest but most stable performance before failing at 800 hours. Thermionic emission and Lambert W-based modeling confirmed hydrogen exposure reduces interfacial barrier height via a proton-induced dipole mechanism common to both diode types. TEM of aged Pt Schottky diodes revealed Pt grain growth and microvoid formation as key degradation mechanisms. TOF-SIMS confirmed nitrogen dopants remain confined to the Cr2O3:N layer, supporting N-doping as a stable, lower-performance alternative to Mg-doping

cond-mat.mtrl-sci↗

arXiv2025

An optically enhanced crystalline silicon allotrope: hydrogen passivated type II silicon clathrate

While Si clathrates have been explored as promising direct bandgap semiconductors, their practical optoelectronic performance has been limited by high doping levels and structural defects. Hydrogen has long been used to improve the optoelectronic quality of conventional Si, yet its role in clathrate structures remains unexplored. In this study, we demonstrate that hydrogen (deuterium) can be incorporated into type II Si clathrate framework using remote plasma treatment. This process leads to the formation of NaD and SiD complexes, which significantly reduce both the Na donor density and dangling bond defects. Electron paramagnetic resonance confirms nearly a tenfold decrease in Na-related donor states, resulting in the lowest doping level reported in Si clathrates to date. Following passivation, the integrated photoluminescence intensity increases by a factor of 40, accompanied by a blue shift of the main emission peak, consistent with a transition closer to the intrinsic band edge. A new emission peak at 930 nm, attributed to hydrogen-related recombination centers, also appears. These improvements remain stable up to 400 oC. Altogether, this work establishes hydrogen passivation as a viable strategy for enhancing light emission in Si clathrates and opens a new pathway toward their application in Si-based light-emitting diodes and other direct-bandgap optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2023

TOBY: A Tool for Exploring Data in Academic Survey Papers

This paper describes TOBY, a visualization tool that helps a user explore the contents of an academic survey paper. The visualization consists of four components: a hierarchical view of taxonomic data in the survey, a document similarity view in the space of taxonomic classes, a network view of citations, and a new paper recommendation tool. In this paper, we will discuss these features in the context of three separate deployments of the tool.

cs.DL↗

arXiv2022

Virtual, Augmented, and Mixed Reality for Human-Robot Interaction: A Survey and Virtual Design Element Taxonomy

Virtual, Augmented, and Mixed Reality for Human-Robot Interaction (VAM-HRI) has been gaining considerable attention in research in recent years. However, the HRI community lacks a set of shared terminology and framework for characterizing aspects of mixed reality interfaces, presenting serious problems for future research. Therefore, it is important to have a common set of terms and concepts that can be used to precisely describe and organize the diverse array of work being done within the field. In this paper, we present a novel taxonomic framework for different types of VAM-HRI interfaces, composed of four main categories of virtual design elements (VDEs). We present and justify our taxonomy and explain how its elements have been developed over the last 30 years as well as the current directions VAM-HRI is headed in the coming decade.

cs.RO↗

arXiv2018

Scalable Entity Resolution Using Probabilistic Signatures on Parallel Databases

Accurate and efficient entity resolution is an open challenge of particular relevance to intelligence organisations that collect large datasets from disparate sources with differing levels of quality and standard. Starting from a first-principles formulation of entity resolution, this paper presents a novel Entity Resolution algorithm that introduces a data-driven blocking and record-linkage technique based on the probabilistic identification of entity signatures in data. The scalability and accuracy of the proposed algorithm are evaluated using benchmark datasets and shown to achieve state-of-the-art results. The proposed algorithm can be implemented simply on modern parallel databases, which allows it to be deployed with relative ease in large industrial applications.

cs.DB↗

arXiv2018

Privacy-Preserving Platform for Transactive Energy Systems

Transactive energy systems (TES) are emerging as a transformative solution for the problems faced by distribution system operators due to an increase in the use of distributed energy resources and a rapid acceleration in renewable energy generation. These, on one hand, pose a decentralized power system controls problem, requiring strategic microgrid control to maintain stability for the community and for the utility. On the other hand, they require robust financial markets operating on distributed software platforms that preserve privacy. In this paper, we describe the implementation of a novel, blockchain-based transactive energy system. We outline the key requirements and motivation of this platform, describe the lessons learned, and provide a description of key architectural components of this system.

cs.DC↗

arXiv2017

Providing Privacy, Safety, and Security in IoT-Based Transactive Energy Systems using Distributed Ledgers

Power grids are undergoing major changes due to rapid growth in renewable energy resources and improvements in battery technology. While these changes enhance sustainability and efficiency, they also create significant management challenges as the complexity of power systems increases. To tackle these challenges, decentralized Internet-of-Things (IoT) solutions are emerging, which arrange local communities into transactive microgrids. Within a transactive microgrid, "prosumers" (i.e., consumers with energy generation and storage capabilities) can trade energy with each other, thereby smoothing the load on the main grid using local supply. It is hard, however, to provide security, safety, and privacy in a decentralized and transactive energy system. On the one hand, prosumers' personal information must be protected from their trade partners and the system operator. On the other hand, the system must be protected from careless or malicious trading, which could destabilize the entire grid. This paper describes Privacy-preserving Energy Transactions (PETra), which is a secure and safe solution for transactive microgrids that enables consumers to trade energy without sacrificing their privacy. PETra builds on distributed ledgers, such as blockchains, and provides anonymity for communication, bidding, and trading.

cs.DC↗

arXiv2004

Determining the superconducting gap structure in Sr2RuO4 from sound attenuation studies below Tc

This work presents a quantitative theoretical study of the sound attenuation in the unconventional multiband superconductor Sr2RuO4 below the superconducting transition temperature Tc. Sound attenuation in this material is shown to have the remarkable property of being able to identify different nodal structures on different bands. The nodal structures on the γband on the one hand, and on the αand βbands on the other, are both found to be characterized by the existence of point nodes, but are significantly different in their quantitative aspects.

cond-mat.supr-con↗