414 research outputs found
Hydrodynamic signatures of stationary Marangoni-driven surfactant transport
We experimentally study steady Marangoni-driven surfactant transport on the
interface of a deep water layer. Using hydrodynamic measurements, and without
using any knowledge of the surfactant physico-chemical properties, we show that
sodium dodecyl sulphate and Tergitol 15-S-9 introduced in low concentrations
result in a flow driven by adsorbed surfactant. At higher surfactant
concentration, the flow is dominated by the dissolved surfactant. Using
Camphoric acid, whose properties are {\it a priori} unknown, we demonstrate
this method's efficacy by showing its spreading is adsorption dominated
Probabilistic Guarantees for Nonlinear Safety-Critical Optimal Control
Leveraging recent developments in black-box risk-aware verification, we
provide three algorithms that generate probabilistic guarantees on (1)
optimality of solutions, (2) recursive feasibility, and (3) maximum controller
runtimes for general nonlinear safety-critical finite-time optimal controllers.
These methods forego the usual (perhaps) restrictive assumptions required for
typical theoretical guarantees, e.g. terminal set calculation for recursive
feasibility in Nonlinear Model Predictive Control, or convexification of
optimal controllers to ensure optimality. Furthermore, we show that these
methods can directly be applied to hardware systems to generate controller
guarantees on their respective systems
Safety-Critical Controller Verification via Sim2Real Gap Quantification
The well-known quote from George Box states that: "All models are wrong, but
some are useful." To develop more useful models, we quantify the inaccuracy
with which a given model represents a system of interest, so that we may
leverage this quantity to facilitate controller synthesis and verification.
Specifically, we develop a procedure that identifies a sim2real gap that holds
with a minimum probability. Augmenting the nominal model with our identified
sim2real gap produces an uncertain model which we prove is an accurate
representor of system behavior. We leverage this uncertain model to synthesize
and verify a controller in simulation using a probabilistic verification
approach. This pipeline produces controllers with an arbitrarily high
probability of realizing desired safe behavior on system hardware without
requiring hardware testing except for those required for sim2real gap
identification. We also showcase our procedure working on two hardware
platforms - the Robotarium and a quadruped
The Atmosphere-Ocean Interface Layer of NASA's Goddard Earth Observing System Model and Data Assimilation System Volume 51
The Goddard Earth Observing System (GEOS) general circulation model (GCM) includes modules for sea surface temperature (SST) diurnal warming and cool-skin layers. To support the application of a coupled atmosphere-ocean data assimilation capability, the GCM needs to be flexible enough to support both coupled atmosphere ocean general circulation model (AOGCM) and atmosphere-only (AGCM) configurations, with only minor configuration changes at the user interface. This document presents a formulation of an atmosphere-ocean interface layer (AOIL) that serves this purpose. Previous work by Akella et al. (2017) described a version of a model for near-surface temperature variations, including both both diurnal warming and cool-skin effects, that has been used since 2017 in the near-real-time GEOS FP (forward processing) weather analysis and forecasting system. The diurnal cycle of SST in that version of the GEOS atmospheric data assimilation system (ADAS) undergoes a sharp decay in the late afternoon (local time). The updated AOIL presented here includes a modification of the similarity function used in the diurnal warming model. Results from offline model runs illustrate an improvement in the near-surface (less than 0:5m depth) diurnal cycle compared to the original formulation. The new formulation requires minimal parameter tuning, and the improvements are robust across long (several month) simulation periods. This new model formulation, however, retains some deficiences from the previous module, such as a small warm bias in calm wind conditions for water depths below 1m. Our future work would include surface salinification and sea-ice into the AOIL
Formal Verification of Safety Critical Autonomous Systems via Bayesian Optimization
As control systems become increasingly more complex, there exists a pressing need to find systematic ways of verifying them. To address this concern, there has been significant work in developing test generation schemes for black-box control architectures. These schemes test a black-box control architecture's ability to satisfy its control objectives, when these objectives are expressed as operational specifications through temporal logic formulae. Our work extends these prior, model based results by lower bounding the probability by which the black-box system will satisfy its operational specification, when subject to a pre-specified set of environmental phenomena. We do so by systematically generating tests to minimize a Lipschitz continuous robustness measure for the operational specification. We demonstrate our method with experimental results, wherein we show that our framework can reasonably lower bound the probability of specification satisfaction
Lipschitz Continuity of Signal Temporal Logic Robustness Measures: Synthesizing Control Barrier Functions from One Expert Demonstration
Control Barrier Functions (CBFs) allow for efficient synthesis of controllers
to maintain desired invariant properties of safety-critical systems. However,
the problem of identifying a CBF remains an open question. As such, this paper
provides a constructive method for control barrier function synthesis around
one expert demonstration that realizes a desired system specification
formalized in Signal Temporal Logic (STL). First, we prove that all STL
specifications have Lipschitz-continuous robustness measures. Second, we
leverage this Lipschitz continuity to synthesize a time-varying control barrier
function. By filtering control inputs to maintain the positivity of this
function, we ensure that the system trajectory satisfies the desired STL
specification. Finally, we demonstrate the effectiveness of our approach on the
Robotarium
Barrier-Based Test Synthesis for Safety-Critical Systems Subject to Timed Reach-Avoid Specifications
We propose an adversarial, time-varying test-synthesis procedure for
safety-critical systems without requiring specific knowledge of the underlying
controller steering the system. From a broader test and evaluation context,
determination of difficult tests of system behavior is important as these tests
would elucidate problematic system phenomena before these mistakes can engender
problematic outcomes, e.g. loss of human life in autonomous cars, costly
failures for airplane systems, etc. Our approach builds on existing,
simulation-based work in the test and evaluation literature by offering a
controller-agnostic test-synthesis procedure that provides a series of
benchmark tests with which to determine controller reliability. To achieve
this, our approach codifies the system objective as a timed reach-avoid
specification. Then, by coupling control barrier functions with this class of
specifications, we construct an instantaneous difficulty metric whose minimizer
corresponds to the most difficult test at that system state. We use this
instantaneous difficulty metric in a game-theoretic fashion, to produce an
adversarial, time-varying test-synthesis procedure that does not require
specific knowledge of the system's controller, but can still provably identify
realizable and maximally difficult tests of system behavior. Finally, we
develop this test-synthesis procedure for both continuous and discrete-time
systems and showcase our test-synthesis procedure on simulated and hardware
examples
Understanding tradeoffs in incremental deployment of new network architectures
Despite the plethora of incremental deployment mechanisms proposed, rapid adoption of new network-layer protocols and architectures remains difficult as reflected by the widespread lack of IPv6 traffic on the Internet. We show that all de-ployment mechanisms must address four key questions: How to select an egress from the source network, how to select an ingress into the destination network, how to reach that egress, and how to reach that ingress. By creating a de-sign space that maps all existing mechanisms by how they answer these questions, we identify the lack of existing mech-anisms in part of this design space and propose two novel approaches: the “4ID ” and the “Smart 4ID”. The 4ID mech-anism utilizes new data plane technology to flexibly decide when to encapsulate packets at forwarding time. The Smart 4ID mechanism additionally adopts an SDN-style control plane to intelligently pick ingress/egress pairs based on a wider view of the local network. We implement these mech-anisms along with two widely used IPv6 deployment mech-anisms and conduct wide-area deployment experiments over PlanetLab. We conclude that Smart 4ID provide better overall performance and failure semantics, and that inno-vations in the data plane and control plane enable straight-forward incremental deployment
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