68 research outputs found
Robust Control Barrier Functions under High Relative Degree and Input Constraints for Satellite Trajectories
This paper presents methodologies for constructing Control Barrier Functions
(CBFs) for nonlinear, control-affine systems, in the presence of input
constraints and bounded disturbances. More specifically, given a constraint
function with high-relative-degree with respect to the system dynamics, the
paper considers three methodologies, two for relative-degree 2 and one for
higher relative-degrees, for converting the constraint function into a CBF.
Three special forms of Robust CBFs (RCBFs) are developed as functions of the
input constraints, system dynamics, and disturbance bounds, and are applied to
provably safe satellite control for asteroid exploration missions. The
resultant RCBF conditions on the control input are enforced in a switched
fashion, which expands the set of allowable trajectories and simplifies control
input computation in the presence of multiple constraints. The proposed methods
are verified in simulations demonstrating the developed RCBFs in an asteroid
flyby scenario for a satellite with low-thrust actuators, and in asteroid
proximity operations for a satellite with high-thrust actuators.Comment: 19 pages. Submitted to Automatic
Compositions of Multiple Control Barrier Functions Under Input Constraints
This paper presents a methodology for ensuring that the composition of
multiple Control Barrier Functions (CBFs) always leads to feasible conditions
on the control input, even in the presence of input constraints. In the case of
a system subject to a single constraint function, there exist many methods to
generate a CBF that ensures constraint satisfaction. However, when there are
multiple constraint functions, the problem of finding and tuning one or more
CBFs becomes more challenging, especially in the presence of input constraints.
This paper addresses this challenge by providing tools to 1) decouple the
design of multiple CBFs, so that a CBF can be designed for each constraint
function independently of other constraints, and 2) ensure that the set
composed from all the CBFs together is a viability domain. Thus, a quadratic
program subject to all the CBFs simultaneously is always feasible. The utility
of this methodology is then demonstrated in simulation for a nonlinear
orientation control system.Comment: 8 pages, 7 figures, under review for 2023 American Control Conferenc
Safety-Critical Control for Systems with Impulsive Actuators and Dwell Time Constraints
This paper presents extensions of control barrier function (CBF) and control
Lyapunov function (CLF) theory to systems wherein all actuators cause impulsive
changes to the state trajectory, and can only be used again after a minimum
dwell time has elapsed. These rules define a hybrid system, wherein the
controller must at each control cycle choose whether to remain on the current
state flow or to jump to a new trajectory. We first derive a sufficient
condition to render a specified set forward invariant using extensions of CBF
theory. We then derive related conditions to ensure asymptotic stability in
such systems, and apply both conditions online in an optimization-based control
law with aperiodic impulses. We simulate both results on a spacecraft docking
problem with multiple obstacles.Comment: Accepted to IEEE Control Systems Letters, extended version includes
full proof of Corollary
W270 Safety of Herbicides Compared to Other Commonly Used Chemicals Pub Number W270
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Version 2.
Stabilizing machine learning models with Age-Period-Cohort inputs for scoring and stress testing
Machine learning models have been used extensively for credit scoring, but the architectures employed suffer from a significant loss in accuracy out-of-sample and out-of-time. Further, the most common architectures do not effectively integrate economic scenarios to enable stress testing, cash flow, or yield estimation. The present research demonstrates that providing lifecycle and environment functions from Age-Period-Cohort analysis can significantly improve out-of-sample and out-of-time performance as well as enabling the model's use in both scoring and stress testing applications. This method is demonstrated for behavior scoring where account delinquency is one of the provided inputs, because behavior scoring has historically presented the most difficulties for combining credit scoring and stress testing. Our method works well in both origination and behavior scoring. The results are also compared to multihorizon survival models, which share the same architectural design with Age-Period-Cohort inputs and coefficients that vary with forecast horizon, but using a logistic regression estimation of the model. The analysis was performed on 30-year prime conforming US mortgage data. Nonlinear problems involving large amounts of alternate data are best at highlighting the advantages of machine learning. Data from Fannie Mae and Freddie Mac is not such a test case, but it serves the purpose of comparing these methods with and without Age-Period-Cohort inputs. In order to make a fair comparison, all models are given a panel structure where each account is observed monthly to determine default or non-default
Pre-transplant antibody screening and anti-CD154 costimulation blockade promote long-term xenograft survival in a pig-to-primate kidney transplant model
Xenotransplantation has the potential to alleviate the organ shortage that prevents many patients with end-stage renal disease from enjoying the benefits of kidney transplantation. Despite significant advances in other models, pig-to-primate kidney xenotransplantation has met limited success. Preformed anti-pig antibodies are an important component of the xenogeneic immune response. To address this, we screened a cohort of 34 rhesus macaques for anti-pig antibody levels. We then selected animals with both low and high titers of anti-pig antibodies to proceed with kidney transplant from galactose-α1,3-galactose knockout/CD55 transgenic pig donors. All animals received T-cell depletion followed by maintenance therapy with costimulation blockade (either anti-CD154 mAb or belatacept), mycophenolate mofetil, and steroid. The animal with the high titer of anti-pig antibody rejected the kidney xenograft within the first week. Low-titer animals treated with anti-CD154 antibody, but not belatacept exhibited prolonged kidney xenograft survival (>133 and >126 vs. 14 and 21 days, respectively). Long-term surviving animals treated with the anti-CD154-based regimen continue to have normal kidney function and preserved renal architecture without evidence of rejection on biopsies sampled at day 100. This description of the longest reported survival of pig-to-non-human primate kidney xenotransplantation, now >125 days, provides promise for further study and potential clinical translation
Synthesis of Biobased Diethyl Terephthalate via Diels-Alder Addition of Ethylene to 2,5-Furandicarboxylic Acid Diethyl Ester: An Alternative Route to 100% Biobased Poly(ethylene terephthalate)
Poly(ethylene terephthalate) (PET) is a ubiquitous thermoplastic currently produced from nonrenewable fossil resources; as such, sustainable biobased routes to the key terephthalate monomer are being widely pursued. Herein is demonstrated a greener solventless route to biobased diethyl terephthalate via a one-pot heterogeneous Lewis acid catalyzed Diels-Alder addition and dehydration of 2,5-furandicarboxylic acid diethyl ester with ethylene, giving yields of terephthalate up to 59% for the key Diels-Alder addition step. A metrics-based comparison against alternative published biobased routes (available from sugars, cellulose and hemicellulose) shows that the clean synthetic pathway developed herein gives a practical atom economy, overall yield and selectivity, making it a viable alternative to routes currently under development
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