Decomposition Algorithm for Global Reachability on a Time-Varying Graph

A decomposition algorithm has been developed for global reachability analysis on a space-time grid. By exploiting the upper block-triangular structure, the planning problem is decomposed into smaller subproblems, which is much more scalable than the original approach. Recent studies have proposed the use of a hot-air (Montgolfier) balloon for possible exploration of Titan and Venus because these bodies have thick haze or cloud layers that limit the science return from an orbiter, and the atmospheres would provide enough buoyancy for balloons. One of the important questions that needs to be addressed is what surface locations the balloon can reach from an initial location, and how long it would take. This is referred to as the global reachability problem, where the paths from starting locations to all possible target locations must be computed. The balloon could be driven with its own actuation, but its actuation capability is fairly limited. It would be more efficient to take advantage of the wind field and ride the wind that is much stronger than what the actuator could produce. It is possible to pose the path planning problem as a graph search problem on a directed graph by discretizing the spacetime world and the vehicle actuation. The decomposition algorithm provides reachability analysis of a time-varying graph. Because the balloon only moves in the positive direction in time, the adjacency matrix of the graph can be represented with an upper block-triangular matrix, and this upper block-triangular structure can be exploited to decompose a large graph search problem. The new approach consumes a much smaller amount of memory, which also helps speed up the overall computation when the computing resource has a limited physical memory compared to the problem size

Risk-Constrained Dynamic Programming for Optimal Mars Entry, Descent, and Landing

A chance-constrained dynamic programming algorithm was developed that is capable of making optimal sequential decisions within a user-specified risk bound. This work handles stochastic uncertainties over multiple stages in the CEMAT (Combined EDL-Mobility Analyses Tool) framework. It was demonstrated by a simulation of Mars entry, descent, and landing (EDL) using real landscape data obtained from the Mars Reconnaissance Orbiter. Although standard dynamic programming (DP) provides a general framework for optimal sequential decisionmaking under uncertainty, it typically achieves risk aversion by imposing an arbitrary penalty on failure states. Such a penalty-based approach cannot explicitly bound the probability of mission failure. A key idea behind the new approach is called risk allocation, which decomposes a joint chance constraint into a set of individual chance constraints and distributes risk over them. The joint chance constraint was reformulated into a constraint on an expectation over a sum of an indicator function, which can be incorporated into the cost function by dualizing the optimization problem. As a result, the chance-constraint optimization problem can be turned into an unconstrained optimization over a Lagrangian, which can be solved efficiently using a standard DP approach

Trajectory planning for unmanned vehicles using robust receding horizon control

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2007.Includes bibliographical references (p. 211-223).This thesis presents several trajectory optimization algorithms for a team of cooperating unmanned vehicles operating in an uncertain and dynamic environment. The first, designed for a single vehicle, is the Robust Safe But Knowledgeable (RSBK) algorithm, which combines several previously published approaches to recover the main advantages of each. This includes a sophisticated cost-to-go function that provides a good estimate of the path beyond the planning horizon, which is extended in this thesis to account for three dimensional motion; constraint tightening to ensure robustness to disturbances, which is extended to a more general class of disturbance rejection controllers compared to the previous work, with a new off-line design procedure; and a robust invariant set which ensures the safety of the vehicle in the event of environmental changes beyond the planning horizon. The system controlled by RSBK is proven to robustly satisfy all vehicle and environmental constraint under the action of bounded external disturbances. Multi-vehicle teams could also be controlled using centralized RSBK, but to reduce computational effort, several distributed algorithms are presented in this thesis. The main challenge in distributing the planning is to capture the complex couplings between vehicles.(cont.) A decentralized form of RSBK algorithm is developed by having each vehicle optimize over its own decision variables and then locally communicate the solutions to its neighbors. By integrating a grouping algorithm, this approach enables simultaneous computation by vehicles in the team while guaranteeing the robust feasibility of the entire fleet. The use of a short planning horizon within RSBK enables the use of a very simple initialization algorithm when compared to previous work, which is essential if the technique is to be used in dynamic and uncertain environments. Improving the level of cooperation between the vehicles is another challenge for decentralized planning, but this thesis presents a unique strategy by enabling each vehicle to optimize its own decision as well as a feasible perturbation of its neighboring vehicles' plans. The resulting cooperative form of the distributed RSBK is shown to result in solutions that sacrifice local performance if it benefits the overall team performance. This desirable performance improvement is achieved with only a small increase in the computation and communication requirements. These algorithms are tested and demonstrated in simulation and on two multi-vehicle testbeds using rovers and quadrotors.(cont.) The experimental results demonstrate that the proposed algorithms successfully overcome the implementation challenges, such as limited onboard computation and communication resources, as well as the various sources of real-world uncertainties arising from modeling error of the vehicle dynamics, tracking error of the low-level controller, external disturbance, and sensing noise.by Yoshiaki Kuwata.Ph.D

Remote Viewer for Maritime Robotics Software

This software is a viewer program for maritime robotics software that provides a 3D visualization of the boat pose, its position history, ENC (Electrical Nautical Chart) information, camera images, map overlay, and detected tracks

Target Trailing With Safe Navigation With Colregs for Maritime Autonomous Surface Vehicles

Systems and methods for operating autonomous waterborne vessels in a safe manner. The systems include hardware for identifying the locations and motions of other vessels, as well as the locations of stationary objects that represent navigation hazards. By applying a computational method that uses a maritime navigation algorithm for avoiding hazards and obeying COLREGS using Velocity Obstacles to the data obtained, the autonomous vessel computes a safe and effective path to be followed in order to accomplish a desired navigational end result, while operating in a manner so as to avoid hazards and to maintain compliance with standard navigational procedures defined by international agreement. The systems and methods have been successfully demonstrated on water with radar and stereo cameras as the perception sensors, and integrated with a higher level planner for trailing a maneuvering target

Association of Progressive CD4+ T Cell Decline in SIV Infection with the Induction of Autoreactive Antibodies

The progressive decline of CD4+ T cells is a hallmark of disease progression in human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infection. Whereas the acute phase of the infection is dominated by virus-mediated depletion of memory CD4+ T cells, chronic infection is often associated with a progressive decline of total CD4+ T cells, including the naïve subset. The mechanism of this second phase of CD4+ T cell loss is unclear and may include immune activation–induced cell death, immune-mediated destruction, and regenerative or homeostatic failure. We studied patterns of CD4+ T cell subset depletion in blood and tissues in a group of 20 rhesus macaques inoculated with derivatives of the pathogenic SIVsmE543-3 or SIVmac239. Phenotypic analysis of CD4+ T cells demonstrated two patterns of CD4+ T cell depletion, primarily affecting either naïve or memory CD4+ T cells. Progressive decline of total CD4+ T cells was observed only in macaques with naïve CD4+ T cell depletion (ND), though the depletion of memory CD4+ T cells was profound in macaques with memory CD4+ T cell depletion (MD). ND macaques exhibited lower viral load and higher SIV-specific antibody responses and greater B cell activation than MD macaques. Depletion of naïve CD4+ T cells was associated with plasma antibodies autoreactive with CD4+ T cells, increasing numbers of IgG-coated CD4+ T cells, and increased incidence of autoreactive antibodies to platelets (GPIIIa), dsDNA, and phospholipid (aPL). Consistent with a biological role of these antibodies, these latter antibodies were accompanied by clinical features associated with autoimmune disorders, thrombocytopenia, and catastrophic thrombotic events. More importantly for AIDS pathogenesis, the level of autoreactive antibodies significantly correlated with the extent of naïve CD4+ T cell depletion. These results suggest an important role of autoreactive antibodies in the CD4+ T cell decline observed during progression to AIDS

Team MIT Urban Challenge Technical Report

This technical report describes Team MITs approach to theDARPA Urban Challenge. We have developed a novel strategy forusing many inexpensive sensors, mounted on the vehicle periphery,and calibrated with a new cross-­modal calibrationtechnique. Lidar, camera, and radar data streams are processedusing an innovative, locally smooth state representation thatprovides robust perception for real­ time autonomous control. Aresilient planning and control architecture has been developedfor driving in traffic, comprised of an innovative combination ofwell­proven algorithms for mission planning, situationalplanning, situational interpretation, and trajectory control. These innovations are being incorporated in two new roboticvehicles equipped for autonomous driving in urban environments,with extensive testing on a DARPA site visit course. Experimentalresults demonstrate all basic navigation and some basic trafficbehaviors, including unoccupied autonomous driving, lanefollowing using pure-­pursuit control and our local frameperception strategy, obstacle avoidance using kino-­dynamic RRTpath planning, U-­turns, and precedence evaluation amongst othercars at intersections using our situational interpreter. We areworking to extend these approaches to advanced navigation andtraffic scenarios

The ability of contemporary cardiologists to judge the ischemic impact of a coronary lesion visually

Background: Landmark trials showed that invasive pressure measurement (Fractional Flow Reserve, FFR) was a better guide to coronary stenting than visual assessment. However, present-day interventionists have benefited from extensive research and personal experience of mapping anatomy to hemodynamics. Aims: To determine if visual assessment of the angiogram performs as well as invasive measurement of coronary physiology. Methods: 25 interventional cardiologists independently visually assessed the single vessel coronary disease of 200 randomized participants in The Objective Randomized Blinded Investigation with optimal medical Therapy of Angioplasty in stable angina trial (ORBITA). They gave a visual prediction of the FFR and Instantaneous Wave-free Ratio (iFR), denoted vFFR and viFR respectively. Each judged each lesion on 2 occasions, so that every lesion had 50 vFFR, and 50 viFR assessments. The group consensus visual estimates (vFFR-group and viFR-group) and individual cardiologists' visual estimates (vFFR-individual and viFR-individual) were tested alongside invasively measured FFR and iFR for their ability to predict the placebo-controlled reduction in stress echo ischemia with stenting. Results: Placebo-controlled ischemia improvement with stenting was predicted by vFFR-group (p < 0.0001) and viFR-group (p < 0.0001), vFFR-individual (p < 0.0001) and viFR-individual (p < 0.0001). There were no significant differences between the predictive performance of the group visual estimates and their invasive counterparts: p = 0.53 for vFFR vs FFR and p = 0.56 for viFR vs iFR. Conclusion: Visual assessment of the angiogram by contemporary experts, provides significant additional information on the amount of ischaemia which can be relieved by placebo-controlled stenting in single vessel coronary artery disease

Real-time trajectory design for UAVs using RHC

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2003.Includes bibliographical references (p. 147-151).by Yoshiaki Kuwata.S.M

Combined EDL-Mobility Planning for Planetary Missions

This paper presents an analysis framework for planetary missions that have coupled mobility and EDL (Entry-Descent-Landing) systems. Traditional systems engineering approaches to mobility missions such as MERs (Mars Exploration Rovers) and MSL (Mars Science Laboratory) independently study the EDL system and the mobility system, and does not perform explicit trade-off between them or risk minimization of the overall system. A major challenge is that EDL operation is inherently uncertain and its analysis results such as landing footprint are described using PDF (Probability Density Function). The proposed approach first builds a mobility cost-to-go map that encodes the driving cost of any point on the map to a science target location. The cost could include variety of metrics such as traverse distance, time, wheel rotation on soft soil, and closeness to hazards. It then convolves the mobility cost-to-go map with the landing PDF given by the EDL system, which provides a histogram of driving cost, which can be used to evaluate the overall risk of the mission. By capturing the coupling between EDL and mobility explicitly, this analysis framework enables quantitative tradeoff between EDL and mobility system performance, as well as the characterization of risks in a statistical way. The simulation results are presented with a realistic Mars terrain dat