Ernesto Nunes

A critical-time-point approach to all-start-time lagrangian shortest paths: a summary of results

Given a spatio-temporal network, a source, a destination, and a start-time interval, the All-start-time Lagrangian Shortest Paths (ALSP) problem determines a path set which includes the shortest path for every start time in the given interval. ALSP is important for critical societal applications related to air travel, road travel, and other spatiotemporal networks. However, ALSP is computationally challenging due to the non-stationary ranking of the candidate paths, meaning that a candidate path which is optimal for one start time may not be optimal for others. Determining a shortest path for each start-time leads to redundant computations across consecutive start times sharing a common solution. The proposed approach reduces this redundancy by determining the critical time points at which an optimal path may change. Theoretical analysis and experimental results show that this approach performs better than naive approaches particularly when there are few critical time points.

Exploiting Spatial Locality and Heterogeneity of Agents for Search and Rescue Teamwork

We propose coordination mechanisms for multiple heterogeneous physical agents that operate in city-scale disaster scenarios, where they need to find and rescue people and extinguish fires. Large-scale disasters are characterized by limited and unreliable communications; dangerous events that may disable agents; uncertainty about the location, duration, and type of tasks; and stringent temporal constraints on task completion times. In our approach, agents form teams with other agents that are in the same geographical area. Our algorithms either yield stable teams formed up front and never change, fluid teams where agents can change teams as need arises, or teams that restrict the types of agents that can belong to the same team. We compare our teaming algorithms against a baseline algorithm in which agents operate independently of others and two state-of-the-art coordination mechanisms. Our algorithms are tested in city-scale disaster simulations using the RoboCup Rescue simulator. Our experiments with different city maps show that, in general, forming teams leads to increased task completion and, specifically, that our teaming method that restricts the types of agents in a team outperforms the other methods.

A taxonomy for task allocation problems with temporal and ordering constraints

Previous work on assigning tasks to robots has proposed extensive categorizations of allocation of tasks with and without constraints. The main contribution of this paper is a specific categorization of problems that have temporal and ordering constraints. We propose a novel taxonomy that emphasizes the differences between temporal and ordering constraints, and organizes the current literature according to the nature of those constraints. We summarize widely used models and methods from the task allocation literature and related areas, such as vehicle routing and scheduling problems, showing similarities and differences.

A framework for predicting trajectories using global and local information

We propose a novel framework for predicting the paths of vehicles that move on a road network. The framework leverages global and local patterns in spatio-temporal data. From a large corpus of GPS trajectories, we predict the subsequent path of an in-progress vehicle trajectory using only spatio-temporal features from the data. Our framework consists of three components: (1) a component that abstracts GPS location data into a graph at the neighborhood or street level, (2) a component that generates policies obtained from the graph data, and (3) a component that predicts the subsequent path of an in-progress trajectory. Hierarchical clustering is used to construct the city graph, where the clusters facilitate a compact representation of the trajectory data to make processing large data sets tractable and efficient. We propose four alternative policy generation algorithms: a frequency-based algorithm (FreqCount), a correlation-based algorithm (EigenStrat), a spectral clusteringbased algorithm (LapStrat), and a Markov Chain-based algorithm (MCStrat). The algorithms explore either global patterns (FreqCount and EigenStrat) or local patterns (MCStrat) in the data, with the exception of LapStrat which explores both. We present an analysis of the performance of the alternative prediction algorithms using a large real-world taxi data set.

Decentralized multi-robot allocation of tasks with temporal and precedence constraints

AbstractWe present an auction-based method for a team of robots to allocate and execute tasks that have temporal and precedence constraints. Temporal constraints are expressed as time windows, within which a task must be executed. The robots use our priority-based iterated sequential single-item auction algorithm to allocate tasks among themselves and keep track of their individual schedules. A key innovation is in decoupling precedence constraints from temporal constraints and dealing with them separately. We demonstrate the performance of the allocation method and show how it can be extended to handle failures and delays during task execution. We leverage the power of simulation as a tool to analyze the robustness of schedules. Data collected during simulations are used to compute well-known indexes that measure the risk of delay and failure in the robots’ schedules. We demonstrate the effectiveness of our method in simulation and with real robot experiments.

Decentralized allocation of tasks with temporal and precedence constraints to a team of robots

We propose an auction-based method for a team of robots to allocate and execute tasks that have temporal and precedence constraints. The robots use our priority-based iterated sequential single-item auction algorithm to allocate tasks among themselves and keep track of their individual schedules. The key idea is to decouple precedence constraints from temporal constraints and deal with them separately. In this paper we demonstrate how the allocation scheme can be extended to handle failures and delays during task execution. We demonstrate the effectiveness of our method in simulation and with real robot experiments.