Building Volumetric Maps with Cooperative Mobile Robots and Useful Information Sharing: a Distributed Control Approach based on Entropy

Rui Paulo Rocha

Ph.D. thesis, Faculty of Engineering of University of Porto, Portugal.
Submitted on Oct. 13, 2005 and approved by unanimity on May 8, 2006.

Abstract

This thesis addresses the problem of how to share efficiently information within a robotic system comprised of several mobile robots, which are programmed to exhibit cooperative behavior in the context of building volumetric maps of unknown environments. More specifically, it addresses the following issues: representing a probabilistic map and improving it through efficient exploration, based on information gain maximization; distributed control of teams of cooperative mobile robots, based on an information utility criteria; and coordinated exploration, aiming at avoiding redundant sensory information and robots' interference.

Robots have been developed essentially to help or substitute humans in tasks which are either repetitive or dangerous. For many of these tasks, especially those that are intrinsically distributed and complex, a team of several cooperative mobile robots ─ a cooperative multi-robot system (MRS) ─ is required to either make viable the mission accomplishment or, at least, accomplish the mission with better performance than a single mobile robot.

In spite of potential advantages related with space distribution, time distribution, complex problems decomposition, robustness, reliability and cost, a MRS requires that each robot maintains a sufficient and consistent level of awareness about the mission assigned to the team and about its teammates, in order to attain effective cooperation. The main challenge is that information is distributed and thus each robot has only partial and, sometimes, inconsistent knowledge about the environment. Sharing efficiently information via communication is thus crucial for robots' cooperation.

Building maps is indeed a relevant robotics' application domain. Firstly, in many other application domains than building maps (e.g. search and rescue, surveillance, planetary exploration, etc.), a robot usually needs a map to support safe and efficient navigation based on a world model. Secondly, robots may substitute humans on building detailed models, such as: fastidious maps of indoor environments (e.g. buildings); maps of buried utilities (e.g.gas pipes, power or communication lines, etc.), in order to avoid getting too close to them in construction activities; or detailed maps of hazardous environments (e.g. abandoned underground mines, nuclear facilities, etc.), in order to support monitoring or maintenance procedures.

The contributions of this thesis include a compact grid-based probabilistic representation model of a volumetric map, which allows to explicitly model uncertainty through the entropy concept. A frontier-based exploration method is also formulated using entropy, so that each robot uses its current map to select a new exploration viewpoint with maximum information gain.

This probabilistic framework is used to devise a distributed architecture model for building volumetric maps with teams of cooperative mobile robots, whereby each robot is altruistically committed to share useful measurements with its teammates. The information sharing is based on the formal definition of a measure of information utility, developed upon the concepts of entropy and mutual information, whereby sensory data is as useful as it contributes to improve the robot's map. The distributed architecture is further refined with a mechanism to coordinate the exploration actions of different robots, thus improving the team's performance.

The proposed methods were implemented in mobile robots, equipped with wireless communication and a stereo-vision system providing range measurements. These robots were used to carry out a set of experiments in a physical environment, which successfully validated the proposed framework and demonstrated the performance improvement yielded by the robots' cooperation. The results obtained with mobile robots were complemented with extensive computer simulations, which demonstrated those methods with varying team sizes.

Keywords ─ Cooperation, mobile robotics, volumetric maps, entropy, information utility, mutual information.

Project

· Web page of the research project.

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BibTeX

@PHDTHESIS(Rocha_05,

AUTHOR = "Rui P. Rocha",

TITLE = "Building Volumetric Maps with Cooperative Mobile Robots and Useful Information Sharing: a Distributed Control Approach based on Entropy",

SCHOOL = "Faculty of Engineering of University of Porto, Portugal",

YEAR = "2005",

MONTH = "Oct."

)

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Last update: 14/06/2006