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Control and coordination of mobile multi-agent systems
KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, various control problems originating from the field of mobile robotics are considered. In particular, the thesis deals with problems that are related to the interaction and coordination of multiple mobile units. The scientific contributions are presented in five papers that together constitute the main part of the thesis. The papers are preceded by a longer introductory part, in which some important results from control theory, data processing and robotics are reviewed.

In the first of the appended papers, two stabilizing tracking controls are proposed for a non-holonomic robot platform of unicycle type. Tolerance to errors and other properties of the controllers are discussed and a reactive obstacle avoidance control, that can easily be incorporated with the proposed tracking controls, is suggested. In Paper B, the results from Paper~A are extended to multi-agent systems. It is demonstrated how the tracking controls from Paper A can be used as building blocks when putting together formations of robots, in which each robot maintains a fixed position relative its neighbors during translation. In addition, switching between the different control functions is shown to be robust, implying that it is possible to change the shape of a formation on-line.

In the first two papers, the tracking problem is facilitated by the assumption that the approximate velocity of the target/leader is known to the tracking robot. Paper C treats the the case where the target velocity is neither directly measurable with the available sensor setup, nor possible to obtain through communication with neighboring agents. Straight-forward computation of the target velocity from available sensor data unfortunately tend to enhance measurement errors and give unreliable estimates. To overcome the difficulties, an alternative approach to velocity estimation is proposed, motivated by the local observability of the given control system.

Paper D deals with another problematic aspect of data acquisition. When using range sensors, one often obtains a mixed data set with measurements originating from many different sources. This problem would, for instance, be encountered by a robot moving in a formation, where it was surrounded by other agents. There exist established techniques for sorting mixed data sets off-line, but for time-depending systems where data need to be sorted on-line and only small time delays can be tolerated, established methods fail. The solution presented in the paper is a prediction-correction type algorithm, referred to as CCIA (Classification Correction and Identification algorithm).

Finally, in Paper E, we consider the problem of maintaining connectivity in a multi-agent system. Often inter-agent communication abilities are associated with some proximity constraints, so when the robots move in relation to each other, communication links both break and form. In the paper we present a framework for analysis that makes it possible to compute a set of general constraints which, if satisfied, are sufficient to guarantee maintained communication for a given multi-agent system. Constraints are computed for two sorts of consensus-based systems and the results are verified in simulations.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , x, 60 p.
Series
Trita-MAT. OS, ISSN 1401-2294 ; 09:05
Keyword [en]
Mobile robotics, tracking, obstacle avoidance, formation control, nonlinear observers, multi-agent coordination, connectivity graphs.
National Category
Computational Mathematics
Identifiers
URN: urn:nbn:se:kth:diva-10910ISBN: 978-91-7415-396-5 (print)OAI: oai:DiVA.org:kth-10910DiVA: diva2:231590
Public defence
2009-09-05, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100715Available from: 2009-08-26 Created: 2009-08-14 Last updated: 2010-07-15Bibliographically approved
List of papers
1. Robust tracking control and obstacle avoidance for non-holonomic mobile agents
Open this publication in new window or tab >>Robust tracking control and obstacle avoidance for non-holonomic mobile agents
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Mobile robotics is a subfield of robotics that has received much attention in the last decades and the problem of tracking a moving object has been considered in many different settings. In this paper we consider a unicycle robot with non-holonomic motion constraints that tracks a target, using only local information. The presence of non-holonomic constraints is a complicating factor that is often deliberately overlooked in theoretical work. Still, for the results to be directly implementable, the non-holonomic constraints must be taken into account. In the paper, two types of decentralized tracking control algorithms, which do not require global information about the system, are proposed. Robustness to measurement noise is demonstrated in simulations and, in addition, a reactive obstacle avoidance control that can easily be integrated with the tracking functions is suggested.

National Category
Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-10905 (URN)
Note
QC 20100714Available from: 2009-08-14 Created: 2009-08-14 Last updated: 2010-07-15Bibliographically approved
2. Adaptive formation control for non-holonomic multi-agent systems
Open this publication in new window or tab >>Adaptive formation control for non-holonomic multi-agent systems
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

The current paper treats the problem of formation keeping for a non-holonomic multi-agent system. In the paper it is shown that tracking controls developed for single-agent systems can be used as base functions for multi-agent formations. To get an idea of the performance that can be expected for such formations,  robustness to noise and error propagation are studied in simulations for a line formation with simple internal structure. In addition, it is shown that switching between different tracking angles and tracking controls can safely be performed. The results on switching stability imply that it is possible to change the structure of a formation on-line. Together, the results obtained in the paper open up for a variety of applications where a team of robots cooperate to perform tasks faster and with higher precision than would be possible for a single agent. To illustrate the potential of the results, two case studies are included in the paper. They are both independent of the rest of the paper and are added as an Appendix.

National Category
Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-10906 (URN)
Note
QC 20100715Available from: 2009-08-14 Created: 2009-08-14 Last updated: 2010-07-15Bibliographically approved
3. Estimation of neighbor velocity in a leader-follower network
Open this publication in new window or tab >>Estimation of neighbor velocity in a leader-follower network
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

In applications where mobile robots are set to follow other moving objects, it is often assumed that not only the relative position, but also the velocity of the object in question is available to the tracking robots. In this paper, we consider a leader-follower system where the followers are equipped only with vision and range sensors. Thus, the followers can detect the positions of their neighbors, but they are unable both to measure the velocity of other agents directly and to obtain estimates of the unknown velocities through communication with other agents. In order to implement the desired leader-follower controls, the followers must use available sensor data to compute individual estimates of their respective leaders' velocities. One option would be to use the Extended Kalman Filter to obtain the estimates, but in this paper we take another approach and propose two separate state observers that can be used by the followers to obtain estimates of the missing velocities and that are also shown to stabilize the tracking controls used by the robots. To evaluate the observers, the tracking performance is compared in simulations for a system that uses observer estimates of the leader's velocity in the control equations and a system that computes the velocity estimates using the standard Extended Kalman Filter algorithm.

National Category
Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-10907 (URN)
Note
QC 20100715Available from: 2009-08-14 Created: 2009-08-14 Last updated: 2010-07-15Bibliographically approved
4. A classification algorithm for mixed time-varying data sets
Open this publication in new window or tab >>A classification algorithm for mixed time-varying data sets
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Keyword
Data classification, EM algorithm, Time-varying system, Prediction-Correction, Hybrid system.
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-10908 (URN)
Note
QC 20100715Available from: 2009-08-14 Created: 2009-08-14 Last updated: 2010-07-15Bibliographically approved
5. Sufficient conditions for connectivity maintenance and rendezvous in leader-follower networks
Open this publication in new window or tab >>Sufficient conditions for connectivity maintenance and rendezvous in leader-follower networks
2010 (English)In: Automatica, ISSN 0005-1098, Vol. 46, no 1, 133-139 p.Article in journal (Refereed) Published
Abstract [en]

In this paper we derive a set of constraints that are sufficient to guarantee maintained connectivity in a leader-follower multi-agent network with proximity based communication topology. In the scenario we consider, only the leaders are aware of the global mission, which is to converge to a known destination point. Thus, the followers need to stay in contact with the group of leaders in order to reach the goal. In the paper we show that we can maintain the initial network structure, and thereby connectivity, by setting up bounds on the ratio of leaders-to-followers and on the magnitude of the goal attraction force experienced by the leaders. The results are first established for an initially complete communication graph and then extended to an incomplete graph. The results are illustrated by computer simulations.

Keyword
Connectivity graphs, Multi-agent networks, Consensus
National Category
Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-10909 (URN)10.1016/j.automatica.2009.10.014 (DOI)000274405900016 ()2-s2.0-73049096074 (Scopus ID)
Note

QC 20150727

Available from: 2009-08-14 Created: 2009-08-14 Last updated: 2015-07-27Bibliographically approved

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