Daniel J. Stilwell

Platoons of underwater vehicles

We have presented a decentralized control design methodology for regulating global functions of cooperating mobile systems. The application of relatively standard system-theoretic tools, leads to a novel broadcast-only communication structure. The feedback mechanism between vehicles is the measurement of the global variables and broadcast of their integrated values. More generally, the methods presented allow the designer to determine what explicit communication strategies are sufficient for a stabilizing decentralized control to exist. Using a simplified model, we showed that it is indeed possible to regulate global variables of a platoon of autonomous underwater vehicles, in particular, the center of the platoon and the distribution of the vehicles about the center. A relatively small amount of explicit communication is required between vehicles and that no vehicle must regulate its actual position. Further, the approach presented is scalable to any number of cooperating vehicles without the need for additional communication, although there is a practical limit on the size of the platoon.

Consensus Seeking Over Random Weighted Directed Graphs

We examine the consensus problem for a group of agents that communicate via a stochastic information network. Communication among agents is modeled as a weighted directed random graph that switches periodically. The existence of any edge is probabilistic and independent from the existence of any other edge. We further allow each edge to be weighted differently. Sufficient conditions for asymptotic almost sure consensus are presented for the case of positive weights and for the case of arbitrary weights.

Sufficient Conditions for Fast Switching Synchronization in Time-Varying Network Topologies

In previous work [J. D. Skufca and E. Bollt, Mathematical Biosciences and Engineering, 1 (2004), pp. 347-359], empirical evidence indicated that a time-varying network could propagate sufficient in...

Toward the development of a material transport system using swarms of ant-like robots

A decentralized method for controlling a homogeneous swarm of autonomous mobile robots that collectively transport a single palletized load is proposed. The small tank-like robots have no advanced sensory or communications capabilities. They have no information on the position or number of other robots transporting the small pallet. Instead, all information needed by the robots is derived from the dynamics inherent when the system of robots is contacting a common rigid body. Each robot derives the required local information from a force sensor mounted at the point at which it contacts the pallet. A distributed control law is derived, and the resulting stable behavior of the system is verified by computer simulation.<<ETX>>

Tracking and formation control of multiple autonomous agents: A two-level consensus approach

Simultaneous tracking and formation control is addressed for a team of autonomous agents that evolve dynamically in a space containing a measurable vector field. Each agent measures the local value of the field along its trajectory and occasionally shares relevant information with other agents, in order to estimate the spatial average obtained from averaging measurements across all agents. Using shared information, agents control their trajectories in a cooperative manner, with the dual goals of driving the average field measurement to a specified value and maintaining a desired formation about the average. Two approaches to virtual leader estimation are considered. The first involves the synthesis of a common virtual leader state, whereas the second involves decentralized estimation of the virtual leader by individual agents. Under the second approach, control is posed as a two-level consensus problem, where agents reach agreement on the virtual leader state at one level and reach formation about the virtual leader at the other level. The decentralized approach is effective even when communication among agents is limited, in the sense that the associated network graph can be disconnected in frozen time.

Interpolation of observer state feedback controllers for gain scheduling

The authors propose a method of interpolating linear time-invariant controllers with observer state feedback structure in order to generate a continuously varying family of controllers that stabilizes a family of linear plants. Gain scheduling is a motivation for this work, and the interpolation method yields guidelines for the design of gain scheduled controllers. The method is illustrated with the design of a missile autopilot using loop-shaping H-infinity controllers.

Brief Stability preserving interpolation methods for the synthesis of gain scheduled controllers

Synthesis of gain scheduled controllers for nonlinear plants often requires that a parameter-varying controller be generated from a finite set of linear time-invariant controllers. We propose interpolation methods for this task with the property that stability of the linearized closed-loop system is preserved for all values of the scheduling parameter. In addition, slow-variation arguments are presented that establish local stability of the nonlinear closed-loop system with gain scheduled controller.

Synchronization in Random Weighted Directed Networks

We assess synchronization of oscillators that are coupled via a time-varying stochastic network, modeled as a weighted directed random graph that switches at a given rate between a set of possible graphs. The existence of any graph edge is probabilistic and independent from the existence of any other edge. We further allow each edge to be weighted differently. Even if the network is always instantaneously not connected, we show that sufficient information is propagated through the network to allow almost sure local synchronization as long as the expected value of the network is connected, and that the switching rate is sufficiently fast.

Toward underwater navigation based on range measurements from a single location

Navigation technology is considered that enables an underwater vehicle to compute its trajectory in real-time by utilizing range measurements from a single known location. This concept is especially attractive for small underwater vehicles where more traditional navigation technology is prohibitive due to volume constraints. By assessing local observability of the underwater vehicle and available measurements about potential vehicle trajectories, we explicitly characterize those trajectories that can be asymptotically estimated. It is shown that all but a small class of trajectories can be estimated.

A complete solution to underwater navigation in the presence of unknown currents based on range measurements from a single location

An underwater navigation algorithm is considered that enables an underwater vehicle to compute its trajectory in the presence of unknown currents by utilizing range measurements from a single known location. There exists a small set of trajectories for which the algorithm does not converge. By assessing local observability about potential vehicle trajectories, we characterize those trajectories that cannot be asymptotically estimated. The navigation algorithm is illustrated using computer simulation and a hardware experiment.