Papers by Daniel Stilwell
We announce the development of a prototype miniature low-cost autonomous underwater vehicle (AUV)... more We announce the development of a prototype miniature low-cost autonomous underwater vehicle (AUV). The vehicle was designed to explore technology for one-way military applications, including minefield mapping/tagging, and cooperative underwater activities with similar units, but could also find application in shallow water mapping and remote sensing. The prototype vehicle measures approximately 24 inches long (61 m) by 3.5 inches in
American Control Conference, 2005
We examine the stability and behavior of a platoon of autonomous vehicles which operate in a dece... more We examine the stability and behavior of a platoon of autonomous vehicles which operate in a decentralized manner and communicate to achieve formation control objectives. We are interested in communication networks that are time-varying, with each vehicle interacting with different sets of vehicles as the system evolves. We are particularly interested in networks which are disconnected, and in which basic
IEEE Transactions on Automatic Control, 1997
We propose a method of interpolating linear time-invariant controllers with observer state feedba... more We 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 scheduling method is illustrated with the design of
This dissertation addresses attitude control and inertial navigation of autonomous underwater veh... more This dissertation addresses attitude control and inertial navigation of autonomous underwater vehicles (AUVs). We present theoretical justification for using simplified models, derive system identification algorithms, and verify our results through extensive field trials. Although this research focuses on small AUVs with limited instrumentation, the results are useful for underwater vehicles of any size. standing team. I wish everybody good luck in their future endeavors.
Ocean Engineering, 2009
Motivated by the desire to improve the navigation performance of small autonomous underwater vehi... more Motivated by the desire to improve the navigation performance of small autonomous underwater vehicles, we seek simple methods for modeling the local flow that affects the vehicle's trajectory. We propose a low-complexity, planar flow field model that consists of a uniform flow component and a singular flow component. Assuming this simplified flow field model, we develop identification algorithms that can be performed quickly using small, sparse data sets collected by a platoon of vehicles. The basic approach involves estimating the uniform flow component, localizing an assumed flow singularity, and identifying the parameters which characterize the singular flow. In order to identify the singular flow component, we propose a least squares and a constrained least squares approach. In the latter case, the minimization is constrained to preserve the average divergence and circulation of the measured flow. Preserving divergence and circulation predictably results in a larger overall error because it eliminates two free model parameters, but it produces a flow model which more realistically captures the flow in the larger region of the measurement points. Numerical simulations and an initial experiment illustrate the parameter identification process as well as the benefits of incorporating a flow field model for vehicle navigation.
A continuous-time gain-scheduled controller must be transformed to a corresponding discrete-time ... more A continuous-time gain-scheduled controller must be transformed to a corresponding discrete-time controller for sampled-data implementation. We show that certain linearization properties of a continuoustime gain scheduled controller are inherited by its sampled-data implementation. We also show that a similar relationship exists for multi-rate gain scheduled controllers arising in flight control applications. 864 scheduled controller can be constructed if the conditions of Theorem 3.4 are satisfied for the original family discrete-time linear controllers.
Ocean Engineering, 2010
Although the use of low-order equivalent models is common and extensively studied for control of ... more Although the use of low-order equivalent models is common and extensively studied for control of aircraft systems, similar analysis has not been performed for submersible systems. Toward an improved understanding of the utility of low-order equivalent models for submersible systems, we examine control design for pitch-axis motion of an autonomous underwater vehicle (AUV). Derived from first principles, the pitch-axis motion of a streamlined AUV is described by third-order dynamics. However, second-order approximate models are common for system identification and control design. In this work, we provide theoretical justification for both the use of and limitations of a second-order model, and we verify our results in practice via a series of case studies. We conclude that a second-order pitchaxis model should often be sufficient for system identification and control design.
International Conference on Intelligent RObots and Systems - IROS, 2005
An underwater navigation algorithm is considered that enables an underwater vehicle to compute it... more An underwater navigation algorithm is considered that enables an underwater vehicle to compute its trajectory and unknown currents by utilizing range measurements from a single known location. By assessing local observability about potential vehicle trajectories, we characterize those trajectories that cannot be asymptotically estimated. Analysis is also presented to clarify the relationship between finite time observability, a theme of this
Conference on Decision and Control, 2000
Development of decentralized control systems for platoons of robotic vehicles often requires dire... more Development of decentralized control systems for platoons of robotic vehicles often requires direct communication between vehicles. Using basic tools from the field of decentralized control, we develop a framework from which existence of vehicle controllers can be assessed for a given communication structure. This approach to control design allows one to investigate minimum inter-vehicle communication solutions. Simulation results are presented
Conference on Decision and Control, 2002
An approach to real-time, decentralized trajectory generation for platoons of autonomous vehicles... more An approach to real-time, decentralized trajectory generation for platoons of autonomous vehicles is developed from well-known control techniques for redundant robotic manipulators. The decentralized structure of this approach permits each vehicle to independently compute its trajectory in real-time using only locally generated information and low-bandwidth feedback generated by a system exogenous to the platoon. Our work is motivated by applications
IEEE Conference on Control Applications, 2001
In this paper, we discuss the parallelism between redundant manipulator control and the control o... more In this paper, we discuss the parallelism between redundant manipulator control and the control of platoon-level functions for systems of autonomous vehicles. We cast the platoon formation problem in an analogous manner to traditional redundancy resolution, and discuss techniques for control of the platoon
International Conference on Robotics and Automation, 2000
Decentralized control of multiple mobile robots for cooperative tasks often requires not only env... more Decentralized control of multiple mobile robots for cooperative tasks often requires not only environmental sensing but communication among the robot subsystems. In this work, we develop observer-based methods for characterizing the implicit and explicit communications required for a swarm of robots to successfully achieve classes of control objectives
Stability and L2 gain properties of linear parameter-varying (LPV) systems are obtained under ass... more Stability and L2 gain properties of linear parameter-varying (LPV) systems are obtained under assumed bounds on either the maximum or average value of the parameter rate
Synthesis of gain scheduled controllers often requires that a parameter-varying controller be gen... more Synthesis of gain scheduled controllers 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 each fixed value of the scheduling parameter. In addition, slow-variation arguments are presented that establish stability of the nonlinear
2007 IEEE 11th International Conference on Computer Vision, 2007
... Xiaojin Gong, Anbumani Subramanian, Christopher L. Wyatt, Daniel J. Stilwell Department of El... more ... Xiaojin Gong, Anbumani Subramanian, Christopher L. Wyatt, Daniel J. Stilwell Department of Electrical and Computer Engineering, Virginia Tech Blacksburg, VA, USA, 24061 {xiaojin, anbumani, clwyatt, stilwell}@vt.edu Abstract ...
2006 Fortieth Asilomar Conference on Signals, Systems and Computers, 2006
... An on board GPS and a wireless ethernet link enable remote operation of the vehicle. ... The ... more ... An on board GPS and a wireless ethernet link enable remote operation of the vehicle. ... The images were acquired from three different aquatic environments taken mostly during after noon ... shorelinedetection algorithm can be used in the vehicle for autonomous navigation in real ...
IEEE Transactions on Systems, Man, and Cybernetics, 2005
An approach to real-time trajectory generation for platoons of autonomous vehicles is developed f... more An approach to real-time trajectory generation for platoons of autonomous vehicles is developed from well-known control techniques for redundant robotic manipulators. The partially decentralized structure of this approach permits each vehicle to independently compute its trajectory in real-time using only locally generated information and low-bandwidth feedback generated by a system exogenous to the platoon. Our work is motivated by applications
AIAA Guidance, Navigation, and Control Conference and Exhibit, 2006
We examine the consensus problem for a group of agents that communicate via a network whose topol... more We examine the consensus problem for a group of agents that communicate via a network whose topology switches randomly. The presence of a communication link between a pair of agents is random. In order to account for general information flow, communication among agents is directed, and each link possess a possibly different non-negative weight. At random instants, the communication network topology switches randomly between a finite set of topologies. Sufficient conditions for asymptotic almost sure consensus are presented.
2007 American Control Conference, 2007
We examine the synchronization problem for a group of dynamic agents that communicate via a movin... more We examine the synchronization problem for a group of dynamic agents that communicate via a moving neighborhood network. Each agent is modeled as a random walker in a finite lattice and is equipped with an oscillator. The communication network topology changes randomly and is dictated by the agents' locations in the lattice. Information sharing (talking) is possible only for geographically neighboring agents. The complex system is a time-varying jump nonlinear system. We introduce the concept of long-time expected communication network defined as the ergodic limit of the stochastic time-varying network. We show that if the long-time expected network supports synchronization, then so does the stochastic network when the agents diffuse sufficiently fast in the lattice.
2006 American Control Conference, 2006
Stability of periodic linear switched systems is studied. By combining the method of averaging wi... more Stability of periodic linear switched systems is studied. By combining the method of averaging with exponential splitting, less conservative bounds on stabilizing switching rates are obtained than with other approximation techniques. Such bounds are useful for analysis and design of switching control laws. The stability analysis is generalized to a wider class of time-varying systems.
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Papers by Daniel Stilwell