Vladimir Djapic

Leader-following control of multiple nonholonomic systems over directed communication graphs

This paper considers the leader-following control problem of multiple nonlinear systems with directed communication topology and a leader. If the state of each system is measurable, distributed state feedback controllers are proposed using neighbours’ state information with the aid of Lyapunov techniques and properties of Laplacian matrix for time-invariant communication graph and time-varying communication graph. It is shown that the state of each system exponentially converges to the state of a leader. If the state of each system is not measurable, distributed observer-based output feedback control laws are proposed. As an application of the proposed results, formation control of wheeled mobile robots is studied. The simulation results show the effectiveness of the proposed results.

Clock synchronization and ranging estimation for control and cooperation of multiple UUVs

This paper presents the initial implementation of an acoustic synchronization and ranging system to enable the control and cooperation of multiple Unmanned Underwater Vehicles (UUVs). Our solution is based on acoustic clock synchronization and one-way ranging. It requires minimum overhead while providing accurate and quick estimation of the relative distances among underwater nodes. The use of one-way ranging allows to scale up to large teams of UUVs and reduces the energy consumption of localization techniques. Our solution has been implemented in SUNSET, leveraging on the accurate timing information and scheduled transmissions provided by SeaModem acoustic modems. Chip Scale Atomic Clocks have been integrated in the SeaModem to overcome the typical drift of real-time clocks thus enabling accurate one-way ranging estimation during long term missions. The performance of the proposed system have been extensively evaluated in two at-sea campaigns considering different testing scenarios. We have shown that our scheme is able to maintain high ranging accuracy over time without requiring the high overhead and energy consumption of two way ranging techniques. We have also shown that the proposed scheme for acoustic synchronization is very effective in synchronizing real-time and atomic clocks of underwater nodes, whenever needed. Our results confirm that the proposed solution for synchronization and one-way ranging allows to enable the control of multiple UUVs keeping at the bay the overhead in the network and the time needed to estimate relative distances.

Distributed formation tracking control of multiple mobile robotic systems

In this paper formation tracking control of multiple wheeled mobile robots is studied. The reference trajectory is considered as a virtual leader vehicle system while the real multiple vehicle systems are considered as follower agents. Chained-form systems and theories of cascaded systems and communication graph are introduced to design control methods for kinematic systems. In addition to distributed control algorithms for kinematic multi-vehicle systems, formation control of vehicles dynamics is addressed with the aid of backstepping method, parametrical uncertainties of vehicles mechanics are estimated by sliding mode control. Simulation results are presented to verify the proposed control laws.

JANUS: Lingua Franca

JANUS is an emergent underwater communications standard for `initial contact. JANUS can serve as an underwater beacon to broadcast information. It is free for public use. The current status of the standard and the areas of its application are presented. The results from multiple governmental programs, recent experiments and early commercial adoptions are provided. Several at-sea data telemetry projects are discussed including between fixed and mobile nodes capable of communicating using multiple physical layer solutions. An overview of the progress and the ongoing efforts for the public and open standard are considered. The utility of JANUS is outlined as an aid for autonomous underwater vehicle navigation by exploiting the infrastructure from cargo ships, wind farms, underwater instrumentation, etc. The relevance of the SUNRISE and Forward-Deployed Energy & Communications Outpost programs are discussed.

Heterogeneous Autonomous Mobile Maritime Expeditionary Robots

For the ONR-funded Heterogeneous Adaptive Maritime Mobile Expeditionary Robots (HAMMER) project, we work on cooperative autonomy for a fleet of unmanned vehicles working together in the aerial, water surface, and underwater domains. Each of these systems work well independently, but our goal is to integrate their performance into one system of vehicles that can safely perform cooperative tasks. The challenges we are working on include creating reliable communications links between vehicles in the harsh low bandwidth maritime environment, integrating novel onboard sensors and inter-vehicle communication to create filters to estimate the state of the network, and creating autonomous takeoff-and-landing algorithms between the aerial/underwater vehicles and the surface “mothership” vehicle. The surface vehicle is envisioned to be capable of transporting the aerial and underwater vehicles as well as providing mission-lengthening power. Possible applications of this system include automated deployment and recovery of data-collecting unmanned underwater vehicles and an ad hoc wireless network where the aerial vehicle relays time-sensitive data collected from the surface or underwater vehicle to a human on a ship many miles away. In a separate but related project, we are also determining human-autonomy teaming required for future Naval programs, assessing the state-of-the-art algorithms, and creating open challenge problems to academia to fill gaps based on the Navys need.

Challenges in underwater navigation: Exploring magnetic sensors anomaly sensing and navigation

This work combines magnetic field sensing and underwater navigation without the need of a priori maps. The initial work utilizes distributed cooperative localization and control used estimate the states of nodes (autonomous underwater vehicles - AUVs) that carry magnetic sensors. Because of their size, weight, power consumption, and cost fluxgate magnetometers are considered. The distributed estimation method and formation control algorithms allow for the creation of various shapes (vertical, horizontal, and longitudinal) of gradiometer sensor using multiple gliders/AUVs that each carry magnetometers as opposed to traditionally considered arrays on a single AUV. This procedure can be applied in Mine Countermeasures (MCM) - military or unexploded ordnances (UXO) both to detect / localize the anomalies (mines) or cable and pipeline survey - civilian, thus serving as a “payload” sensor. At the same time, the same anomailes can be used as features for navigation corrections.

Autonomous takeoff & landing of small UAS from the USV

A successful combination of the Interoperable Heterogeneous Unmanned Autonmous Systems for future missions at sea is becoming a reality. An Unmanned Surface Vehicle (USV), as the central node and main transport mechanism, can be used to carry Unmanned Aerial System (UAS) and Unmanned Underwater Vehicles (UUVs) to distances over several hundred miles. The system is designed to be modular and can easily be scaled up if needed. In this paper, the initial implementation of an advanced UAS takeoff/landing from/to a 16 foot catamaran USV (70×40 inch landing area) provides solutions that can enable the interoperability, coordination and cooperation of autonomous mobile marine robots in GPS-denied environment are presented. To enable such a heterogeneous network three main research areas have been blended: 1) image processing, 2) state estimation, and 3) cooperative control and autonomy. This paper focuses on the first two components and addresses the integration of the third one. Due to the challenges of the maritime environment, novel software and hardware have to be investigated to provide the level of flexibility and features that are required for efficient and robust landing. Robust and reliable communications have to be developed and implemented to enable an efficient sharing of data and control messages among heterogeneous surface and aerial platforms.