Ignacio Mas

Cluster Space Specification and Control of Mobile Multirobot Systems

The cluster space state representation of mobile multirobot systems is introduced as a means of enabling enhanced control of mobile multirobot systems. A conceptual framework is proposed for the selection of appropriate cluster space state variables for an n-robot system, the development of formal kinematics that associate the cluster space state variables with robot-specific variables, and the implementation of a cluster space control system architecture. The cluster space approach is then demonstrated for examples of two- and three-robot clusters consisting of differential drive robots operating in a plane. In these examples, we demonstrate cluster space variable selection, review the critical kinematic relationships, and present experimental results that demonstrate the ability of the systems to meet control specifications while allowing a single operator to easily specify and supervise the motion of the clusters.

Entrapment/escorting and patrolling missions in multi-robot cluster space control

The tasks of entrapping/escorting and patrolling around an autonomous target are presented making use of the multi-robot cluster space control approach. The cluster space control technique promotes simplified specification and monitoring of the motion of mobile multi-robot systems of limited size. Previous work has established the conceptual foundation of this approach and has experimentally verified and validated its use for 2-robot, 3-robot and 4-robot systems, with varying implementations ranging from automated trajectory control to human-in-the-loop piloting. In this publication, we show that the problem of entrapping/escorting/patrolling is trivial to define and manage from a cluster space perspective. Using a 3-robot experimental testbed, results are shown for the given tasks. We also revise the definition of the cluster space framework for a three-robot formation and incorporate a robot-level obstacle avoidance functionality.

Dynamic Guarding of Marine Assets Through Cluster Control of Automated Surface Vessel Fleets

There is often a need to mark or patrol marine areas in order to prevent boat traffic from approaching critical regions, such as the location of a high-value vessel, a dive site, or a fragile marine ecosystem. In this paper, we describe the use of a fleet of robotic kayaks that provides such a function: the fleet circum- navigates the critical area until a threatening boat approaches, at which point the fleet establishes a barrier between the ship and the protected area. Coordinated formation control of the fleet is implemented through the use of the cluster-space control architecture, which is a full-order controller that treats the fleet as a virtual, articulating, kinematic mechanism. An application-specific layer interacts with the cluster-space controller in order for an operator to directly specify and monitor guarding-related parameters, such as the spacing between boats. This system has been experimentally verified in the field with a fleet of robotic kayaks. In this paper, we describe the control architecture used to establish the guarding behavior, review the design of the robotic kayaks, and present experimental data regarding the functionality and performance of the system.

Obstacle Avoidance Policies for Cluster Space Control of Nonholonomic Multirobot Systems

The cluster space control technique promotes simplified specification and monitoring of the motion of mobile multirobot systems of limited size. In this publication, we summarize the definition of the cluster space framework and introduce a multirobot cluster space controller specific for unicycle-like nonholonomic mobile robots. The controller produces cluster commands that translate into valid robot-level motions. We then study the closed-loop system stability in the Lyapunov sense. Two different obstacle avoidance algorithms are proposed and the stability of the resulting systems is also addressed. Experimental tests with a three-robot system and simulation results with a ten-robot system verify the functionality of the proposed approaches.

Cluster space specification and control of a 3-robot mobile system

The cluster space control technique promotes simplified specification and monitoring of the motion of mobile multi-robot systems of limited size. Previous work has established the conceptual foundation of this approach and has experimentally verified and validated its use for two diverse 2- robot systems and with varying implementations ranging from automated trajectory control to human-in-the-loop piloting. In this paper, we present the cluster space control of a 3-robot system. In doing so, we develop the fundamental kinematic relationships, illustrate the closed-loop control framework, describe the simulation and hardware testbed environments used for verification, and present initial experimental results of the successfully implemented system.

Cluster Space Control of Autonomous Surface Vessels

Flexibility, coverage, and redundancy are only three of the many advantages that multi-robot systems offer over single-robot systems. Individual unit coordination is a key technical consideration in fielding real-world application multi-robot systems. Simplified mobile multi-robot system motion specification and monitoring are promoted through the cluster space control technique. This approach has been established and experimentally verified in previous work for use with varying implementations ranging from human-in-the-loop piloting to automated trajectory control and for land-based systems consisting of 2-4 robots. A new low-cost autonomous surface vessels (ASVs) design and fabrication is described by the authors in this paper. A multi-boat system that uses the cluster space control technique to make it capable of autonomous navigation is included in the technical system. A centralized controller is also included that uses a shore-based computer to relay drive commands and receive ASV data in its implementation. A pilot may specify that a third boat maintain formation with two ASVs or may remotely drive a two-ASV cluster by using these drive commands when using the cluster space control approach. Depending on the needs of a specific application, there can be arbitrary translation, rotation, and resizing of the resulting multi-ASV clusters. There is discussion of plans for future works and provision of experimental results demonstrating these capabilities.

Centralized and decentralized multi-robot control methods using the cluster space control framework

The cluster space control technique promotes simplified specification and monitoring of the motion of mobile multi-robot systems. Previous work has established the conceptual foundation of this approach and has experimentally verified and validated its use. In this publication, we summarize the definition of the cluster space framework for planar robots and study some of the most common formation control methods found in the literature from the cluster space perspective. In doing this, we show that our proposed formation control framework can be implemented in various ways; as a centralized or distributed system, and with different levels of scalability depending on the particular cluster definition chosen. In particular, lead-follower, potential functions and virtual structures approaches are analyzed with the intent of addressing the generality and flexibility of the cluster space formation control approach. Experimental results illustrate the different implementations which are then compared and contrasted.

Gradient-Based Cluster Space Navigation for Autonomous Surface Vessels

This paper presents an experimentally demonstrated gradient-based multirobot technique for adaptively navigating within a parameter field. To implement this technique, simultaneous measurements of the parameter are made at different locations within the field by a spatially controlled cluster of mobile robots. These measurements are shared in order to compute a local gradient of the field. Depending on the task to be achieved, the multirobot cluster is directed with respect to this direction. Moving in or opposite to the gradient direction allows efficient navigation to local maxima/minima in the field, a capability of interest for applications such as detecting pollution sources or the location of resource-starved areas. Moving perpendicular to the gradient direction allows parameter contours to be navigated, a behavior useful for applications such as defining the extent of a field or establishing a safety perimeter at a defined field level. This paper describes the multirobot control technique which combines a full degree-of-freedom “cluster space” multirobot controller with a gradient-based adaptive navigation capability. Verification of the technique through field experiments using a fleet of three robotic kayaks is also presented. Finally, a discussion of results, a review of challenges, and a review of ongoing and future work are presented.

Dynamic Control of Mobile Multirobot Systems: The Cluster Space Formulation

The formation control technique called cluster space control promotes simplified specification and monitoring of the motion of mobile multirobot systems of limited size. Previous paper has established the conceptual foundation of this approach and has experimentally verified and validated its use for various systems implementing kinematic controllers. In this paper, we briefly review the definition of the cluster space framework and introduce a new cluster space dynamic model. This model represents the dynamics of the formation as a whole as a function of the dynamics of the member robots. Given this model, generalized cluster space forces can be applied to the formation, and a Jacobian transpose controller can be implemented to transform cluster space compensation forces into robot-level forces to be applied to the robots in the formation. Then, a nonlinear model-based partition controller is proposed. This controller cancels out the formation dynamics and effectively decouples the cluster space variables. Computer simulations and experimental results using three autonomous surface vessels and four land rovers show the effectiveness of the approach. Finally, sensitivity to errors in the estimation of cluster model parameters is analyzed.

Error characterization in the vicinity of singularities in multi-robot cluster space control

The cluster space control technique promotes simplified specification and monitoring of the motion of mobile multi-robot systems of limited size. Previous work has established the conceptual foundation of this approach and has experimentally verified and validated its use for 2-robot, 3-robot and 4-robot systems, with varying implementations ranging from automated trajectory control to human-in-the-loop piloting. In this publication, we review the cluster space framework and its application to a 3-robot system and present the problem of robot space to cluster space error propagation and its impact on cluster position uncertainty and control performance. A theoretical formulation of cluster position error is presented. Using a 3-robot system testbed, the results are verified through experimental measurements. The cluster space Jacobian matrix condition number is proposed as a metric for acceptable cluster configuration errors.