@article {2017k-NesRobBueFra,
	title = {Decentralized Simultaneous Multi-target Exploration using a Connected Network of Multiple Robots},
	journal = {Autonomous Robots},
	volume = {41},
	year = {2017},
	month = {03/2016},
	pages = {989-1011},
	abstract = {This paper presents a novel decentralized control strategy for a multi-robot system that enables parallel multi-target exploration while ensuring a time-varying connected topology in cluttered 3D environments. Flexible continuous connectivity is guaranteed by building upon a recent connectivity maintenance method, in which limited range, line-of-sight visibility, and collision avoidance are taken into account at the same time.
Completeness of the decentralized multi-target exploration algorithm is guaranteed by dynamically assigning the robots with different motion behaviors during the exploration task. One major group is subject to a suitable downscaling of the main traveling force based on the traveling efficiency of the current leader and the direction alignment between traveling and connectivity force. This supports the leader in always reaching its current target and, on a larger time horizon, that the whole team realizes the overall task in finite time.
Extensive Monte~Carlo simulations with a group of several quadrotor UAVs show the scalability and effectiveness of the proposed method and experiments validate its practicability. },
	keywords = {Connectivity maintenance, Exploration, Motion control of multiple robots, Patrolling / Surveillance},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/1505.05441v3.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016g-NesRobBueFra-exp.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016g-NesRobBueFra-empty.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016g-NesRobBueFra-office.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016g-NesRobBueFra-town.mp4},
	author = {Thomas Nestmeyer and Paolo Robuffo Giordano and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@article {2014i-ZelRobBueFra,
	title = {Decentralized Rigidity Maintenance Control with Range Measurements for Multi-Robot Systems},
	journal = {The International Journal of Robotics Research},
	volume = {34},
	year = {2014},
	pages = {105-128},
	abstract = {This work proposes a fully decentralized strategy for maintaining the formation rigidity of a multi-robot system using only range measurements, while still allowing the graph topology to change freely over time. In this direction, a first contribution of this work is the new concept of weighted frameworks and rigidity, and of the rigidity eigenvalue, which when positive ensures the infinitesimal rigidity of a weighted framework. We then propose a distributed algorithm for estimating a common relative position reference frame amongst a team of robots with only range measurements in addition to one agent endowed with the capability of measuring the bearing to two other agents. This first estimation step is embedded into a subsequent distributed algorithm for estimating the rigidity eigenvalue associated with the weighted framework. The estimate of the rigidity eigenvalue is finally used to generate a local control action for each agent that both maintains the rigidity property and enforces additional constraints such as collision avoidance and sensing/communication range limits and occlusions. As an additional feature of our approach, the communication and sensing links among the robots are also left free to change over time while preserving rigidity of the whole framework. The proposed scheme is then experimentally validated with a robotic testbed consisting of 6 quadrotor UAVs operating in a cluttered environment.},
	keywords = {Motion control of multiple robots, Rigidity mainenance},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/1309.0535v3.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/1309.0535v2.pdf},
	author = {Daniel Zelazo and Antonio Franchi and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2014f-ZelFraRob,
	title = {Rigidity Theory in SE(2) for Unscaled Relative Position Estimation using only Bearing},
	booktitle = {2014 European Control Conference},
	year = {2014},
	month = {06/2014},
	pages = {2703-2708},
	address = {Strasbourg, France},
	abstract = {This work considers the problem of estimating the unscaled relative positions of a multi-robot team in a common reference frame from bearing-only measurements. Each robot has access to a relative bearing measurement taken from the local body frame of the robot, and the robots have no knowledge of a common reference frame. An extension of rigidity theory is made for frameworks embedded in the special Euclidean group SE(2) = R^2  {\texttimes} S1. We introduce definitions describing rigidity for SE(2) frameworks and provide necessary and sufficient conditions for when such a framework is infinitesimally rigid in SE(2). We then introduce the directed bearing rigidity matrix and show that an SE(2) framework is infinitesimally rigid if and only if the rank of this matrix is equal to 2|V| - 4, where |V| is the number of agents in the ensemble. The directed bearing rigidity matrix and its properties are then used in the implementation and convergence proof of a distributed estimator to determine the unscaled relative positions in a common frame. Simulation results are given to support the
analysis.},
	keywords = {Localization of ground robots, Motion control of multiple robots, Rigidity mainenance},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014f-ZelFraRob-preprint.pdf},
	author = {Daniel Zelazo and Antonio Franchi and Paolo Robuffo Giordano}
}
@booklet {2013l-NesRobFra,
	title = {Human-assisted Parallel Multi-target Visiting in a Connected Topology},
	howpublished = {6th Int. Work. on Human-Friendly Robotics},
	year = {2013},
	month = {10/2012},
	address = {Rome, Italy},
	keywords = {Bilateral Shared Control of Mobile Robots, Connectivity maintenance, Motion control of multiple robots},
	author = {Thomas Nestmeyer and Paolo Robuffo Giordano and Antonio Franchi}
}
@article {2011q-SonFraChuKimBueRob,
	title = {Human-Centered Design and Evaluation of Haptic Cueing for Teleoperation of Multiple Mobile Robots},
	journal = {IEEE Transactions on Systems, Man, \& Cybernetics. Part B: Cybernetics},
	volume = {43},
	year = {2013},
	month = {04/2013},
	pages = {597-609},
	abstract = {In this paper, we investigate the effect of haptic cueing on human operator{\textquoteright}s performance in the field of bilateral teleoperation of multiple mobile robots, in particular multiple unmanned aerial vehicles (UAVs). Two aspects of human performance are deemed important in this area, namely the maneuverability of mobile robots and perceptual sensitivity of the remote environment. We introduce metrics that allow us to address these aspects in two psychophysical studies, which are reported here. Three fundamental haptic cue types were evaluated. The Force cue conveys information on the proximity of the commanded trajectory to obstacles in the remote environment. The Velocity cue represents the mismatch between the commanded and actual velocity of the UAVs and can implicitly provide a rich amount of information regarding the actual behavior of the UAVs. Finally, the Velocity+Force cue is a linear combination of the two.
Our experimental results show that while maneuverability is best supported by the Force cue feedback, perceptual sensitivity is best served by the Velocity cue feedback. In addition, we show that large gains in the haptic feedbacks do not always guarantee an enhancement in teleoperator{\textquoteright}s performance.},
	keywords = {Evaluation, Force feedback, Haptics, Multi-robot systems, Psychophysical evaluation of haptic feedback, Psychophysics, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013a-SonFraChuKimBueRob_1.pdf},
	author = {Hyoung Il Son and Antonio Franchi and Lewis L. Chuang and Junsuk Kim and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@booklet {2013h-NesRieLaeHarBotRobFra,
	title = {Interactive Demo: Haptic Remote Control of Multiple UAVs with Autonomous Cohesive Behavior},
	howpublished = {Int. Work. on Towards Fully Decentralized Multi-Robot Systems: Hardware, Software and Integration, at 2013 IEEE Int. Conf. on Robotics and Automation},
	year = {2013},
	month = {05/2013},
	address = {Karlsruhe, Germany},
	keywords = {Bilateral Shared Control of Mobile Robots, Connectivity maintenance, Middleware for robotics, UAV hardware platforms},
	url = {http://icra2013mrs.tuebingen.mpg.de/},
	author = {Thomas Nestmeyer and Martin Riedel and Johannes L{\"a}chele and Simon Hartmann and Fiete Botschen and Paolo Robuffo Giordano and Antonio Franchi}
}
@booklet {2012f-NesRobFra,
	title = {Multi-target Simultaneous Exploration with Continual Connectivity},
	howpublished = {2th Int. Work. on Crossing the Reality Gap - From Single to Multi- to Many Robot Systems, at 2013 IEEE Int. Conf. on Robotics and Automation},
	year = {2013},
	month = {05/2013},
	address = {Karlsruhe, Germany},
	keywords = {Connectivity maintenance, Exploration, Motion control of multiple robots},
	author = {Thomas Nestmeyer and Paolo Robuffo Giordano and Antonio Franchi}
}
@article {2013l-RobFraSecBue,
	title = {A Passivity-Based Decentralized Strategy for Generalized Connectivity Maintenance},
	journal = {The International Journal of Robotics Research},
	volume = {32},
	year = {2013},
	month = {03/2013},
	pages = { 299-323},
	abstract = {The design of decentralized controllers coping with the typical constraints on the inter-robot sensing/communication capabilities represents a promising direction in multi-robot research thanks to the inherent scalability and fault tolerance of these approaches. In these cases, connectivity of the underlying interaction graph plays a fundamental role: it represents a necessary condition for allowing a group or robots achieving a common task by resorting to only local information. Goal of this paper is to present a novel decentralized strategy able to enforce connectivity maintenance for a group of robots in a flexible way, that is, by granting large freedom to the group internal configuration so as to allow establishment/deletion of interaction links at anytime as long as global connectivity is preserved. A peculiar feature of our approach is that we are able to embed into a unique connectivity preserving action a large number of constraints and requirements for the group: (i) presence of specific inter-robot sensing/communication models, (ii) group requirements such as formation control, and (iii) individual requirements such as collision avoidance. This is achieved by defining a suitable global potential function of the second smallest eigenvalue λ2 of the graph Laplacian, and by computing, in a decentralized way, a gradient-like controller built on top of this potential. Simulation results obtained with a group of quadorotor UAVs and UGVs, and experimental results obtained with four quadrotor UAVs, are finally presented to thoroughly illustrate the features of our approach on a concrete case study.},
	keywords = {Bilateral Shared Control of Mobile Robots, Connectivity maintenance, Motion control of multiple robots},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013l-RobFraSecBue-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/IJRRConn_MM_1_LQ.mp4},
	author = {Paolo Robuffo Giordano and Antonio Franchi and Cristian Secchi and Heinrich H. B{\"u}lthoff}
}
@article {2013b-LeeFraSonBueRob,
	title = {Semi-Autonomous Haptic Teleoperation Control Architecture of Multiple Unmanned Aerial Vehicles},
	journal = {IEEE/ASME Transaction on Mechatronics, Focused Section on Aerospace Mechatronics},
	volume = {18},
	year = {2013},
	month = {08/2013},
	pages = {1334-1345},
	abstract = {We propose a novel semi-autonomous haptic teleoperation control architecture for multiple unmanned aerial vehicles (UAVs), consisting of three control layers: 1) UAV control layer, where each UAV is abstracted by, and is controlled to follow
the trajectory of, its own kinematic Cartesian virtual point (VP); 2) VP control layer, which modulates each VP{\textquoteright}s motion according to the teleoperation commands and local artificial potentials (for VP-VP/VP-obstacle collision avoidance and VP-VP connectivity preservation); and 3) teleoperation layer, through which a single remote human user can command all (or some) of the VPs{\textquoteright} velocity while haptically perceiving the state of all (or some) of the UAVs and obstacles. Master-passivity/slave-stability and some asymptotic performance measures are proved. Experimental results using four custom-built quadrotor-type UAVs are also presented to illustrate the theory.},
	keywords = {Bilateral Shared Control of Mobile Robots, Motion control of multiple robots, UAV hardware platforms},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013b-LeeFraSonBueRob.pdf},
	author = {Dongjun Lee and Antonio Franchi and Hyoung Il Son and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@booklet {2013i-LaeRieRobFra,
	title = {SwarmSimX and TeleKyb: Two ROS-integrated Software Frameworks for Single- and Multi-Robot Applications},
	howpublished = {Int. Work. on Towards Fully Decentralized Multi-Robot Systems: Hardware, Software and Integration, at 2013 IEEE Int. Conf. on Robotics and Automation},
	year = {2013},
	month = {05/2013},
	address = {Karlsruhe, Germany},
	keywords = {Middleware for robotics, Simulators for robotics},
	url = {http://icra2013mrs.tuebingen.mpg.de/},
	author = {Johannes L{\"a}chele and Martin Riedel and Paolo Robuffo Giordano and Antonio Franchi}
}
@conference {2013j-GraRieBueRobFra,
	title = {The TeleKyb Framework for a Modular and Extendible ROS-based Quadrotor Control},
	booktitle = {6th European Conference on Mobile Robots},
	year = {2013},
	month = {09/2013},
	address = {Barcelona, Spain},
	abstract = {The free and open source Tele-Operation Platform of the MPI for Biological Cybernetics (TeleKyb) is an end- to-end software framework for the development of bilateral teleoperation systems between human interfaces (e.g., haptic force feedback devices or gamepads) and groups of quadrotor Unmanned Aerial Vehicles (UAVs). Among drivers for devices and robots from various hardware manufactures, TeleKyb provides a high-level closed-loop robotic controller for mobile robots that can be extended dynamically with modules for state estimation, trajectory planning, processing, and tracking. Since all internal communication is based on the Robot Operating System (ROS), TeleKyb can be easily extended to meet future needs. The capabilities of the overall framework are demonstrated in both an experimental validation of the controller for an individual quadrotor and a complex experimental setup involving bilateral human-robot interaction and shared formation control of multiple UAVs.},
	keywords = {Middleware for robotics, UAV hardware platforms},
	url = {http://www.ros.org/wiki/telekyb},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013j-GraRieBueRobFra-preprint.pdf},
	author = {Volker Grabe and Martin Riedel and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano and Antonio Franchi}
}
@conference {2012j-SpiFraOriBueRob,
	title = {Aerial Grasping of a Moving Target with a Quadrotor UAV},
	booktitle = {2012 IEEE/RSJ Int. Conf. on Intelligent Robots and Systems},
	year = {2012},
	month = {10/2012},
	pages = {4985-4992},
	address = {Vilamoura, Portugal},
	abstract = {For a quadrotor aircraft, we study the problem of planning a trajectory that connects two arbitrary states while allowing the UAV to grasp a moving target at some intermediate time.
To this end, two classes of canonical grasping maneuvers are defined and characterized. A planning strategy relying on differential flatness is then proposed to concatenate one or more grasping maneuvers by means of spline-based subtrajectories, with the additional objective of minimizing the total transfer time.
The proposed planning algorithm is not restricted to pure hovering-to-hovering motions and takes into account practical constraints, such as the finite duration of the grasping phase. 
The effectiveness of the proposed approach is shown by means of physically-based simulations.},
	keywords = {Motion Planning, Optimal Trajectory Planning},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012j-SpiFraOriBueRob.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012j-SpiFraOriBueRob.mp4},
	author = {Riccardo Spica and Antonio Franchi and Giuseppe Oriolo and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2012b-SecFraBueRob,
	title = {Bilateral Teleoperation of a Group of UAVs with Communication Delays and Switching Topology},
	booktitle = {2012 IEEE Int. Conf. on Robotics and Automation},
	year = {2012},
	month = {05/2012},
	address = {St. Paul, MN},
	abstract = {In this paper, we present a passivity-based decentralized approach for bilaterally teleoperating a group of UAVs composing the slave side of the teleoperation system. In particular, we explicitly consider the presence of time delays, both among the master and slave, and within UAVs composing the group. Our focus is on analyzing suitable (passive) strategies that allow a stable teloperation of the group despite presence of delays, while still ensuring high flexibility to the group topology (e.g., possibility to autonomously split or join during the motion). The performance and soundness of the approach is validated by means of human/hardware-in-the-loop simulations (HHIL).},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Distributed algorithms, Force feedback, Motion control of multiple robots, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012d-SecFraBueRob.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012d-SecFraBueRob.mp4},
	author = {Cristian Secchi and Antonio Franchi and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@article {2012e-FraSecSonBueRob,
	title = {Bilateral Teleoperation of Groups of Mobile Robots with Time-Varying Topology},
	journal = {IEEE Transaction on Robotics},
	volume = {28},
	year = {2012},
	month = {10/2012},
	pages = {1019	-1033},
	abstract = {In this paper, a novel decentralized control strategy for bilaterally teleoperating heterogeneous groups of mobile robots from different domains (aerial, ground, marine and under- water) is proposed. By using a decentralized control architecture, the group of robots, treated as the slave-side, is made able to navigate in a cluttered environment while avoiding obstacles, inter-robot collisions and following the human motion commands. Simultaneously, the human operator acting on the master side is provided with a suitable force feedback informative of the group response and of the interaction with the surrounding environment. Using passivity based techniques, we allow the behavior of the group to be as flexible as possible with arbitrary split and join events (e.g., due to inter-robot visibility/packet losses or specific task requirements) while guaranteeing the stability of the system. We provide a rigorous analysis of the system stability and steady-state characteristics, and validate performance through human/hardware-in-the-tloop simulations by considering a heterogeneous fleet of UAVs and UGVs as case study. Finally, we also provide an experimental validation with 4 quadrotor UAV},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Distributed algorithms, Force feedback, Haptics, Motion control of multiple robots, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012e-FraSecSonBueRob.pdf},
	author = {Antonio Franchi and Cristian Secchi and Hyoung Il Son and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2012i-FraRob,
	title = {Decentralized Control of Parallel Rigid Formations with Direction Constraints and Bearing Measurements},
	booktitle = {51th IEEE Conference on Decision and Control },
	year = {2012},
	month = {12/2012},
	pages = {5310-5317},
	address = {Maui, HI},
	abstract = {In this paper we analyze the relationship between scalability, minimality and rigidity, and its application to cooperative control. As a case study, we address the problem of multi-agent formation control by proposing a distributed control strategy that stabilizes a formation described with bearing (direction) constraints, and that only requires bearing measurements and parallel rigidity of the interaction graph. We also consider the possibility of having different graphs modeling the interaction network in order to explicitly take into account the conceptual difference between sensing, communication, control, and parameters stored in the network. We then show how the information can be {\textquoteleft}moved{\textquoteright} from a graph to another making use of decentralized estimation, provided the parallel rigidity property. Finally we present simulative examples in order to show the validity of the theoretical analysis in some illustrative cases.},
	keywords = {Decentralized control, Distributed algorithms, Estimation, Formation control, Motion control of multiple robots, Multi-robot systems},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012i-FraRob-preprint.pdf},
	author = {Antonio Franchi and Paolo Robuffo Giordano}
}
@conference {2012h-RieFraRobBueSon,
	title = {Experiments on Intercontinental Haptic Control of Multiple UAVs},
	booktitle = {12th Int. Conf. on Intelligent Autonomous Systems},
	year = {2012},
	month = {06/2012},
	pages = {227-238},
	address = {Jeju Island, Korea},
	abstract = {In this paper we propose and experimentally validate a bilateral teleoperation framework where a group of UAVs are controlled over an unreliable network with typical intercontinental time delays and packet losses. This setting is meant to represent a realistic and challenging situation for the stability the bilateral closed-loop system.
In order to increase human telepresence, the system provides the operator with both a video stream coming from the onboard cameras mounted on the UAVs, and with a suitable haptic cue, generated by a force-feedback device, informative of the UAV tracking performance and presence of impediments on the remote site.

In addition to the theoretical background, we describe the hardware and software implementation of this intercontinental teleoperation: this is composed of a semi-autonomous group of multiple quadrotor UAVs, a 3-DOF haptic interface, and a network connection based on a VPN tunnel between Germany and South Korea. The whole software framework is based upon the Robotic Operating System (ROS) communication standard.},
	keywords = {Bilateral Shared Control of Mobile Robots, Haptics, Middleware for robotics, Multi-robot systems, Teleoperation, UAV hardware platforms},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012h-RieFraRobBueSon.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012h-RieFraRobBueSon.mp4},
	author = {Martin Riedel and Antonio Franchi and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano and Hyoung Il Son}
}
@conference {2012k-MasFraBueRob,
	title = {Interactive Planning of Persistent Trajectories for Human-Assisted Navigation of Mobile Robots},
	booktitle = {2012 IEEE/RSJ Int. Conf. on Intelligent Robots and Systems},
	year = {2012},
	month = {10/2012},
	pages = {2641-2648},
	address = {Vilamoura, Portugal},
	abstract = {This work extends the framework of bilateral shared control of mobile robots with the aim of increasing the robot autonomy and decreasing the operator commitment. We consider persistent autonomous behaviors where a cyclic motion must be executed by the robot. The human operator is in charge of modifying online some geometric properties of the desired path. This is then autonomously processed by the robot in order to produce an actual path guaranteeing: i) tracking feasibility, ii) collision avoidance with obstacles, iii) closeness to the desired path set by the human operator, and iv) proximity to some points of interest. A force feedback is implemented to inform the human operator of the global deformation of the pathrather than using the classical mismatch between desired and executed motion commands. Physically-based simulations, with human/hardware-in-the-loop and a quadrotor UAV as robotic platform, demonstrate the feasibility of the method. },
	keywords = {Bilateral Shared Control of Mobile Robots, Force feedback, Haptics, Motion Planning, Optimal Trajectory Planning, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012k-MasFraBueRob.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012k-MasFraBueRob.mp4},
	author = {Carlo Masone and Antonio Franchi and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@booklet {2012q-RieFraBueRob,
	title = {Intercontinental Haptic Control and Advanced Supervisory Interfaces for Groups of Multiple UAVs},
	howpublished = {5th Int. Work. on Human-Friendly Robotics},
	year = {2012},
	month = {10/2012},
	address = {Bruxelles, Belgium},
	keywords = {Bilateral Shared Control of Mobile Robots, Middleware for robotics, Motion control of multiple robots, UAV hardware platforms},
	author = {Martin Riedel and Heinrich H. B{\"u}lthoff and Antonio Franchi and Paolo Robuffo Giordano}
}
@article {2012q-FraMasGraRylBueRob,
	title = {Modeling and Control of UAV Bearing-Formations with Bilateral High-Level Steering},
	journal = {The International Journal of Robotics Research, Special Issue on 3D Exploration, Mapping, and Surveillance},
	volume = {31},
	year = {2012},
	month = {10/2012},
	pages = {1504-1525},
	abstract = {In this paper we address the problem of controlling the motion of a group of UAVs bound to keep a formation defined in terms of only relative angles (i.e., a bearing-formation). This problem can naturally arise within the context of several multi-robot applications such as, e.g., exploration, coverage, and surveillance. First, we introduce and thoroughly analyze the concept and properties of bearing-formations, and provide a class of minimally linear sets of bearings sufficient to uniquely define such formations. We then propose a bearing-only formation controller requiring only bearing measurements, converging almost globally, and maintaining bounded inter-agent distances despite the lack of direct metric information. The controller still leaves the possibility to impose group motions tangent to the current bearing-formation. These can be either autonomously chosen by the robots because of any additional task (e.g., exploration), or exploited by an assisting human co-operator. For this latter {\textquoteleft}human-in-the-loop{\textquoteright} case, we propose a multi-master/multi-slave bilateral shared control system providing the co-operator with some suitable force cues informative of the UAV performance. The proposed theoretical framework is extensively validated by means of simulations
and experiments with quadrotor UAVs equipped with onboard cameras. Practical limitations, e.g., limited field-of-view, are also considered.},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Force feedback, Formation control, Haptics, Motion control of multiple robots, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012q-FraMasGraRylBueRob-preprint.pdf},
	author = {Antonio Franchi and Carlo Masone and Volker Grabe and Markus Ryll and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2012g-ZelFraAlgBueRob,
	title = {Rigidity Maintenance Control for Multi-Robot Systems},
	booktitle = {2012 Robotics: Science and Systems Conference},
	year = {2012},
	month = {07/2012},
	address = {Sydney, Australia},
	abstract = {Rigidity of formations in multi-robot systems is important for formation control, localization, and sensor fusion.  This work proposes a rigidity maintenance gradient controller for a multi-agent robot team.  To develop such a controller, we first provide an alternative characterization of the rigidity matrix and use that to introduce the novel concept of the rigidity eigenvalue.  We provide a necessary and sufficient condition relating the positivity of the rigidity eigenvalue to the rigidity of the formation.  The rigidity maintenance controller is based on the gradient of the rigidity eigenvalue with respect to each robot position.  This gradient has a naturally distributed structure, and is thus amenable to a distributed implementation.  Additional requirements such as obstacle and inter-agent collision avoidance, as well as typical constraints such as limited sensing/communication ranges and line-of-sight occlusions, are also explicitly considered.  Finally, we present a simulation with a group of seven quadrotor UAVs to demonstrate and validate the theoretical results.},
	keywords = {Decentralized control, Formation control, Motion control of multiple robots, Multi-robot systems, Rigidity mainenance},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012g-ZelFraAlgBueRob-preprint.pdf},
	author = {Daniel Zelazo and Antonio Franchi and Frank Allg{\"o}wer and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@article {2012f-FraSecRylBueRob,
	title = {Shared Control: Balancing Autonomy and Human Assistance with a Group of Quadrotor UAVs.},
	journal = {IEEE Robotics and Automation Magazine, Special Issue on Aerial Robotics and the Quadrotor Platform},
	volume = {19},
	year = {2012},
	month = {09/2012},
	pages = {57-68},
	abstract = {In this paper, we present a complete control framework and associated experimental testbed for the bilateral shared control of a group of quadrotor UAVs. This control architecture is applicable to any task and allows to integrate: i) a decentralized topological motion control (responsible for the mutual interactions in the UAV formation), ii) a human assistance module (allowing human intervention, whenever needed, on some aspects of the UAV group behavior), and iii) a force-feedback possibility (increasing the telepresence of the human assistants by providing suitable haptic cues informative of the UAV behavior). 
We will show, as worked-out case studies, how to specialize the topological motion controller to the relevant cases of constant, unconstrained and connected group topologies, and how to let a human operator intervening at the level of single UAVs or of the whole group dynamics.
A detailed description of the experimental testbed is also provided with emphasis on the quadrotor UAV hardware and software architectures. Finally, the versatility of the proposed framework is demonstrated by means of experiments with real UAVs. Although quadrotors are used as actual robotic platforms, the proposed framework can be straightforwardly extended to many different kinds of UAVs with similar motion characteristics.},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Force feedback, Formation control, Haptics, Middleware for robotics, Motion control of multiple robots, Multi-robot systems, UAV hardware platforms},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012f-FraSecRylBueRob-preprint.pdf},
	author = {Antonio Franchi and Cristian Secchi and Markus Ryll and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@inbook {2012m-LaeFraBueRob,
	title = {SwarmSimX: Real-time Simulation Environment for Multi-robot Systems},
	booktitle = {3rd Int. Conf. on Simulation, Modeling, and Programming for Autonomous Robots},
	year = {2012},
	month = {11/2012},
	address = {Tsukuba, Japan},
	abstract = {In this paper we present a novel simulation environment called SwarmSimX with the ability to simulate dozens of robots in a realistic 3D environment. The software architecture of SwarmSimX allows new robots, sensors, and other libraries to be loaded at runtime, extending the functionality of the simulation environment significantly. In addition, SwarmSimX allows an easy exchange of the underlying libraries used for the visual and physical simulation to incorporate different libraries (e.g., improved or future versions). A major feature is also the possibility to perform the whole simulation in real-time allowing for human-in-the-loop or hardware-in-the-loop scenarios. SwarmSimX has been already employed in several works presenting haptic shared control of multiple mobile robots (e.g., quadrotor UAVs). Additionally, we present here two validation tests showing the physical delity and the real-time performance of SwarmSimX. For the tests we used NVIDIA PhysX and Ogre3D as physics and rendering libraries, respectively},
	keywords = {Multi-robot systems, Simulators for robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2012m-LaeFraBueRob-preprint.pdf},
	author = {Johannes L{\"a}chele and Antonio Franchi and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2011e-RobFraSecBue,
	title = {Bilateral Teleoperation of Groups of UAVs with Decentralized Connectivity Maintenance},
	booktitle = {2011 Robotics: Science and Systems Conference},
	year = {2011},
	month = {06/2011},
	address = {Los Angeles, CA},
	abstract = {In this paper, we present a decentralized passivity-based control strategy for the bilateral teleoperation of a fleet of Unmanned Aerial Vehicles (UAVs). The human operator at the master side can command the fleet motion and receive suitable force cues informative about the remote environment. By properly controlling the energy exchanged within the slave side (the UAV fleet), we guarantee that the connectivity of the fleet is preserved and we prevent inter-agent and obstacle collisions. At the same time, we allow the behavior of the UAVs to be as flexible as possible with arbitrary split and join maneuvers. The results of the paper are validated through semi-experiments.},
	keywords = {Bilateral Shared Control of Mobile Robots, Connectivity maintenance, Decentralized control, Distributed algorithms, Estimation, Force feedback, Haptics, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011e-RobFraSecBue-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011e-RobFraSecBue-almost_preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011e-RobFraSecBue.mp4},
	author = {Paolo Robuffo Giordano and Antonio Franchi and Cristian Secchi and Heinrich H. B{\"u}lthoff}
}
@conference {2011i-FraMasBueRob,
	title = {Bilateral Teleoperation of Multiple UAVs with Decentralized Bearing-only Formation Control},
	booktitle = {2011 IEEE/RSJ Int. Conf. on Intelligent Robots and Systems},
	year = {2011},
	month = {09/2011},
	pages = {2215-2222},
	address = {San Francisco, CA},
	abstract = {We present a decentralized system for the bilateral teleoperation of groups of UAVs which only relies
on relative bearing measurements, i.e., without the need of distance information or global localization. The properties of a 3D bearing-formation are analyzed, and a minimal set of bearings needed for its definition is provided. We also design a novel decentralized formation control almost globally convergent and able to maintain bounded and non-vanishing inter-distances among the agents despite the absence of direct distance measurements. Furthermore, we develop a multimaster/multi-slave teleoperation setup in order to control the overall behavior of the group and to convey to the human operator suitable force cues, while ensuring stability in presence of delays and packet losses over the master-slave communication channel. The theoretical framework is validated by means of extensive human/hardware in-the-loop simulations using two force-feedback devices and a group of quadrotors.},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Force feedback, Formation control, Haptics, Motion control of multiple robots, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011i-FraMasBueRob-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011i-FraMasBueRob.mp4},
	author = {Antonio Franchi and Carlo Masone and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2011g-FraBueRob,
	title = {Distributed Online Leader Selection in the Bilateral Teleoperation of Multiple UAVs},
	booktitle = {50th IEEE Conference on Decision and Control },
	year = {2011},
	month = {12/2011},
	pages = {3559-3565},
	address = {Orlando, FL},
	abstract = {For several applications like data collection, surveillance, search and rescue and exploration of wide areas, the use of a group of simple robots rather than a single complex robot has proven to be very effective and promising, and the problem of coordinating a group of agents has received a lot of attention over the last years. In this paper, we consider the challenge of establishing a bilateral force-feedback teleoperation channel between a human operator (the master side) and a remote multi-robot system (the slave side) where a special agent, the leader, is selected and directly controlled by the master. In particular, we study the problem of distributed online optimal leader selection, i.e., how to choose, and possibly change, the leader online in order to maximize some suitable criteria related to the tracking performance of the whole group w.r.t. the master commands. Human/hardware-in-the-loop simulation results with a group of UAVs support the theoretical claims of the paper.},
	keywords = {Decentralized control, Distributed algorithms, Force feedback, Haptics, Leader selection, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011g-FraBueRob-preprint.pdf},
	author = {Antonio Franchi and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2011d-SonKimChuFraRobLeeBue,
	title = {An Evaluation of Haptic Cues on the Tele-Operator{\textquoteright}s Perceptual  Awareness of Multiple UAVs{\textquoteright} Environments},
	booktitle = { IEEE  World Haptics Conference},
	year = {2011},
	month = {06/2011},
	pages = {149-154},
	address = {Istanbul, Turkey},
	abstract = {The use of multiple unmanned aerial vehicles (UAVs) is increasingly being incorporated into a wide range of teleoperation applications. To date, relevant research has largely been focused on the development of appropriate control schemes. In this paper, we extend previous research by investigating how control performance could be improved by providing the teleoperator with haptic feedback cues. First, we describe a control scheme that allows a teleoperator to manipulate the flight of multiple UAVs in a remote environment. Next, we present three designs of haptic cue feedback that could increase the teleoperator{\textquoteright}s environmental awareness of such a remote environment. These cues are based on the UAVs{\textquoteright} i) velocity information, ii) proximity to obstacles, and iii) a combination of these two sources of information. Finally, we present an experimental evaluation of these haptic cue designs. Our evaluation is based on the teleoperator{\textquoteright}s perceptual sensitivity to the physical environment inhabited by the multiple UAVs. We conclude that a teleoperator{\textquoteright}s perceptual sensitivity is best served by haptic feedback cues that are based on the velocity information of multiple UAVs.},
	keywords = {Evaluation, Force feedback, Haptics, Multi-robot systems, Psychophysics, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011d-SonKimChuFraRobLeeBue-preprint.pdf},
	author = {Hyoung Il Son and Junsuk Kim and Lewis L. Chuang and Antonio Franchi and Paolo Robuffo Giordano and Dongjun Lee and Heinrich H. B{\"u}lthoff}
}
@conference {2011k-RobFraSecBue,
	title = {Experiments of Passivity-Based Bilateral Aerial Teleoperation of a Group of UAVs with Decentralized Velocity Synchronization},
	booktitle = {2011 IEEE/RSJ Int. Conf. on Intelligent Robots and Systems},
	year = {2011},
	month = {09/2011},
	pages = {163-170},
	address = {San Francisco, CA},
	abstract = {In this paper, we present an experimental validation of a novel decentralized passivity-based control strategy for teleoperating a group of Unmanned Aerial Vehicles (UAVs): the slave side, consisting of the UAVs, is endowed with large group autonomy by allowing time-varying topology and interrobot/obstacle collision avoidance. The master side, represented by a human operator, controls the group motion and receives suitable force feedback cues informing her/him about the remote slave motion status. Passivity theory is exploited for guaranteeing stability of the slave side and of the overall teleoperation channel. Results of experiments involving the use of 4 quadcopters are reported and discussed, confirming the soundness of the paper theoretical claims.},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Distributed algorithms, Force feedback, Haptics, Motion control of multiple robots, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011k-RobFraSecBue-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011k-RobFraSecBue.mp4},
	author = {Paolo Robuffo Giordano and Antonio Franchi and Cristian Secchi and Heinrich H. B{\"u}lthoff}
}
@conference {2011b-LeeFraRobSonBue,
	title = {Haptic Teleoperation of Multiple Unmanned Aerial Vehicles over the Internet},
	booktitle = { 2011 IEEE Int. Conf. on Robotics and Automation},
	year = {2011},
	month = {05/2011},
	pages = {1341-1347},
	address = {Shanghai, China},
	abstract = {We propose a novel haptic teleoperation control framework for multiple unmanned aerial vehicles (UAVs) over the Internet, consisting of the three control layers: 1) UAV control layer, where each UAV is abstracted by, and is controlled to follow the trajectory of, its own kinematic virtual point (VP); 2) VP control layer, which modulates each VP{\textquoteright}s motion according to the teleoperation commands and local artificial potentials (for inter-VP/VP-obstacle collision avoidance and inter-VP connectivity preservation); and 3) teleoperation layer, through which a remote human user can command all (or some) of the VPs{\textquoteright} velocity while haptically perceiving the state of all (or some) of the UAVs over the Internet.
Master-passivity/slave-stability and some asymptotic performance measures are proved. Semi-experiment results are presented to validate the theory.},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Force feedback, Formation control, Haptics, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011b-LeeFraRobSonBue-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011b-LeeFraRobSonBue-low_qlty.mp4},
	author = {Dongjun Lee and Antonio Franchi and Paolo Robuffo Giordano and Hyoung Il Son and Heinrich H. B{\"u}lthoff}
}
@conference {2011h-SonChuFraKimLeeLeeBueRob,
	title = {Measuring an Operator{\textquoteright}s Maneuverability Performance in the Haptic Teleoperation of Multiple Robots},
	booktitle = {2011 IEEE/RSJ Int. Conf. on Intelligent Robots and Systems},
	year = {2011},
	month = {09/2011},
	pages = {3039-3046},
	address = {San Francisco, CA},
	abstract = {In this paper, we investigate the maneuverability performance of human teleoperators on multi-robots. First, we propose that maneuverability performance can be assessed by a frequency response function that jointly considers the input force of the operator and the position errors of the multi-robot system that is being maneuvered. Doing so allows us to evaluate maneuverability performance in terms of the human teleoperator{\textquoteright}s interaction with the controlled system. This allowed us to effectively determine the suitability of different haptic cue algorithms in improving teleoperation maneuverability. Performance metrics based on the human teleoperator{\textquoteright}s frequency response function indicate that maneuverability performance is best supported by a haptic feedback algorithm which is based on an obstacle avoidance force.},
	keywords = {Evaluation, Force feedback, Haptics, Multi-robot systems, Psychophysics, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011h-SonChuFraKimLeeLeeBueRob-preprint.pdf},
	author = {Hyoung Il Son and Lewis L. Chuang and Antonio Franchi and Junsuk Kim and Dongjun Lee and Seong-Whan Lee and Heinrich H. B{\"u}lthoff and Paolo Robuffo Giordano}
}
@conference {2011a-FraRobSecSonBue,
	title = {A Passivity-Based Decentralized Approach for the Bilateral Teleoperation of a Group of UAVs with Switching Topology},
	booktitle = {2011 IEEE Int. Conf. on Robotics and Automation},
	year = {2011},
	month = {05/2011},
	pages = {898-905},
	address = {Shanghai, China},
	abstract = {In this paper, a novel distributed control strategy for teleoperating a fleet of Unmanned Aerial Vehicles (UAVs) is proposed. Using passivity based techniques, we allow the behavior of the UAVs to be as flexible as possible with arbitrary split and join decisions while guaranteeing stability of the system. Furthermore, the overall teleoperation system is also made passive and, therefore, characterized by a stable behavior both in free motion and when interacting with unknown {passive} obstacles. The performance of the system is validated through semi-experiments.},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Distributed algorithms, Force feedback, Haptics, Motion control of multiple robots, Multi-robot systems, Teleoperation},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011a-FraRobSecSonBue-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2011a-FraRobSecSonBue-low_qlty.mp4},
	author = {Antonio Franchi and Paolo Robuffo Giordano and Cristian Secchi and Hyoung Il Son and Heinrich H. B{\"u}lthoff}
}
@booklet {2010d-RobFraSonSecLeeBue,
	title = {Towards Bilateral Teleoperation of Multi-Robot Systems},
	howpublished = {3rd Int. Work. on Human-Friendly Robotics},
	year = {2010},
	month = {10/2010},
	address = {Tuebingen, Germany},
	keywords = {Bilateral Shared Control of Mobile Robots, Decentralized control, Distributed algorithms, Force feedback, Haptics, Multi-robot systems, Teleoperation},
	author = {Paolo Robuffo Giordano and Antonio Franchi and Hyoung Il Son and Cristian Secchi and Dongjun Lee and Heinrich H. B{\"u}lthoff}
}
