| 1. |
- Gurholt, Tiril P., et al.
(författare)
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Intracranial and subcortical volumes in adolescents with early‐onset psychosis : A multisite mega‐analysis from the ENIGMA consortium
- 2020
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Ingår i: Human Brain Mapping. - Stockholm : Wiley. - 1065-9471 .- 1097-0193. ; 43:1, s. 373-384
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Tidskriftsartikel (refereegranskat)abstract
- Early-onset psychosis disorders are serious mental disorders arising before the age of 18 years. Here, we investigate the largest neuroimaging dataset, to date, of patients with early-onset psychosis and healthy controls for differences in intracranial and subcortical brain volumes. The sample included 263 patients with early-onset psychosis (mean age: 16.4 ± 1.4 years, mean illness duration: 1.5 ± 1.4 years, 39.2% female) and 359 healthy controls (mean age: 15.9 ± 1.7 years, 45.4% female) with magnetic resonance imaging data, pooled from 11 clinical cohorts. Patients were diagnosed with early-onset schizophrenia (n = 183), affective psychosis (n = 39), or other psychotic disorders (n = 41). We used linear mixed-effects models to investigate differences in intracranial and subcortical volumes across the patient sample, diagnostic subgroup and antipsychotic medication, relative to controls. We observed significantly lower intracranial (Cohen's d = −0.39) and hippocampal (d = −0.25) volumes, and higher caudate (d = 0.25) and pallidum (d = 0.24) volumes in patients relative to controls. Intracranial volume was lower in both early-onset schizophrenia (d = −0.34) and affective psychosis (d = −0.42), and early-onset schizophrenia showed lower hippocampal (d = −0.24) and higher pallidum (d = 0.29) volumes. Patients who were currently treated with antipsychotic medication (n = 193) had significantly lower intracranial volume (d = −0.42). The findings demonstrate a similar pattern of brain alterations in early-onset psychosis as previously reported in adult psychosis, but with notably low intracranial volume. The low intracranial volume suggests disrupted neurodevelopment in adolescent early-onset psychosis.
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| 2. |
- Schillinger, P., et al.
(författare)
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Adaptive heterogeneous multi-robot collaboration from formal task specifications
- 2021
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Ingår i: Robotics and Autonomous Systems. - : Elsevier BV. - 0921-8890 .- 1872-793X. ; 145
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Tidskriftsartikel (refereegranskat)abstract
- Efficiently coordinating different types of robots is an important enabler for many commercial and industrial automation tasks. Here, we present a distributed framework that enables a team of heterogeneous robots to dynamically generate actions from a common, user-defined goal specification. In particular, we discuss the integration of various robotic capabilities into a common task allocation and planning formalism, as well as the specification of expressive, temporally-extended goals by non -expert users. Models for task allocation and execution both consider non-deterministic outcomes of actions and thus, are suitable for a wide range of real-world tasks including formally specified reactions to online observations. One main focus of our paper is to evaluate the framework and its integration of software modules through a number of experiments. These experiments comprise industry-inspired scenarios as motivated by future real-world applications. Finally, we discuss the results and learnings for motivating practically relevant, future research questions. (C) 2021 Elsevier B.V. All rights reserved.
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| 3. |
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| 4. |
- Bechlioulis, C. P., et al.
(författare)
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Robust control of large vehicular platoons with prescribed transient and steady state performance
- 2014
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Ingår i: Proceedings of the IEEE Conference on Decision and Control. - : IEEE conference proceedings. ; , s. 3689-3694
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Konferensbidrag (refereegranskat)abstract
- In this paper, we consider the longitudinal control problem for a platoon of vehicles with unknown nonlinear dynamics. More specifically, we design a decentralized model-free control protocol in the sense that each vehicle utilizes only local relative information regarding its front vehicle, obtained by its on-board sensors, to calculate its own control signal, without incorporating any prior knowledge of the model nonlinearities/disturbances or any approximation structures to acquire such knowledge. Additionally, the transient and steady state response is a priori determined by certain designer-specified performance functions and is fully decoupled by the agents' dynamic model, the number of vehicles composing the platoon and the control gains selection, which relaxes significantly the control design procedure. Moreover, introducing certain inter-vehicular distance constraints during the transient response shaping, collisions between successive vehicles as well as connectivity breaks owing to limited sensing capabilities are provably avoided. Finally, the proposed methodology results in a low complexity design. Actually, it is a static scheme involving very few and simple calculations to output the control signal, thus making its distributed implementation straightforward.
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| 5. |
- Bechlioulis, C. P., et al.
(författare)
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Sensor-based motion control of autonomous underwater vehicles, part II : Robust motion control strategies
- 2020
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Ingår i: Autonomous Underwater Vehicles. - : Institution of Engineering and Technology (IET). ; , s. 45-78
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The first section of this chapter presents an NMPC strategy for underwater robotic vehicles operating under various constraints. The purpose of the controller is to guide the vehicle towards specific way -points. Various constraints such as obstacles, workspace boundaries and control input saturation as well as predefined upper bound of the vehicle velocity (requirements for several underwater tasks such as seabed inspection scenario and mosaicking) are considered during the control design. The proposed scheme incorporates the full dynamics of the vehicle in which the ocean currents are also involved. The controller is designed in order to find the optimal thrusts required for minimizing the way -point tracking error. Moreover, the controlinputs calculated by the proposed approach are formulated in a way that the vehicle will exploit the ocean currents, when they are in favor of the way -point tracking mission, which results in reduced energy consumption by the thrusters. In the second part of this chapter, novel position- and trajectory -tracking control schemes for AUVs are presented. The proposed controllers do not utilize the vehicle’s dynamic model parameters and guarantee prescribed transient and steady-state performance despite the presence of external disturbances and kinematic constraints for the case of underactuated vehicles. Moreover, through the appropriate selection of certain performance functions, the proposed scheme can also guarantee the satisfaction of motion and performance constraints imposed by the desired task.
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| 6. |
- Dimarogonas, Dimos V., et al.
(författare)
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A connection between formation infeasibility and velocity alignment in kinematic multi-agent systems
- 2008
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 44:10, s. 2648-2654
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a feedback control strategy that achieves convergence of a multi-agent system to a desired formation configuration is proposed for both the cases of agents with single integrator and nonholonomic unicycle-type kinematics. When inter-agent objectives that specify the desired formation cannot occur simultaneously in the state space the desired formation is infeasible. it is shown that under certain assumptions, formation infeasibility forces the agents' velocity vectors to a common value at steady state. This provides a connection between formation infeasibility and flocking behavior for the multi-agent system. We finally also obtain an analytic expression of the common velocity vector in the case of formation infeasibility.
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| 7. |
- Dimarogonas, Dimos V., et al.
(författare)
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Connectedness Preserving Distributed Swarm Aggregation for Multiple Kinematic Robots
- 2008
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Ingår i: IEEE Transactions on robotics. - : Institute of Electrical and Electronics Engineers (IEEE). - 1552-3098 .- 1941-0468. ; 24:5, s. 1213-1223
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Tidskriftsartikel (refereegranskat)abstract
- A distributed swarm aggregation algorithm is developed for a team of multiple kinematic agents. Specifically, each agent is assigned a control law, which is the sum of two elements: a repulsive potential field, which is responsible for the collision avoidance objective, and an attractive potential field, which forces the agents to converge to a configuration where they are close to each other. Furthermore, the attractive potential field forces the agents that are initially located within the sensing radius of an agent to remain within this area for all time. In this way, the connectivity properties of the initially formed communication graph are rendered invariant for the trajectories of the closed-loop system. It is shown that under the proposed control law, agents converge to a configuration where each agent is located at a bounded distance from each of its neighbors. The results are also extended to the case of nonholonomic kinematic unicycle-type agents and to the case of dynamic edge addition. In the latter case, we derive a smaller bound in the swarm size than in the static case.
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| 8. |
- Dimarogonas, Dimos V., et al.
(författare)
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Inverse Agreement Protocols With Application to Distributed Multi-Agent Dispersion
- 2009
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Ingår i: IEEE Transactions on Automatic Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9286 .- 1558-2523. ; 54:3, s. 657-663
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Tidskriftsartikel (refereegranskat)abstract
- We propose a distributed inverse agreement control law for multiple kinematic agents that forces the team members to disperse In the workspace. Both the cases of an unbounded and a circular, bounded workspace are considered. In the first case, we show that the closed-loop system reaches a configuration In which the minimum distance between any pair of agents is larger than a specific lower bound. It is proved that this lower bound coincides with the agents' sensing radius. In the case of a bounded circular workspace, the control law is redefined to force the agents to remain within the workspace boundary. Moreover the proposed control design guarantees collision avoidance between the team members in all cases. The results are supported through relevant computer simulations.
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| 9. |
- Dimarogonas, Dimos V., et al.
(författare)
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Leader-follower cooperative attitude control of multiple rigid bodies
- 2009
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Ingår i: Systems & control letters (Print). - : Elsevier BV. - 0167-6911 .- 1872-7956. ; 58:6, s. 429-435
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we extend our previous results on coordinated control of rotating rigid bodies to the case of teams with heterogeneous agents. We assume that only a certain subgroup of the agents (the leaders) is vested with the main control objective, that is, maintain constant relative orientation amongst themselves. The other members of the team must meet relaxed control specifications, namely, maintain their respective orientations within certain bounds, dictated by the orientation of the leaders. The proposed control laws respect the limited information each rigid body has with respect to the rest of its peers (leaders or followers), as well as with the rest of the team. Each rigid body is equipped with a feedback control law that utilizes the Laplacian matrix of the associated communication graph, and which encodes the limited communication capabilities between the team members. Similarly to the single integrator case, the convergence of the system relies on the connectivity of the communication graph.
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| 10. |
- Eqtami, A., et al.
(författare)
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Aperiodic model predictive control via perturbation analysis
- 2012
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Ingår i: Decision and Control (CDC), 2012 IEEE 51st Annual Conference on. - : IEEE. ; , s. 7193-7198
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Konferensbidrag (refereegranskat)abstract
- In this paper, an enhanced event-based scheme for model predictive control (MPC) of constrained discrete-time systems with additive disturbances is investigated. The re/calculation of the MPC control law is triggered whenever an event depending on the error of the measured state with respect to the nominal state of the system occurs. Between the controller updates, the last computed control trajectory is applied to the system, in conjunction with a correction term. This term consists of a perturbation solution of the nominal system which itself depends on the aforementioned error. The overall framework yields less conservative results with respect to our previous work. The results are illustrated through a simulated example.
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| 11. |
- Eqtami, A., et al.
(författare)
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Event-based model Predictive control for the cooperation of distributed agents
- 2012
-
Ingår i: 2012 American Control Conference (ACC). - : IEEE Computer Society. - 9781457710957 ; , s. 6473-6478
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Konferensbidrag (refereegranskat)abstract
- This paper proposes an event-based framework for the control of a team of cooperating distributed agents. The agents are dynamically decoupled and they are controlled locally by Nonlinear Model Predictive Controllers (NMPC). The event-driven framework allows for triggering the solution of the optimal control problem of the NMPC only when it is needed. The scheduling of the control updates for each of the agents depends on an error of the state information received from the neighboring agents. Sufficient conditions for triggering are provided and the results are illustrated through a simulated example.
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| 12. |
- Eqtami, A., et al.
(författare)
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Self-triggered model predictive control for nonholonomic systems
- 2013
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Ingår i: 2013 European Control Conference, ECC 2013. - : IEEE. - 9783033039629 ; , s. 638-643
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Konferensbidrag (refereegranskat)abstract
- This paper proposes a Model Predictive Control (MPC) framework combined with a self-triggering mechanism for constrained uncertain systems. Under the proposed scheme, the control input as well as the next control update time are provided at each triggering instant. Between two consecutive triggering instants, the control trajectory given by the MPC is applied to the plant in an open-loop fashion. This results to less frequent computations while preserving stability and convergence of the closed-loop system. A scenario for the stabilization of a nonholonomic robot subject to constraints and disturbances is considered, with the aim of reaching a specific triggering mechanism. The robot under the proposed control framework is driven to a compact set where it is ultimately bounded. The efficiency of the proposed approach is illustrated through a simulated example.
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| 13. |
- Eqtami, A., et al.
(författare)
-
Self-triggered model predictive control framework for the cooperation of distributed nonholonomic agents
- 2013
-
Ingår i: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC). - : IEEE conference proceedings. - 9781467357173 ; , s. 7384-7389
-
Konferensbidrag (refereegranskat)abstract
- In this paper we propose a decentralized Model Predictive Control (MPC) framework with a self-triggering mechanism, for a team of cooperating agents. The nonholonomic agents are controlled locally and exchange information with their neighbors. The aim at scheduling the control updates based on a self-triggering criterion is twofold: To reduce the updates of the control law for each agent and to reduce the communication effort between the agents. The input-to-state (abbr. ISS) stability of the agents is proven, the condition for triggering is provided and the theoretic results are then depicted by a simulated example.
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| 14. |
- Filippidis, I., et al.
(författare)
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Decentralized multi-agent control from local LTL specifications
- 2012
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Ingår i: Decision and Control (CDC), 2012 IEEE 51st Annual Conference on. - : IEEE. ; , s. 6235-6240
-
Konferensbidrag (refereegranskat)abstract
- We propose a methodology for decentralized multi-agent control from Linear Temporal Logic (LTL) specifications. Each agent receives an independent specification to formally synthesize its own hybrid controller. Mutual satisfiability is not a priori guaranteed. Due to limited communication, the agents utilize meeting events to exchange their controller automata and verify satisfiability through model checking. Local interaction only when common atomic propositions exist reduces the overall computational cost, facilitating scalability. Provably correct collision avoidance and convergence is ensured by Decentralized Multi-Agent Navigation Functions.
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| 15. |
- Heshmati-alamdari, S., et al.
(författare)
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A Robust Force Control Approach for Underwater Vehicle Manipulator Systems
- 2017
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Ingår i: IFAC-PapersOnLine. - : Elsevier. - 2405-8963. ; 50:1, s. 11197-11202
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Tidskriftsartikel (refereegranskat)abstract
- In various interaction tasks using Underwater Vehicle Manipulator Systems (UVMSs) (e.g. sampling of the sea organisms, underwater welding), important factors such as: i) uncertainties and complexity of UVMS dynamic model ii) external disturbances (e.g. sea currents and waves) iii) imperfection and noises of measuring sensors iv) steady state performance as well as v) inferior overshoot of interaction force error, should be addressed during the force control design. Motivated by the above factors, this paper presents a model-free control protocol for force controlling of an Underwater Vehicle Manipulator System which is in contact with an unknown compliant environment, without incorporating any knowledge of the UVMS's dynamic model, exogenous disturbances and sensor's noise model. Moreover, the transient and steady state response as well as reduction of overshooting force error are solely determined by certain designer-specified performance functions and are fully decoupled by the UVMS's dynamic model, the control gain selection, as well as the initial conditions. Finally, a simulation study clarifies the proposed method and verifies its efficiency.
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| 16. |
- Heshmati-Alamdari, S., et al.
(författare)
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A self-triggered visual servoing model predictive control scheme for under-actuated underwater robotic vehicles
- 2014
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Ingår i: Proceedings - IEEE International Conference on Robotics and Automation. - : IEEE conference proceedings. ; , s. 3826-3831
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Konferensbidrag (refereegranskat)abstract
- This paper presents a novel Vision-based Nonlinear Model Predictive Control (NMPC) scheme for an under-actuated underwater robotic vehicle. In this scheme, the control loop does not close periodically, but instead a self-triggering framework decides when to provide the next control update. Between two consecutive triggering instants, the control sequence computed by the NMPC is applied to the system in an open-loop fashion, i.e, no state measurements are required during that period. This results to a significant smaller number of requested measurements from the vision system, as well as less frequent computations of the control law, reducing in that way the processing time and the energy consumption. The image constraints (i.e preserving the target inside the camera's field of view), the external disturbances induced by currents and waves, as well as the vehicle's kinematic constraints due to under-actuation, are being considered during the control design. The closed-loop system has analytically guaranteed stability and convergence properties, while the performance of the proposed control scheme is experimentally verified using a small under-actuated underwater vehicle in a test tank.
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| 17. |
- Karras, G. C., et al.
(författare)
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Sensor-based motion control of autonomous underwater vehicles, part I : Modeling and low-complexity state estimation
- 2020
-
Ingår i: Autonomous Underwater Vehicles. - : Institution of Engineering and Technology. ; , s. 15-43
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The goal of this chapter is to provide the reader with the appropriate analytical methods, in order to derive a simple yet accurate vehicle model, design a state estimation algorithm that could be easily integrated into the embedded system framework of an underwater robotic vehicle and perform a fast online dynamic parameter identification.
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| 18. |
- Kyriakopoulos, K J, et al.
(författare)
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European robotic projects
- 2005
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Ingår i: IEEE robotics & automation magazine. - : Institute of Electrical and Electronics Engineers (IEEE). - 1070-9932 .- 1558-223X. ; 12:2, s. 4-4
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 19. |
- Maniatopoulos, S., et al.
(författare)
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A decentralized event-based predictive navigation scheme for Air-Traffic Control
- 2012
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Ingår i: 2012 American Control Conference (ACC). - : IEEE Computer Society. - 9781457710957 ; , s. 2503-2508
-
Konferensbidrag (refereegranskat)abstract
- This paper presents an approach for the multiagent navigation and conflict resolution problem, that considers the issue of performance. We present a decentralized predictive navigation scheme that combines the Decentralized Navigation Functions methodology with the Model Predictive Control (MPC) framework while preserving the former's collision avoidance and convergence guarantees. Aircrafts, flying at constant altitude, are modeled as unicycles. Performance criteria are encoded in a cost functional. Due to decentralization, each agent does not take into account the decisions of others in the control law calculation, resulting in performance discrepancies. Therefore, we employ event-triggered executions in our scheme. The improved performance is demonstrated through simulations.
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| 20. |
- Roussos, G. P., et al.
(författare)
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3D navigation and collision avoidance for a non-holonomic vehicle
- 2008
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Ingår i: Proceedings of the American Control Conference. ; , s. 3512-3517
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Konferensbidrag (refereegranskat)abstract
- This paper expands the methodology of Navigation Functions for the control of a spherical aircraft-like 3-dimensional nonholonomic vehicle. A Dipolar Navigation Function is used to generate a feasible, non-holonomic trajectory for the vehicle that leads from an arbitrary position to the target, in combination with a discontinuous feedback control law that steers the vehicle. The motion model used incorporates the nonholonomic constraints imposed on an aircraft, preventing any movement along the lateral or perpendicular axis, as well as preventing high yaw rotation rates. The control strategy provides guaranteed collision avoidance and convergence, and is supported by non-trivial simulation results.
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