| 1. |
- Majchrzak, P., et al.
(författare)
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Switching of the electron-phonon interaction in 1T-VSe2 assisted by hot carriers
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:24
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Tidskriftsartikel (refereegranskat)abstract
- We apply an intense infrared laser pulse in order to perturb the electronic and vibrational states in the three-dimensional charge density wave material 1T-VSe2. Ultrafast snapshots of the light-induced hot carrier dynamics and nonequilibrium quasiparticle spectral function are collected using time- and angle-resolved photoemission spectroscopy. The hot carrier temperature and time-dependent electronic self-energy are extracted from the time-dependent spectral function, revealing that incoherent electron-phonon interactions heat the lattice above the charge density wave critical temperature on a timescale of (200±40) fs. Density functional perturbation theory calculations establish that the presence of hot carriers alters the overall phonon dispersion and quenches efficient low-energy acoustic phonon scattering channels, which results in a new quasiequilibrium state that is experimentally observed.
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| 2. |
- 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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| 3. |
- 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
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Ingår i: Autonomous Underwater Vehicles. - : Institution of Engineering and Technology. ; , s. 15-43
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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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| 4. |
- Bechlioulis, Charalampos P., et al.
(författare)
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Robust Image-Based Visual Servoing With Prescribed Performance Under Field of View Constraints
- 2019
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Ingår i: IEEE Transactions on robotics. - : Institute of Electrical and Electronics Engineers (IEEE). - 1552-3098 .- 1941-0468. ; 35:4, s. 1063-1070
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we propose a visual servoing scheme that imposes predefined performance specifications on the image feature coordinate errors and satisfies the visibility constraints that inherently arise owing to the camera's limited field of view, despite the inevitable calibration and depth measurement errors. Its efficiency is demonstrated via comparative experimental and simulation studies.
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| 5. |
- Biswas, Deepnarayan, et al.
(författare)
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Ultrafast Triggering of Insulator-Metal Transition in Two-Dimensional VSe2
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:5, s. 1968-1975
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Tidskriftsartikel (refereegranskat)abstract
- The transition-metal dichalcogenide VSe2 exhibits an increased charge density wave transition temperature and an emerging insulating phase when thinned to a single layer. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these phases in single-layer VSe2 using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap, which we disentangle from the ensuing hot carrier dynamics by fitting a model spectral function to the time-dependent photoemission intensity. This procedure leads to an estimated time scale of 480 fs for the closure of the gap, which suggests that the phase transition in single-layer VSe2 is driven by electron-lattice interactions rather than by Mott-like electronic effects. The ultrafast optical switching of these interactions in SL VSe2 demonstrates the potential for controlling phase transitions in 2D materials with light.
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| 6. |
- Fahrleitner-Pammer, A., et al.
(författare)
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Fracture rate and back pain during and after discontinuation of teriparatide : 36-month data from the European Forsteo Observational Study (EFOS)
- 2011
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Ingår i: Osteoporosis International. - : Springer Science and Business Media LLC. - 0937-941X .- 1433-2965. ; 22:10, s. 2709-2719
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Tidskriftsartikel (refereegranskat)abstract
- Summary: In this observational study in postmenopausal women with severe osteoporosis, the incidence of fractures was decreased during 18 months of teriparatide treatment with no evidence of further change in the subsequent 18-month post-teriparatide period when most patients took other osteoporosis medications. Fracture reduction was accompanied by reductions in back pain. Introduction: To describe fracture outcomes and back pain in postmenopausal women with severe osteoporosis during 18 months of teriparatide treatment and 18 months post-teriparatide in normal clinical practice. Methods: The European Forsteo Observational Study (EFOS) was a prospective, multinational, observational study. Data on incident clinical fractures and back pain (100 mm Visual Analogue Scale [VAS] and questionnaire) were collected. Fracture data were summarised in 6-month intervals and analysed using logistic regression with repeated measures. Changes from baseline in back pain VAS were analysed using a repeated measures model. Results: A total of 208 (13.2%) of 1,576 patients sustained 258 fractures during 36 months of follow-up: 34% were clinical vertebral fractures and 66% non-vertebral fractures. The adjusted odds of fracture were reduced during teriparatide treatment and there was no evidence of further change in the 18-month post-teriparatide period, during which 63.3% patients took bisphosphonates. A 74% decrease in the adjusted odds of fracture in the 30- to < 36-month period compared with the first 6-month period was observed (p < 0.001). Back pain decreased during teriparatide treatment and this decrease was sustained after teriparatide discontinuation. Adjusted mean back pain VAS decreased by 26.3 mm after 36 months (p < 0.001) from baseline mean of 57.8 mm. Conclusions: In a real-life clinical setting, the risk of fracture decreased during teriparatide treatment, with no evidence of further change after teriparatide was discontinued. The changes in back pain seen during treatment were maintained for at least 18 months after teriparatide discontinuation. These results should be interpreted in the context of the design of an observational study.
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| 7. |
- Heshmati-Alamdari, Shahab, et al.
(författare)
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A robust interaction control approach for underwater vehicle manipulator systems
- 2018
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Ingår i: Annual Reviews in Control. - : Elsevier. - 1367-5788 .- 1872-9088. ; 46, s. 315-325
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Forskningsöversikt (refereegranskat)abstract
- In underwater robotic interaction tasks (e.g., sampling of sea organisms, underwater welding, panel handling, etc) various issues regarding the uncertainties and complexity of the robot dynamic model, the external disturbances (e.g., sea currents), the steady state performance as well as the overshooting/undershooting of the interaction force error, should be addressed during the control design. Motivated by the aforementioned considerations, this paper presents a force/position tracking control protocol for an Underwater Vehicle Manipulator System (UVMS) in compliant contact with a planar surface, without incorporating any knowledge of the UVMS dynamic model, the exogenous disturbances or the contact stiffness model. Moreover, the proposed control framework guarantees: (i) certain predefined minimum speed of response, maximum steady state error as well as overshoot/undershoot concerning the force/position tracking errors, (ii) contact maintenance and (iii) bounded closed loop signals. Additionally, the achieved transient and steady state performance is solely determined by certain designer-specified performance functions/parameters and is fully decoupled from the control gain selection and the initial conditions. Finally, both simulation and experimental studies clarify the proposed method and verify its efficiency.
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| 8. |
- Heshmati Alamdari, Shahabodin, et al.
(författare)
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A Robust Predictive Control Approach for Underwater Robotic Vehicles
- 2020
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Ingår i: IEEE Transactions on Control Systems Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 1063-6536 .- 1558-0865. ; 28:6, s. 2352-2363
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Tidskriftsartikel (refereegranskat)abstract
- This article presents a robust nonlinear model predictive control (NMPC) scheme for autonomous navigation of underwater robotic vehicles operating in a constrained workspace including the static obstacles. In particular, the purpose of the controller is to guide the vehicle toward specific way points with guaranteed input and state constraints. Various constraints, such as obstacles, workspace boundaries, predefined upper bounds for the velocity of the robotic vehicle, and thruster saturations, are considered during the control design. Moreover, the proposed control scheme is designed at dynamic level, and it incorporates the full dynamics of the vehicle in which the ocean currents are also involved. Hence, taking the thrusts as the control inputs of the robotic system and formulating them accordingly, the vehicle exploits the ocean current dynamics when these are in favor of the way-point tracking mission, resulting in reduced energy consumption by the thrusters. The robustness of the closed-loop system against parameter uncertainties has been analytically guaranteed with convergence properties. The performance of the proposed control strategy is experimentally verified using a 4 degrees of freedom (DoF) underwater robotic vehicle inside a constrained test tank with sparse static obstacles.
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| 9. |
- Heshmati Alamdari, Shahabodin, et al.
(författare)
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A Self-triggered Position Based Visual Servoing Model Predictive Control Scheme for Underwater Robotic Vehicles
- 2020
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Ingår i: Machines. - : MDPI. - 2075-1702. ; 8:2
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Tidskriftsartikel (refereegranskat)abstract
- An efficient position based visual sevroing control approach for Autonomous Underwater Vehicles (AUVs) by employing Non-linear Model Predictive Control (N-MPC) is designed and presented in this work. In the proposed scheme, a mechanism is incorporated within the vision-based controller that determines when the Visual Tracking Algorithm (VTA) should be activated and new control inputs should be calculated. More specifically, the control loop does not close periodically, i.e., between two consecutive activations (triggering instants), the control inputs calculated by the N-MPC at the previous triggering time instant are applied to the underwater robot in an open-loop mode. This results in a significantly smaller number of requested measurements from the vision tracking algorithm, as well as less frequent computations of the non-linear predictive control law. This results in a reduction in processing time as well as energy consumption and, therefore, increases the accuracy and autonomy of the Autonomous Underwater Vehicle. The latter is of paramount importance for persistent underwater inspection tasks. Moreover, the Field of View constraints (FoV), control input saturation, the kinematic limitations due to the underactuated degree of freedom in sway direction, and the effect of the model uncertainties as well as external disturbances have been considered during the control design. In addition, the stability and convergence of the closed-loop system has been guaranteed analytically. Finally, the efficiency and performance of the proposed vision-based control framework is demonstrated through a comparative real-time experimental study while using a small underwater vehicle.
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| 10. |
- 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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