| 1. |
- Hait, S., et al.
(författare)
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Impact of ferromagnetic layer thickness on the spin pumping in Co60Fe20B20/Ta bilayer thin films
- 2021
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Ingår i: Journal of Materials Science-Materials in Electronics. - : Springer Science and Business Media LLC. - 0957-4522 .- 1573-482X. ; 32, s. 12453-12465
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Tidskriftsartikel (refereegranskat)abstract
- We report the tuneable spin angular momentum transfer (spin pumping) from Co60Fe20B20 (CFB) amorphous alloy into the Ta heavy metal nanolayers. All the films are grown on Si (100) substrate at room temperature using ion-beam sputtering technique. Structural studies reveal that the grown Ta films over amorphous CFB are crystalline even at ultrathin regime. The bilayers possess very low interface roughness (< 0.5 nm) and are continuous throughout the thickness range. Comparative analysis of the spin pumping in CFB (4, 6 and 8 nm) as a function of the Ta thickness (vary from 1 to 10 nm in step of 1 nm) has been performed employing ferromagnetic resonance (FMR) spectroscopy. It is observed that the effective damping increase exponentially with the increase of Ta, (i.e. follows ballistic spin transport) in two series of CFB (4 nm)/Ta (0-10 nm) and CFB(6 nm)/Ta (0-10 nm) bilayers, which is characteristic of normal spin pumping. However, the anomalous behaviour has been observed for CFB (8 nm)/Ta (0-10 nm) bilayer series where the spin current generated in Ta with the thicker CFB behaves oppositely. The results demonstrate the strong dependence of ferromagnet thickness on the spin pumping into the Ta nanolayers. This study paves the way to choose suitable ferromagnetic layer thickness for spin current-induced switching applications in spintronics.
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| 2. |
- Husain, Sajid, et al.
(författare)
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Multi-jump magnetization switching in Co2FeAl full Heusler alloy thin films : Experiments and simulations
- 2019
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Ingår i: Journal of Magnetism and Magnetic Materials. - : ELSEVIER SCIENCE BV. - 0304-8853 .- 1873-4766. ; 486
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Tidskriftsartikel (refereegranskat)abstract
- Co2FeAl (CFA) thin films of 50 nm thickness have been grown on MgO (001) single crystal substrates at room temperature with and without post-annealing (PA) at 300 degrees C and 400 degrees C using dual ion-beam sputtering technique. The XRD pattern of the as-grown film revealed that CFA grows with preferred crystallographic orientation on the MgO (001) substrate. Temperature dependent anisotropy measurements on PA films revealed a dominating contribution from cubic anisotropy as confirmed by the analysis of azimuthal angle dependent longitudinal in-plane magneto-optical Kerr effect (MOKE) measurements. The contributions from the cubic and uniaxial anisotropies have also been quantified employing ferromagnetic resonance spectroscopy. Magnetization reversal is accompanied with a plateau in the MOKE hysteresis recorded at various azimuthal angles in the in-plane applied magnetic field configuration. The occurrence of the observed plateau is explained by the presence of a combination of domain walls such as 90 degrees, 135 degrees and 180 degrees domain walls and/or complex domains which is supported by results from micromagnetic simulations. These results demonstrate the feasibility of manipulating the magnetization switching in one of the two ferromagnetic electrodes of the magnetic tunnel junction devices based on Heusler alloy ferromagnetic films.
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| 3. |
- Akansel, Serkan, et al.
(författare)
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Effect of seed layers on dynamic and static magnetic properties of Fe65Co35 thin films
- 2018
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 51:30
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Tidskriftsartikel (refereegranskat)abstract
- Fe65Co35 thin films have been deposited on SiO2 substrates using sputtering technique with different choices of seed layer; Ru, Ni82.5Fe17.5, Rh, Y and Zr. Best soft magnetic properties were observed with seed layers of Ru, Ni82.5Fe17.5 and Rh. Adding these seed layers, the coercivity of the Fe65Co35 films decreased to values of around 1.5 mT, which can be compared to the value of 12.5 mT obtained for films deposited without seed layer. Further investigations were performed on samples with these three seed layers in terms of dynamic magnetic properties, both on as prepared and annealed samples, using constant frequency cavity and broadband ferromagnetic resonance measurements. Damping parameters of around 8.0X10-3 and 4.5X10-3 were obtained from in-plane and out-of-plane measurements, respectively, for as prepared samples, values that were reduced to 6.5X10-3 and 4.0X10-3 for annealed samples.
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| 4. |
- Batkovic, Ivo, 1992, et al.
(författare)
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Experimental Validation of Safe MPC for Autonomous Driving in Uncertain Environments
- 2023
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Ingår i: IEEE Transactions on Control Systems Technology. - 1063-6536 .- 1558-0865. ; 31:5, s. 2027-2042
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Tidskriftsartikel (refereegranskat)abstract
- The full deployment of autonomous driving systems on a worldwide scale requires that the self-driving vehicle can be operated in a provably safe manner, i.e., the vehicle must be able to avoid collisions in any possible traffic situation. In this article, we propose a framework based on model predictive control (MPC) that endows the self-driving vehicle with the necessary safety guarantees. In particular, our framework ensures constraint satisfaction at all times while tracking the reference trajectory as close as obstacles allow, resulting in a safe and comfortable driving behavior. To discuss the performance and real-time capability of our framework, we provide first an illustrative simulation example, and then, we demonstrate the effectiveness of our framework in experiments with a real test vehicle.
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| 5. |
- Gupta, Ankit, 1988, et al.
(författare)
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A Framework for Vehicle Lateral Motion Control With Guaranteed Tracking and Performance
- 2019
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Ingår i: 2019 IEEE Intelligent Transportation Systems Conference, ITSC 2019. ; , s. 3607-3612
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Konferensbidrag (refereegranskat)abstract
- In this paper, we propose a framework to design a controller for the vehicle lateral dynamics, which guarantees to meet desired safety and performance requirement. Bounded lateral deviation from the reference trajectory is the strict safety requirement considered in this paper.The proposed control design relies on a mild assumption. That is, the trajectory planner generates a trajectory that is piece-wise clothoidal (PWC), with bounded curvature and curvature rate.Closed-loop simulations using a linear quadratic regulator (LQR) and a model predictive control (MPC) controller, developed based on the proposed framework, validates the proposed approach.
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| 6. |
- Gupta, Ankit
(författare)
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Adapting Deep Learning for Microscopy: Interaction, Application, and Validation
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Microscopy is an integral technique in biology to study the fundamental components of life visually. Digital microscopy and automation have enabled biologists to conduct faster and larger-scale experiments with a sharp increase in the data generated. Microscopy images contain rich but sparse information, as typically, only small regions in the images are relevant for further study. Image analysis is a crucial tool for biologists in the objective interpretation and extraction of quantitative measurements from microscopy data. Recently, deep learning techniques have shown superior performance in various image analysis tasks. The models learn feature representations from the data by optimizing for a task. However, the techniques require a significant amount of annotated data to perform well. Domain experts are required to annotate microscopy data, making it expensive and time-consuming. The models offer no insight into their prediction, and the learned features are not directly interpretable. This poses challenges to the reliable utilization of the technique in high-trust applications such as drug discovery or disease detection. High data variability in microscopy and poor generalization performance of deep learning models further increase the difficulty in general usage of the technique. The work in this thesis presents frameworks and methods to solve the practical challenges of applying deep learning in microscopy. The application-specific evaluation approaches were presented to validate the approaches, aiming to increase trust in the system. The major contributions of this work are as follows. Papers I and III present human-in-the-loop frameworks for quick adaption of deep learning to new data and for improving models' performance based on human input in visual explanations provided by the model, respectively. Paper II proposes a template-matching approach to improve user interactions in the framework proposed in Paper I. Papers III and IV present architectural modifications in the deep learning models proposed for better visual explanation and image-to-image translation, respectively. Papers IV and V present biologically relevant evaluations of approaches, i.e., analysis of the deep learning models in relation to the biological task.This thesis is aimed towards better utilization and adaptation of the DL methods and techniques to the microscopy data. We show that the annotation burden for the user can be significantly reduced by intuitive annotation frameworks and using contemporary deep-learning paradigms. We further propose architectural modifications in the models to adapt to the requirements and demonstrate the utility of application-specific analysis in microscopy.
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| 7. |
- Gupta, Ankit, 1988, et al.
(författare)
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Computation of low-complexity control-invariant sets for systems with uncertain parameter dependence
- 2019
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098. ; 101, s. 330-337
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers the problem of computing a low-complexity robust control-invariant (LC-RCI) set for uncertain systems, along with a static linear state-feedback law. The LC-RCI set, assumed to be symmetric around the origin and described by the same number of affine inequalities as twice the dimension of the state vector, is the result of an iterative procedure, where semi-definite programs (SDPs) are solved at each step. The SDPs are formulated to increase the LC-RCI volume at each step, subject to tractable reformulations of the system constraints as well as the invariance condition (in the form of standard or dilated LMIs), and a new approach to determinant maximization. The two proposed algorithms are applicable to systems with rational parameter dependence, which cannot be handled with the existing similar approaches without introducing additional conservatism.
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| 8. |
- Gupta, Ankit, 1988, et al.
(författare)
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Computation of Parameter Dependent Robust Invariant Sets for LPV Models with Guaranteed Performance
- 2023
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098. ; 151
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents an iterative algorithm to compute a Robust Control Invariant (RCI) set, along with an invariance-inducing control law, for Linear Parameter-Varying (LPV) systems. As the real-time measurements of the scheduling parameters are typically available, in the presented formulation, we allow the RCI set description along with the invariance-inducing controller to be scheduling parameter dependent. The considered formulation thus leads to parameter-dependent conditions for the set invariance, which are replaced by sufficient Linear Matrix Inequality (LMI) conditions via Polya's relaxation. These LMI conditions are then combined with a novel volume maximization approach in a Semidefinite Programming (SDP) problem, which aims at computing the desirably large RCI set. In addition to ensuring invariance, it is also possible to guarantee performance within the RCI set by imposing a chosen quadratic performance level as an additional constraint in the SDP problem. The reported numerical example shows that the presented iterative algorithm can generate invariant sets which are larger than the maximal RCI sets computed without exploiting scheduling parameter information.
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| 9. |
- Gupta, Ankit, 1988, et al.
(författare)
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Computation of Robust Control Invariant Sets with Predefined Complexity for Uncertain Systems
- 2021
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1099-1239 .- 1049-8923. ; 31:5, s. 1674-1688
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents an algorithm that computes polytopic robust control-invariant (RCI) sets for rationally parameter-dependent systems with additive disturbances. By means of novel LMI feasibility conditions for invariance along with a newly developed method for volume maximization, an iterative algorithm is proposed for the computation of RCI sets with maximized volumes. The obtained RCI sets are symmetric around the origin by construction and have a user-defined level of complexity. Unlike many similar approaches, fixed state feedback structure is not imposed. In fact, a specific control input is obtained from the LMI problem for each extreme point of the RCI set. The outcomes of the proposed algorithm can be used to construct a piecewise-affine controller based on offline computations.
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| 10. |
- Gupta, Ankit, 1988
(författare)
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Control of Constrained Dynamical Systems with Performance Guarantees: With Application to Vehicle motion Control
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In control engineering, models of the system are commonly used for controller design. A standard control design problem consists of steering the given system output (or states) towards a predefined reference. Such a problem can be solved by employing feedback control strategies. By utilizing the knowledge of the model, these strategies compute the control inputs that shrink the error between the system outputs and their desired references over time. Usually, the control inputs must be computed such that the system output signals are kept in a desired region, possibly due to design or safety requirements. Also, the input signals should be within the physical limits of the actuators. Depending on the constraints, their violation might result in unacceptable system failures (e.g. deadly injury in the worst case). Thus, in safety-critical applications, a controller must be robust towards the modelling uncertainties and provide a priori guarantees for constraint satisfaction. A fundamental tool in constrained control application is the robust control invariant sets (RCI). For a controlled dynamical system, if initial states belong to RCI set, control inputs always exist that keep the future state trajectories restricted within the set. Hence, RCI sets can characterize a system that never violates constraints. These sets are the primary ingredient in the synthesis of the well-known constraint control strategies like model predictive control (MPC) and interpolation-based controller (IBC). Consequently, a large body of research has been devoted to the computation of these sets. In the thesis, we will focus on the computation of RCI sets and the method to generate control inputs that keep the system trajectories within RCI set. We specifically focus on the systems which have time-varying dynamics and polytopic constraints. Depending upon the nature of the time-varying element in the system description (i.e., if they are observable or not), we propose different sets of algorithms. The first group of algorithms apply to the system with time-varying, bounded uncertainties. To systematically handle the uncertainties and reduce conservatism, we exploit various tools from the robust control literature to derive novel conditions for invariance. The obtained conditions are then combined with a newly developed method for volume maximization and minimization in a convex optimization problem to compute desirably large and small RCI sets. In addition to ensuring invariance, it is also possible to guarantee desired closed-loop performance within the RCI set. Furthermore, developed algorithms can generate RCI sets with a predefined number of hyper-planes. This feature allows us to adjust the computational complexity of MPC and IBC controller when the sets are utilized in controller synthesis. Using numerical examples, we show that the proposed algorithms can outperform (volume-wise) many state-of-the-art methods when computing RCI sets. In the other case, we assume the time-varying parameters in system description to be observable. The developed algorithm has many similar characteristics as the earlier case, but now to utilize the parameter information, the control law and the RCI set are allowed to be parameter-dependent. We have numerically shown that the presented algorithm can generate invariant sets which are larger than the maximal RCI sets computed without exploiting parameter information. Lastly, we demonstrate how we can utilize some of these algorithms to construct a computationally efficient IBC controller for the vehicle motion control. The devised IBC controller guarantees to meet safety requirements mentioned in ISO 26262 and the ride comfort requirement by design.
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