| 1. |
- Axehill, Daniel, 1978-
(författare)
-
Applications of Integer Quadratic Programming in Control and Communication
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main topic of this thesis is integer quadratic programming with applications to problems arising in the areas of automatic control and communication. One of the most widespread modern control principles is the discrete-time method Model Predictive Control (MPC). The main advantage with MPC, compared to most other control principles, is that constraints on control signals and states can easily be handled. In each time step, MPC requires the solution of a Quadratic Programming (QP) problem. To be able to use MPC for large systems, and at high sampling rates, optimization routines tailored for MPC are used. In recent years, the range of application of MPC has been extended from constrained linear systems to so-called hybrid systems. Hybrid systems are systems where continuous dynamics interact with logic. When this extension is made, binary variables are introduced in the problem. As a consequence, the QP problem has to be replaced by a far more challenging Mixed Integer Quadratic Programming (MIQP) problem. Generally, for this type of optimization problems, the computational complexity is exponential in the number of binary optimization variables. In modern communication systems, multiple users share a so-called multi-access channel, where the information sent by different users is separated by using almost orthogonal codes. Since the codes are not completely orthogonal, the decoded information at the receiver is slightly correlated between different users. Further, noise is added during the transmission. To estimate the information originally sent, a maximum likelihood problem involving binary variables is solved. The process of simultaneously estimating the information sent by multiple users is called multiuser detection. In this thesis, the problem to efficiently solve MIQP problems originating from MPC is addressed. Two different algorithms are presented. First, a polynomial complexity preprocessing algorithm for binary quadratic programming problems is presented. By using the algorithm, some, or all, binary variables can be computed efficiently already in the preprocessing phase. In simulations, the algorithm is applied to unconstrained MPC problems with a mixture of real and binary control signals. It has also been applied to the multiuser detection problem, where simulations have shown that the bit error rate can be significantly reduced by using the proposed algorithm as compared to using common suboptimal algorithms. Second, an MIQP algorithm tailored for MPC is presented. The algorithm uses a branch and bound method where the relaxed node problems are solved by a dual active set QP algorithm. In this QP algorithm, the KKT-systems are solved using Riccati recursions in order to decrease the computational complexity. Simulation results show that both the QP solver and the MIQP solver proposed have lower computational complexity than corresponding generic solvers.
|
|
| 2. |
- Galis, Vasilis, 1978-, et al.
(författare)
-
Partisan Scholarship in Technoscientific Controversies : Reflections on Research Experience
- 2012
-
Ingår i: Science as Culture. - London : Routledge. - 0950-5431 .- 1470-1189. ; 21:3, s. 335-364
-
Tidskriftsartikel (refereegranskat)abstract
- Several academic traditions have addressed epistemological objectivity and/or partisanship in the study of technoscientific controversies. On the one hand, positivist andrelativist scholars agree that the political commitments of the social researcher should notimpinge on scientific enquiry, while on the other hand, feminist and Marxist scholars notonly take stands in diverse technoscientific debates, but even claim their agendas to bemore credible than those of orthodox scientists. Such perspectives stress that all researchis partisan in one way or another because it involves questions of who controls,manipulates, and establishes decisions, facts, and knowledge. With this in mind, it ispossible to identify different forms of partisan research including capture byparticipants, de facto and overt partisanship, and mercenary scholarship. These differentforms of partisan scholarship are characterised by differences in the motives underlyingepistemological choices of research topic and method, personal commitments to thefields studied, use of research findings in controversies, and positioning of results inwider debates. Two examples help to illustrate partisan scholarship: first, a study of newtechnologies for managing climate change (carbon dioxide capture and storage); andsecond, the construction of the new underground metro system in Athens and itsaccommodation of accessibility standards. Both cases entail partisan positions and raisesimilar concerns about the orthodox epistemological assumptions underpinningsociotechnical systems, especially when it comes to technoscientific controversies.Supporting STS partisan scholarship, therefore, enables greater social and democraticengagement with technoscientific development.
|
|
| 3. |
- Meier, Markus, et al.
(författare)
-
Comparing reconstructed past variations and future projections of the Baltic sea ecosystem first results from multi model ensemble simulations
- 2012
-
Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 7:3, s. 034005-
-
Tidskriftsartikel (refereegranskat)abstract
- Multi-model ensemble simulations for the marine biogeochemistry and food web of the Baltic Sea were performed for the period 1850-2098, and projected changes in the future climate were compared with the past climate environment. For the past period 1850-2006, atmospheric, hydrological and nutrient forcings were reconstructed, based on historical measurements. For the future period 1961-2098, scenario simulations were driven by regionalized global general circulation model (GCM) data and forced by various future greenhouse gas emission and air-and riverborne nutrient load scenarios (ranging from a pessimistic 'business-as-usual' to the most optimistic case). To estimate uncertainties, different models for the various parts of the Earth system were applied. Assuming the IPCC greenhouse gas emission scenarios A1B or A2, we found that water temperatures at the end of this century may be higher and salinities and oxygen concentrations may be lower than ever measured since 1850. There is also a tendency of increased eutrophication in the future, depending on the nutrient load scenario. Although cod biomass is mainly controlled by fishing mortality, climate change together with eutrophication may result in a biomass decline during the latter part of this century, even when combined with lower fishing pressure. Despite considerable shortcomings of state-of-the-art models, this study suggests that the future Baltic Sea ecosystem may unprecedentedly change compared to the past 150 yr. As stakeholders today pay only little attention to adaptation and mitigation strategies, more information is needed to raise public awareness of the possible impacts of climate change on marine ecosystems.
|
|
| 4. |
- Parvini Ahmadi, Shervin, 1989-
(författare)
-
Distributed Optimization for Control and Estimation
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Adopting centralized optimization approaches in order to solve optimization problem arising from analyzing large-scale systems, requires a powerful computational unit. Such units, however, do not always exist. In addition, it is not always possible to form the optimization problem in a centralized manner due to structural constraints or privacy requirements. A possible solution in these cases is to use distributed optimization approaches. Many large-scale systems have inherent structures which can be exploited to develop scalable optimization approaches. In this thesis, chordal graph properties are used in order to design tailored distributed optimization approaches for applications in control and estimation, and especially for model predictive control and localization problems. The first contribution concerns a distributed primal-dual interior-point algorithm for which it is investigated how parallelism can be exploited. In particular, it is shown how the computations of the algorithm can be distributed on different processors so that they can be run in parallel. As a result, the algorithm execution time is accelerated compared to the case where the algorithm is run on a single processor. Simulation studies on linear model predictive control and robust model predictive control confirm the efficiency of the framework. The second contribution is to devise a tailored distributed algorithm for nonlinear least squares with application to a sensor network location problem. It relies on the Levenberg-Marquardt algorithm, in which the computations are distributed using message passing over the computational graph of the problem, which is obtained from what is known as the clique tree of the problem. The results indicate that the algorithm provides not only a good localization accuracy, but also it requires fewer iterations and communications between computational agents in order to converge compared to known first-order methods. The third contribution is a study of extending the message passing idea in order to design tailored distributed algorithm for general non-convex problems. The framework relies on an augmented Lagrangian algorithm in which a primal-dual interior-point method is used for the inner iteration. Application of the framework for general model predictive control of systems with several interconnected sub-systems is extensively investigated. The performance of the framework is then compared with distributed methods based on the alternating direction method of multipliers, where the superiority of the framework is illustrated.
|
|
| 5. |
- Scaini, Anna, et al.
(författare)
-
Pathways from research to sustainable development: Insights from ten research projects in sustainability and resilience
- 2024
-
Ingår i: AMBIO. - : Springer Nature. - 0044-7447 .- 1654-7209. ; 53
-
Tidskriftsartikel (refereegranskat)abstract
- Drawing on collective experience from ten collaborative research projects focused on the Global South, we identify three major challenges that impede the translation of research on sustainability and resilience into better-informed choices by individuals and policy-makers that in turn can support transformation to a sustainable future. The three challenges comprise: (i) converting knowledge produced during research projects into successful knowledge application; (ii) scaling up knowledge in time when research projects are short-term and potential impacts are long-term; and (iii) scaling up knowledge across space, from local research sites to larger-scale or even global impact. Some potential pathways for funding agencies to overcome these challenges include providing targeted prolonged funding for dissemination and outreach, and facilitating collaboration and coordination across different sites, research teams, and partner organizations. By systematically documenting these challenges, we hope to pave the way for further innovations in the research cycle.
|
|
| 6. |
|
|
| 7. |
- Anshelm, Jonas, 1960-, et al.
(författare)
-
Battling Promethean dreams and Trojan horses : Revealing the critical discourses of geoengineering
- 2014
-
Ingår i: Energy Research & Social Science. - : Elsevier. - 2214-6296 .- 2214-6326. ; 2, s. 135-144
-
Tidskriftsartikel (refereegranskat)abstract
- Geoengineering could counteract climate change by either altering the earth's global energy balance by reflecting sunlight or removing CO2 from the atmosphere. Geoengineering evokes various ethical and political challenges that are increasingly reflected in public debate and deliberation. Via a qualitative textual analysis of 1500 articles, we investigate discursive claims critical of geoengineering, considering what subjects are the most controversial, and what worldviews, values, and problematizations are shared by the actors subscribing to this discourse. We argue that the controversy about geoengineering differs, discursively, from other techno-political conflicts. Geoengineering proponents are described as reluctantly favouring research and deployment and displaying an unusual self-reflexivity, as they are well aware of and seriously consider all the technology's risks. Our analysis demonstrates that the discourse critical of geoengineering differs from and questions the dominant pro-geoengineering discourse in several profound ways with lasting implications for energy scholarship and analysis.
|
|
| 8. |
- Anshelm, Jonas, 1960-, et al.
(författare)
-
Climate change and the convergence between ENGOs and buisness: on the loss of utopian energies
- 2011
-
Ingår i: Environmental Values. - : White Horse Press. - 0963-2719 .- 1752-7015. ; 20:1, s. 75-94
-
Tidskriftsartikel (refereegranskat)abstract
- The conflicts permeating the environmental debate since the 1960s have mainly involved two actors: multinational companies and international environmental organizations (ENGOs). Today, there are signs that the antagonism is ending with regards to co-operation and strategy. We argue that this convergence is no longer limited to specific joint projects, but is also prevalent at the idea and policy levels. Both actors have begun describing problems in similar terms, articulating the same goals and recommending the same solutions. Such convergence offers advantages in efforts to counteract climate change but also some problems: declining citizen trust in ENGOs, risk of intellectually impoverished environmental and energy debates, and loss of alternate visions and values.
|
|
| 9. |
- Arnström, Daniel, 1994-
(författare)
-
On Complexity Certification of Active-Set QP Methods with Applications to Linear MPC
- 2021
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In model predictive control (MPC) an optimization problem has to be solved at each time step, which in real-time applications makes it important to solve these efficiently and to have good upper bounds on worst-case solution time. Often for linear MPC problems, the optimization problem in question is a quadratic program (QP) that depends on parameters such as system states and reference signals. A popular class of methods for solving such QPs is active-set methods, where a sequence of linear systems of equations is solved. The primary contribution of this thesis is a method which determines which sequence of subproblems a popular class of such active-set algorithms need to solve, for every possible QP instance that might arise from a given linear MPC problem (i.e, for every possible state and reference signal). By knowing these sequences, worst-case bounds on how many iterations, floating-point operations and, ultimately, the maximum solution time, these active-set algorithms require to compute a solution can be determined, which is of importance when, e.g, linear MPC is used in safety-critical applications. After establishing this complexity certification method, its applicability is extended by showing how it can be used indirectly to certify the complexity of another, efficient, type of active-set QP algorithm which reformulates the QP as a nonnegative least-squares method. Finally, the proposed complexity certification method is extended further to situations when enhancements to the active-set algorithms are used, namely, when they are terminated early (to save computations) and when outer proximal-point iterations are performed (to improve numerical stability).
|
|
| 10. |
- Arnström, Daniel, 1994-
(författare)
-
Real-Time Certified MPC : Reliable Active-Set QP Solvers
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In Model Predictive Control (MPC), optimization problems are solved recurrently to produce control actions. When MPC is used in real time to control safety-critical systems, it is important to solve these optimization problems with guarantees on the worst-case execution time. In this thesis, we take aim at such worst-case guarantees through two complementary approaches:(i) By developing methods that determine exact worst-case bounds on the computational complexity and execution time for deployed optimization solvers.(ii) By developing efficient optimization solvers that are tailored for the given application and hardware at hand.We focus on linear MPC, which means that the optimization problems in question are quadratic programs (QPs) that depend on parameters such as system states and reference signals. For solving such QPs, we consider active-set methods: a popular class of optimization algorithms used in real-time applications.The first part of the thesis concerns complexity certification of well-established active-set methods. First, we propose a certification framework that determines the sequence of subproblems that a class of active-set algorithms needs to solve, for every possible QP instance that might arise from a given linear MPC problem (i.e., for every possible state and reference signal). By knowing these sequences, one can exactly bound the number of iterations and/or floating-point operations that are required to compute a solution. In a second contribution, we use this framework to determine the exact worst-case execution time (WCET) for linear MPC. This requires factors such as hardware and software implementation/compilation to be accounted for in the analysis. The framework is further extended in a third contribution by accounting for internal numerical errors in the solver that is certified. In a similar vein, a fourth contribution extends the framework to handle proximal-point iterations, which can be used to improve the numerical stability of QP solvers, furthering their reliability.The second part of the thesis concerns efficient solvers for real-time MPC. We propose an efficient active-set solver that is contained in the above-mentioned complexity-certification framework. In addition to being real-time certifiable, we show that the solver is efficient, simple to implement, can easily be warm-started, and is numerically stable, all of which are important properties for a solver that is used in real-time MPC applications. As a final contribution, we use this solver to exemplify how the proposed complexity-certification framework developed in the first part can be used to tailor active-set solvers for a given linear MPC application. Specifically, we do this by constructing and certifying parameter-varying initializations of the solver.
|
|
