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- Ferizbegovic, Mina
(författare)
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Dual control concepts for linear dynamical systems
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We study simultaneous learning and control of linear dynamical systems. In such a setting, control policies are derived with respect to two objectives: i) to control the system as well as possible, given the current knowledge of system dynamics (exploitation), and ii) to gather as much information as possible about the unknown system that can improve control (exploration).These two objectives are often in conflict, and this phenomenon is known as the exploration-exploitation trade-off.More specifically, in the context of simultaneous learning and control, we consider: linear quadratic regulation (LQR) problem, model reference control, and data-driven control based on Willems \textit{et al.}'s fundamental lemma. First, we consider the LQR problem with unknown dynamics. We present robust and certainty equivalence (CE) model-based control methods that balance exploration and exploitation. We focus on control policies that can be iteratively updated after sequentially collecting data.We propose robust (with respect to parameter uncertainty) LQR design methods. To quantify uncertainty, we derive a methodbased on Bayesian inference, which is directly applicable to robust control synthesis. To begin, we derive a robust controller to minimize the worst-case cost, with high probability, given the empirical observation of the system. This robust controller synthesis is then used to derive a robust dual controller, which updates its control policy after collecting data. An episode in which data is collected is called exploration, and the episode using an updated control policy called exploitation. The objective is to minimize the worst-case cost of the updated control policy, requiring that a given exploration budget constrains the worst-case cost during exploration. Additionally, we derive methods that balance exploration and exploitation to minimize the cumulative worst-case cost for a fixed number of episodes. In this thesis, we refer to such a problem as robust reinforcement learning. Essentially, it is a robust dual controller aiming to minimize the cumulative worst-case cost, and that updates its control policy in each episode.Numerical experiments show that the proposed methods perform better than existing state-of-the-art algorithms. Moreover, experiments also indicate that the exploration prioritizes the uncertainty reduction in the parameters that matter most for control.A control policy using the CE principle for LQR consists of a sum of an optimal controller calculated using estimated dynamics at time $t$, and an additive external excitation. It has been shown over the years that the optimal asymptotic rate of regret is in many instances $\mathcal{O}(\sqrt{T})$. In particular, this rate can be obtained by adding a white noise external excitation, with a variance decaying as $\gamma/\sqrt{T}$, where $\gamma$ is a predefined constant. As the amount of excitation is pre-determined, such approaches can be viewed as open-loop control of the external excitation. In this thesis, we approach the problem of designing the external excitation from a feedback perspective leveraging the well-known benefits of feedback control for decreasing sensitivity to external disturbances and system-model mismatch, as compared to open-loop strategies. The benefits of this approach over the open-loop approach can be seen in the case of unmodeled dynamics and disturbances. However, even when using the benefits of feedback control, we do not calculate the optimal amount of external excitation. To find the optimal amount of external excitation, we suggest exploration strategies that are based on a time-dependent scaling $\gamma_t$ and can attain cumulative regret similar to or lower than cumulative regret obtained for optimal scaling $\gamma^*$ according to numerical examples.Second, we consider the model reference control problem with the goal of proposing a data-driven robust control design method based on an average risk criterion, which we call Bayes control. We show that this approach has very close ties to the Bayesian kernel-based method, but the conceptual difference lies in the use of a deterministic respective stochastic setting for the system parameters. Finally, we consider data-driven control using Willems \textit{et al.}'s fundamental lemma. First, we propose variations of the fundamental lemma that, instead of a data trajectory, utilize correlation functions in the time domain, as well as power spectra of the input and the output in the frequency domain. Since data-driven control using the fundamental lemma can become a very expensive computation task for large datasets, the proposed variations are easy to computeeven for large datasets and can be efficient as a data compression technique. Second, we study connections of data informativity conditions between the results based on the fundamental lemma (finite time), and classical system identification. We show that finite time informativity conditions for state-space systems are closely linked to the identifiability conditions derived from the fundamental lemma. We prove that the obtained persistency of excitation conditions for infinite time are sufficient conditions for finite time informativity. Moreover, we reveal that the obtained conditions for a finite time in closed-loop are stricter than in classical system identification. This is a consequence of the noiseless data setting in the fundamental lemma that precludes the possibility of noise to excite the system in a feedback setting.
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| 2. |
- Furneaux, Brendan R., 1979-
(författare)
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Fungal communities of West African ectomycorrhizal woodlands
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ectomycorrhizal symbiosis is a widespread mutualist relationship between fungi and plants, often trees. It is most well-known in temperate and boreal forests, but also occurs in a variety of tropical ecosystems, including Sudanian savanna woodlands and gallery forests in West Africa. In addition to their important role in nutrient cycling, many ectomycorrhizal fungi also produce edible mushrooms as their fruit-bodies. In this thesis, I explore spatial patterns of above-ground and below-ground diversity in West African ectomycorrhizal communities, as well as the use of wild edible fungi by the local human population. In Papers I and II, I used soil DNA metabarcoding to investigate fungal communities in their vegetative state. Paper I used measurements of the scale of spatial autocorrelation within the fungal community in Sudanian woodlands in Benin as a test to compare different high-throughput DNA sequencing strategies, including short (≈350 bp) amplicon barcoding using the Illumina, Ion Torrent, and PacBio sequencing platforms, as well as long (≈1550 bp) amplicon barcoding using PacBio. There were some differences in species richness and community composition recovered by the two amplicon lengths, attributable to known primer biases for the short amplicons, but these did not lead to different ecological results. Additionally, Paper I introduced new software packages for analysis of long-amplicon metabarcoding data and integrating phylogenetic information into sequence-based taxonomic identification. In Paper II we sampled both Sudanian woodland and gallery forest sites in five countries across West Africa using long-amplicon metabarcoding with PacBio. We found significant differences in fungal community composition between Sudanian woodland and gallery forest sites, but not between different tree species within each vegetation type. Papers III and IV focused above-ground to study the natural production and human use of the mushrooms themselves. In Paper III we exhaustively collected mushrooms from nine Sudanian woodland plots in Benin during three consecutive rainy seasons. We measured the total biomass produced of each morphospecies, as well as environmental variables related to microclimate, host tree availability, and soil chemistry. Mushroom production and diversity were negatively correlated with soil nitrogen levels, and positively correlated with soil phosphorus levels. Although there were no clear differences in the fungal communities associated with the four host trees present in our plots, greater host tree diversity was associated with greater fungal diversity and productivity. Finally, Paper IV combined interviews with local people from four ethnic groups in five villages near the study sites from Paper III about their knowledge and preferences for edible mushrooms with DNA barcoding of specimens. Knowledge and preferences for different mushrooms varied between ethnic groups, but people living in a village where their ethnic group is a minority tended to absorb knowledge about mushrooms from the majority group in the village. Women over age 35 were the most knowledgeable about mushrooms, but the degree of gender difference varied between groups. We compiled a list of the most choice edible mushrooms in the area, which can inform the possibility for commercial trade in wild mushrooms to supplement the income of rural people and encourage sustainable forest management.
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| 3. |
- Smith, Ashley, 1979-
(författare)
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The Archival Life of Home Movies : Regional Reflections and Negotiated Visions of a Shared Past
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This study investigates the ways in which private home movies are transformed into curated archival objects. Through the concept of the archival home movie, it explores the impact of preservation and content description on access, use, and, thus, regional historiography. Additionally, it maps out the relationship between ordinary home-movie imagery and regional meaning making.Using material from the University of Mississippi’s Home Movie Collection as a case study, the dissertation centers on the practice of researching family films and the possibilities for their contemporary cultural relevance. In recent years, home movies and amateur film have become topics of interest in studies of non-theatrical film as sources for unofficial histories. This dissertation intervenes into these discussions of the cultural value of home movies as a hands-on and self-reflexive investigation into the archive itself, as well as an activation of the archive. This methodology includes stagings of the home movie in live screenings, through which the dissertation investigates modes of spectator engagement with the historical material.Chapter 1 assesses strategies for working with archival home movies that draw from areas such as the study of family photography and photo albums, as well as the study of diaries. The following chapters each move further away from the familial point of origin of the home movie and toward regional re-readings and archival reuses. Using “home” as a starting point, Chapter 2 engages with the domestic elements in a selection of home movies shot on a cotton plantation during the late 1930s and early 1940s. Through an expanded notion of home, it argues that work and free time, public and private, and family and nonfamily are intertwined—both onscreen and off. Chapter 3 positions a collection of archival home movies shot in the Mississippi Delta in the 1950s as mediated witness to the ever-presence and—at times—invisibility of institutionalized racism in the mid-century American South. Chapter 4 maps the creative treatment of one collection of archival home movies in contemporary documentaries, museum installations, and experimental films. Finally, Chapter 5 evaluates the activation of home movies as constructed regional reflections in a series of live screenings associated with Home Movie Day.Previous studies of family photographs and family film have pointed to the ways these media function to obscure discord and present a harmonious picture. Overall, this study of southern home movies demonstrates how a double logic of obfuscation is at work in these films. In addition to a vision of familial harmony, this dissertation argues that the southern home movie also puts forth a vision of racial harmony. This onscreen racial harmony, when presented during the Jim Crow years, should be understood as the result of a specifically white fantasy of racial togetherness that, at the same time, upholds traditional hierarchies.
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