• Publicationsclose
• Research dataclose
• Article close
• Publikationer från KTH close

Variable phase-pole machines have the potential to extend the operational range to higher speeds through magnetic pole changes. The state-of-the-art vector space decomposition cannot model the transient behavior of the pole change for any possible phase-pole configuration as it creates a discontinuity. The proposed harmonic plane decomposition theory solves this issue by generalizing the vector-space decomposition to the fullest extend by using its discrete Fourier transformation interpretation. The theory for indirect rotor field-oriented control is developed using the harmonic-plane decomposition. A controlled, loaded pole change on a wound independently-controlled stator-coils machine using two transition strategies shows the harmonic-plane decomposition-based controller’s ability to maintain torque in the transition. Additionally, the proposed controller accomplishes real harmonic injection and balanced steady-state operation. Accepted by IEEE transactions on industrial electronicsQC 20221213

Ethanol, as the most produced renewable biofuel, is considered a promising low-carbon alternative to petroleum-based fuels in the transport sector due to its high energy density and auto-ignition resistance. The lean-burn combustion in spark-ignition (SI) engines has the potential to further improve thermal efficiency in regard to knock mitigation and the reduction of combustion temperature. However, the characteristics of lean-burn combustion in an ethanol-fueled engine in relation to the combustion losses and the gas-exchange process remain unclear, especially for high-load operation. This study contributes with a deeper understanding of the high-load performance of an ethanol-fueled heavy-duty SI engine using lean-burn combustion. Based on the experimental results from a single-cylinder engine test, a 6-cylinder engine model is built by integrating a validated predictive combustion model to characterize the lean-burn combustion process. The engine’s thermal efficiency and combustion phasing are evaluated for knock limited operation and then compared to the theoretical optimum which is regardless of knock. The energy and exergy balances are applied to evaluate the effect of dilution with excess air ratios up to 1.8. Losses through heat transfer, exhaust flow, and incomplete combustion are quantified. In addition, entropy generated through combustion is discussed to identify the relationship between exergy destruction and different operating conditions. In the context of lean-burn combustion, the thermal efficiency at high-load operation incrementally increases from 40.4% at stoichiometric condition to 47.3% at an excess air ratio of 1.8. At the same time, the exergy destruction through combustion increases by 3.3 percentage points across the selected dilution range. Furthermore, the challenging requirements to realize lean-burn combustion with lower exhaust gas temperatures and higher intake boost pressures is assessed through an exergy analysis of the turbocharging system. QC 20230123

QC 20230130

This paper presents a theory for how geometric image transformations can be handled by a first layer of linear receptive fields, in terms of true covariance properties, which, in turn, enable geometric invariance properties at higher levels in the visual hierarchy. Specifically, we develop this theory for a generalized Gaussian derivative model for visual receptive fields, which is derived in an axiomatic manner from first principles, that reflect symmetry properties of the environment, complemented by structural assumptions to guarantee internally consistent treatment of image structures over multiple spatio-temporal scales. It is shown how the studied generalized Gaussian derivative model for visual receptive fields obeys true covariance properties under spatial scaling transformations, spatial affine transformations, Galilean transformations and temporal scaling transformations, implying that a vision system, based on image and video measurements in terms of the receptive fields according to this model, can to first order of approximation handle the image and video deformations between multiple views of objects delimited by smooth surfaces, as well as between multiple views of spatio-temporal events, under varying relative motions between the objects and events in the world and the observer. We conclude by describing implications of the presented theory for biological vision, regarding connections between the variabilities of the shapes of biological visual receptive fields and the variabilities of spatial and spatio-temporal image structures under natural image transformations. Comment: 34 pages, 14 figures

Teacher educators’ distinct and dual task of educating future teachers includes using digital tools to support students’ ongoing learning while exemplifying appropriate teaching strategies where the use of digital tools, such as response systems (RSs), are commonly occurring. RSs have been used in higher education for a long time, and many studies discuss how larger student groups answering multiple-choice questions during lectures contribute to student participation and learning. However, there is limited research on RSs, particularly related to teacher education. Therefore, this interview study aims to explore for what purposes teacher educators use RSs in teaching and what advantages and limitations they experience. In the thematic analysis, we found that the teacher educators used RSs to teach simultaneously as they were role models on how to use digital tools for learning. They used anonymous open-text answers more than multiple-choice questions to support student participation, immediately assess, and provide feedback in both larger and smaller groups. The complexity of time management connected to the use of RSs was highlighted. RSs were also used to initiate discussions with the teacher students about the purposes, advantages, and limitations of using digital tools for learning. QC 20230403

Condition monitoring of the insulation health of electric machines is usually performed with offline tests. In recent years, online condition monitoring methods for the groundwall insulation have been introduced. The majority of these works are based on leakage current measurements, requiring additional equipment compared to the standard one in converters. The condition monitoring method in this work instead relies on the high-frequency ringing of stator currents occurring after a converter switching transition. This work evaluates the method experimentally in a randomly-wound stator winding of an induction machine prototype equipped with taps. The insulation condition change is forced by inserting external capacitors. Through the analysis of different metrics for the health monitoring and a detailed discussion of the hardware and software requirements for an electric drive, a viable way forward for the practical implementation of the approach is proposed. QC 20230117

Scenario-based approaches have been receiving a huge amount of attention in research and engineering of automated driving systems. Due to the complexity and uncertainty of the driving environment, and the complexity of the driving task itself, the number of possible driving scenarios that an Automated Driving System or Advanced Driving-Assistance System may encounter is virtually infinite. Therefore it is essential to be able to reason about the identification of scenarios and in particular critical ones that may impose unacceptable risk if not considered. Critical scenarios are particularly important to support design, verification and validation efforts, and as a basis for a safety case. In this paper, we present the results of a systematic mapping study in the context of autonomous driving. The main contributions are: (i) introducing a comprehensive taxonomy for critical scenario identification methods; (ii) giving an overview of the state-of-the-art research based on the taxonomy encompassing 86 papers between 2017 and 2020; and (iii) identifying open issues and directions for further research. The provided taxonomy comprises three main perspectives encompassing the problem definition (the why), the solution (the methods to derive scenarios), and the assessment of the established scenarios. In addition, we discuss open research issues considering the perspectives of coverage, practicability, and scenario space explosion. QC 20220525

Fibre-reinforced shotcrete (sprayed concrete) is one of the major components in the support system for tunnels in hard rock. Several empirical design methodologies have been developed over the years due to the complexity and many uncertainties involved in rock support design. Therefore, this paper aims to highlight how the choice of design methodology and fibre type impacts the structural capacity of the lining and the emission of greenhouse gases (GHG). The paper starts with a review of different design methods. Then, an experimental campaign is presented in which the structural performance of shotcrete reinforced with various dosages of fibres made of steel, synthetic and basalt was compared. A case study is presented in which the permanent rock support is designed based on the presented design methods. Here, only the structural requirements were considered, and suitable dosages of fibres were selected based on the experimental results. The emission of GHG was calculated for all design options based on environmental product declarations for each fibre type. The result in this paper indicates that synthetic fibres have the greatest potential to lower the emissions of GHG in the design phase. Moreover, the choice of design method has a significant impact on the required dosage of fibres. QC 20230307

We generate new knowledge about financial crises and their consequences for MNC technological development, thereby addressing a largely uncharted issue at the crossroads of the organization, strategy and international business literatures. Drawing on threat-rigidity theory, we argue that financial crises have an overall negative effect on MNC technological exploration and that the strength of the effect differs across greenfield and acquired subsidiaries. Results from an empirical investigation of 21 MNCs over the 1890–2008 period suggest that the dampening effect of financial crises on technological exploration is confined to home-country units and greenfield subsidiaries, whereas it is found to be of less significance among acquired foreign subsidiaries. We suggest that such differentiation within the MNC is indicative of a previously unobserved advantage from multinationality, which allows it to smoothen the effects of financial crises on long-term technological development and corporate growth. QC 20230123

Conventional cemented carbide is recommended for machining Ti6Al4V. However, polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (pcBN) also show promise. Demands for higher productivity accelerate diffusional dissolution and chemically driven wear mechanisms in these tool materials. This study investigates active wear mechanisms by studying the interactions between Ti6Al4V and PCD, pcBN, and cemented carbide tools in diffusion couples at temperatures from 900° to 1300°C. All tool materials suffered from diffusion to varying degrees, and different chemical reactions occurred. Titanium carbide with minor vanadium alloying (Ti,V)C reaction products act as diffusion barriers when using PCD and cemented carbide, while the reaction products acting as diffusion barrier in pcBN is (Ti,V)B2. The presence of Mo and W in binder sites of pcBN reduces diffusional dissolution of cBN. Diffusion simulations agreed well with microscopy investigations and were enabled by the known temperature and pressure conditions of the static diffusion couples. QC 20221025