[article-cris] Sähkötekniikan korkeakoulu / ELEC

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    Finite-Time Consensus Dual Algorithm for Distributed Optimization over Digraphs
    (2023-07-01) Jiang, Wei; Charalambous, Themistoklis; Bionic and Rehabilitation Engineering; Department of Electrical Engineering and Automation; Department of Electrical Engineering and Automation; Ishii, Hideaki; Ebihara, Yoshio; Imura, Jun-ichi; Yamakita, Masaki
    In this paper, we solve the convex distributed optimization problems, which include unconstrained optimization and a special constrained problem commonly known as a resource allocation problem, over a network of agents among which the communication can be represented by directed graphs (digraphs), by using the finite-time consensus-based and dual-based first-order gradient descent (GD) techniques. The key point is that a special consensus matrix is utilized for problem reformulation to make our dual-based algorithm suitable for digraphs. By the property of distributed finite-time exact (not approximate) consensus, the classical centralized optimization techniques (e.g., Nesterov accelerated GD) can be embedded into our dual-based algorithm conveniently, which means our distributed algorithm can inherit performance of classical centralized algorithms that has been proved to have optimal convergence performance. As a result, our proposed algorithm has faster convergence rate related to the optimization iteration number compared with other distributed optimization algorithms in literature. Since there are finite consensus communication steps inside each consensus process, when the time needed to communicate values between two neighbors is less than a threshold of the time needed to perform local computations, our proposed algorithm is also faster related to the time, as demonstrated in the simulations.
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    An 18–28 GHz dual-mode down-converter IC for 5G applications
    (Springer, 2024) Naghavi, Saeed; Ryynänen, Kaisa; Zahra, Mahwish; Korsman, Aleksi; Stadius, Kari; Kosunen, Marko; Unnikrishnan, Vishnu; Anttila, Lauri; Valkama, Mikko; Ryynänen, Jussi; Department of Electronics and Nanoengineering; Tampere University; Department of Electronics and Nanoengineering
    Emerging spectrum trends require a higher integration of 5G New Radio Frequency Range 1 (FR1) and Frequency Range 2 (FR2) bands to enhance the availability of spectrum and spectrum-sharing opportunities. To enable the reception of both FR1 and FR2 bands in a seamless hardware entity, we propose combining homodyne and heterodyne architectures. This necessitates the incorporation of a down-converter module that transfers the incoming signals from FR2 bands down to FR1, ensuring compatibility with an FR1 direct-conversion receiver (DCR) for the final signal reception. The primary focus of this paper is the design and implementation of the required integrated down-converter. The module includes an integrated balun, a low-noise amplifier (LNA) with a bypass mode, a dual-mode mixer, and an intermediate frequency (IF) amplifier. The introduced bypass mode helps to further elevate the linearity performance compared to the nominal mode. The bypass mode is designed for joint communication and sensing operation to avoidthe compression of the receiver. This work also incorporates a local oscillator (LO) signal distribution network with phase tuning elements using a mixed-signal approach. The circuit is implemented in a 22-nm CMOS process, and the active die area is 0.6 mm 2 . The measurements demonstrate that the implemented chip can efficiently perform the required frequency conversion over a wide frequency range of 18–28 GHz. Conversion gain of 4.5–7.5 dB, noise figure of 15–19.7 dB, 1 dB compression point (IP1dB) of − 16 to − 10 dBm, and input third-order intercept point (IIP3) of − 5 to 0 dBm are achieved. The measured IP1 dB and IIP3 for the bypass mode are +0.5 to +4.5 dBm and +8.5 to +10 dBm, respectively.
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    Multifunctional Grid-Forming Cascade Control for Converters Equipped with an LCL Filter
    (2023) Mourouvin, Rayane; Nurminen, Tuure; Hinkkanen, Marko; Pirsto, Ville; Kukkola, Jarno; Department of Electrical Engineering and Automation; ABB Group; Department of Electrical Engineering and Automation
    A multifunctional control framework is presented for grid-forming converters. This structure enables to operate the voltage source converter (VSC) in voltage-source mode in normal operation while activating a current control loop if overcurrent limits are reached. The framework is obtained by merging a multifunctional cascade controller recently proposed for stand-alone applications and a hybrid synchronization loop, which relies on a combination of a power synchronization loop (PSL) and a phase-locked loop (PLL). These two together make it possible to control a grid converter in either current or voltage source mode while maintaining synchronism with the grid. The effectiveness of the proposed control framework is assessed through experimental validation using a 12.5-kVA converter connected to strong and weak grids in terms of both power reference tracking and rejection of grid voltage sag. In particular, the proper performance of the system is emphasized in both tested grid configurations.
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    Low-Temperature Wafer-Level Bonding with Cu-Sn-In Solid Liquid Interdiffusion for Microsystem Packaging
    (Elsevier, 2024-03-01) Golim, Obert; Vuorinen, Vesa; Wernicke, Tobias; Pawlak, Marta; Paulasto-Kröckel, Mervi; Department of Electrical Engineering and Automation; EV Group E. Thallner GmbH; Department of Electrical Engineering and Automation
    This work demonstrates the potential use of Cu-Sn-In metallurgy for wafer-level low-temperature solid-liquid interdiffusion (LT-SLID) bonding process for microelectromechanical system (MEMS) packaging. Test structures containing seal-ring shaped SLID bonds were employed to bond silicon and glass wafers at temperatures as low as 170 °C. Scanning acoustic microscopy (SAM) was utilized to inspect the quality of as-bonded wafers. The package hermeticity was characterized by cap-deflection measurements and evaluated through finite element modelling. The results indicate the bonds are hermetic, but residual stresses limit the quantitative analysis of the hermeticity. The microstructural studies confirm the bonds contain a single-phase intermetallic Cu6(Sn,In)5 that remains thermally stable up to 500 °C. This work shows Cu-Sn-In based low-temperature bonding method as a viable packaging option for optical MEMS or other temperature-sensitive components.
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    gritulator: Grid Converter Simulator in Python
    (2024-01-30) Mourouvin, Rayane; Kukkola, Jarno; Tiitinen, Lauri; Hinkkanen, Marko; Department of Electrical Engineering and Automation; Department of Electrical Engineering and Automation
    Open-source simulation tools are gaining interest in academia as they allow to share knowledge and experience between researchers. Furthermore, they are useful instruments for educational purposes. This paper presents gritulator, an open-source Python-based simulator, for grid converters. The simulator is available in GitHub at https://github.com/Aalto-Electric-Drives/gritulator. The structure of the simulator, and selected models and control methods are introduced. To illustrate the simulator in use, two converter-control examples are given and their simulated waveforms are compared with corresponding experimental measurements.
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    A new Push-pull Class-Φ Converter for Multi-MHz Dual-frequency Wireless Power Transfer Applications
    (2023) Liu, Yining; Jayathurathnage, Prasad; Tretyakov, Sergei A.; Kyyrä, Jorma; Department of Electrical Engineering and Automation; Department of Electronics and Nanoengineering; Department of Electrical Engineering and Automation; Department of Electronics and Nanoengineering
    We propose a novel converter for simultaneous dual-frequency wireless power transfer (WPT) devices. The proposed converter realizes independent WPT channels at two frequencies: the main frequency fs and the second harmonic 2fs. With the coupled-T-network, a new class-Φ based push-pull inverter is introduced to provide load-independent currents for two transmitter coils. The power channels at two frequencies are decoupled and independent from each other, providing high flexibility for individual or simultaneous WPT at both frequency bands. The experimental WPT system has verified the characteristics of the proposed converter, realizing 64 W: 16 W power transfer for 6.78 MHz and 13.56 MHz receivers, respectively, with 80 % system efficiency.
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    Broadband miniaturized spectrometers with a van der Waals tunnel diode
    (Nature Publishing Group, 2024-01-17) Uddin, Md Gius; Das, Susobhan; Shafi, Abde Mayeen; Wang, Lei; Cui, Xiaoqi; Nigmatulin, Fedor; Ahmed, Faisal; Liapis, Andreas C.; Cai, Weiwei; Yang, Zongyin; Lipsanen, Harri; Hasan, Tawfique; Yoon, Hoon Hahn; Sun, Zhipei; Department of Electronics and Nanoengineering; Shanghai Jiao Tong University; Centre of Excellence in Quantum Technology, QTF; Zhejiang University; University of Cambridge
    Miniaturized spectrometers are of immense interest for various on-chip and implantable photonic and optoelectronic applications. State-of-the-art conventional spectrometer designs rely heavily on bulky dispersive components (such as gratings, photodetector arrays, and interferometric optics) to capture different input spectral components that increase their integration complexity. Here, we report a high-performance broadband spectrometer based on a simple and compact van der Waals heterostructure diode, leveraging a careful selection of active van der Waals materials- molybdenum disulfide and black phosphorus, their electrically tunable photoresponse, and advanced computational algorithms for spectral reconstruction. We achieve remarkably high peak wavelength accuracy of ~2 nanometers, and broad operation bandwidth spanning from ~500 to 1600 nanometers in a device with a ~ 30×20 μm2 footprint. This diode-based spectrometer scheme with broadband operation offers an attractive pathway for various applications, such as sensing, surveillance and spectral imaging.
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    High-Precision Time-to-Digital Conversion for Calibration of Outphasing Radio Transmitters
    (2023) Boopathy, Dhanashree; Cheung, Tze Hin; Spelman, Andrei; Ghosh, Agnimesh; Lampu, Vesa; Anttila, Lauri; Stadius, Kari; Kosunen, Marko; Ryynanen, Jussi; Unnikrishnan, Vishnu; Department of Electronics and Nanoengineering; Tampere University; Department of Electronics and Nanoengineering
    Wireless transceivers for 5G NR FR2 frequencies around 30 GHz support signal bandwidths up to 400 MHz to achieve ambitious data rates. The Phase Modulators (PMs) in the FR2 outphasing transmitters generates delays with delay steps of about a few hundred femtoseconds. To calibrate and linearize the PMs, time-to-digital converters (TDCs) that measure delays with higher accuracy in the order of a few femtoseconds are required. To this end, this work explores two synthesizable time interval averaging (TIA) TDCs which employ averaging to achieve a high accuracy with low-precision hardware. The results show that the delay quantization step of the converter has an effect only on the time taken to achieve the required accuracy, presenting opportunities to reduce area and power consumption. Simulation shows that a TDC with quantization step of 12.5 ns achieves an accuracy of 0.3 fs by averaging 2^28 samples. For a 32 GHz 7-bit PM producing a minimum delay step of 244 fs, this implies a TDC of 8-bit precision for each time step. The hardware synthesized towards a 22 nm FDSOI process occupies 0.0004 mm^2 area and consumes 0.3 mW power.
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    Design Pattern for Industrial Control Applications Based on One-Line IEC 61499 Adapter Connections
    (2023-10-19) Jhunjhunwala, Pranay; Ovsiannikova, Polina; Vyatkin, Valeriy; Department of Electrical Engineering and Automation; Department of Electrical Engineering and Automation
    IEC 61499 is an emerging standard for distributed automation which requires well-defined design practises to improve development efficiency. In this paper, we extend the one-line engineering design pattern and provide step-by-step guidelines on its implementation keeping in mind key features of Industry 4.0. The one-line design pattern simplifies the modification of the application structure and the addition of new components to the system without disrupting the existing control application. It also enables easy fault identification and debugging of the system. The design pattern was developed and validated in three different use cases that demonstrated the applicability of the pattern.
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    The metamorphosis of the Type Ib SN 2019yvr: late-time interaction
    (Oxford University Press, 2024-03-01) Ferrari, Lucía; Folatelli, Gastón; Kuncarayakti, Hanindyo; Stritzinger, Maximilian; Maeda, Keiichi; Bersten, Melina; Román Aguilar, Lili M.; Sáez, M. Manuela; Dessart, Luc; Lundqvist, Peter; Mazzali, Paolo; Nagao, Takashi; Ashall, Chris; Bose, Subhash; Brennan, Seán J.; Cai, Yongzhi; Handberg, Rasmus; Holmbo, Simon; Karamehmetoglu, Emir; Pastorello, Andrea; Reguitti, Andrea; Anderson, Joseph; Chen, Ting Wan; Galbany, Lluís; Gromadzki, Mariusz; Gutiérrez, Claudia P.; Inserra, Cosimo; Kankare, Erkki; Müller Bravo, Tomás E.; Mattila, Seppo; Nicholl, Matt; Pignata, Giuliano; Sollerman, Jesper; Srivastav, Shubham; Young, David R.; Consejo Nacional de Investigaciones Científicas y Técnicas; University of Turku; Aarhus University; Kyoto University; University of California Berkeley; Institut d 'Astrophysique de Paris; Stockholm University; Max-Planck-Institut für Astrophysik; Anne Lähteenmäki Group; Virginia Polytechnic Institute and State University; Chinese Academy of Sciences; INAF - Osservatorio Astronomico di Padova; European Southern Observatory Santiago; National Central University; CSIC - Institute of Space Sciences; University of Warsaw; Cardiff University; Queen's University Belfast; Universidad de Tarapacá; Department of Electronics and Nanoengineering
    We present observational evidence of late-time interaction between the ejecta of the hydrogen-poor Type Ib supernova (SN) 2019yvr and hydrogen-rich circumstellar material (CSM), similar to the Type Ib SN 2014C. A narrow H α emission line appears simultaneously with a break in the light-curve decline rate at around 80–100 d after explosion. From the interaction delay and the ejecta velocity, under the assumption that the CSM is detached from the progenitor, we estimate the CSM inner radius to be located at ∼6.5–9.1 × 1015 cm. The H α emission line persists throughout the nebular phase at least up to +420 d post-explosion, with a full width at half maximum of ∼2000 km s−1. Assuming a steady mass-loss, the estimated mass-loss rate from the luminosity of the H α line is ∼3–7 × 10−5 M yr−1. From hydrodynamical modelling and analysis of the nebular spectra, we find a progenitor He-core mass of 3–4 M, which would imply an initial mass of 13–15 M. Our result supports the case of a relatively low-mass progenitor possibly in a binary system as opposed to a higher mass single star undergoing a luminous blue variable phase.
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    Digital Polar Transmitters for Massive MIMO: Sum-Rate and Power Efficiency Analysis
    (IEEE, 2024-01-01) Xu, Guixian; Lampu, Vesa; Kosunen, Marko; Unnikrishnan, Vishnu; Ryynanen, Jussi; Valkama, Mikko; Anttila, Lauri; Tampere University; Department of Electronics and Nanoengineering; Department of Electronics and Nanoengineering
    In this article, we comprehensively investigate the potential of the digital polar radio transmitter architecture for multi-user massive multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) downlink system. In terms of throughput performance, we derive a lower bound for the average sum-rate achievable with Gaussian signaling inputs and zero-forcing (ZF) precoding based on Bussgang decomposition. By diagonal approximation, we derive an approximate, yet accurate, model for the distortion caused by uniform polar quantization, which can be used to evaluate the corresponding sum-rate in closed form. To assess the power efficiency, we provide power consumption models with realistic parameters and values for the quantized polar and Cartesian transmitters, based on state-of-the-art integrated circuit (IC) designs and measurements. Extensive numerical results demonstrate that the proposed quantized polar transmitter can enable excellent performance in terms of average sum-rate, symbol error rate (SER), and out-of-band (OOB) emission level, compared to the Cartesian architecture. Furthermore, the power consumption comparisons show that the digital polar transmitter can save more than 36% in the energy consumption under 64-antenna setting in typical 5G enhanced mobile broadband use cases, thus making it highly appealing for future power-efficient massive MIMO transmitter implementations.
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    Methods of data streaming from IEC 61499 applications to Cloud storages
    (2023) Lyu, Tuojian; Galkin, Nikolai; Liakh, Tatiana; Yang, Chen Wei; Vyatkin, Valeriy; Department of Electrical Engineering and Automation; Luleå University of Technology; Department of Electrical Engineering and Automation
    This paper presents and discusses two methods for collecting data from decentralised control applications designed in IEC 61499 architecture. The topic is justified by the growing use of Cloud-based storage and presentation services in the Internet-of-Things systems. Enabling IEC 61499 devices with the capabilities of storing data in the Cloud and using web-based data presentation and data analytics is of great practical importance. Moreover, what is the most elegant way to configure such data streaming in IEC 61499 is an open question, to solving which this paper aims to provide state-of-the-art information.
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    Mechatronic Swarm and its Virtual Commissioning
    (2023-04-17) Lyu, Tuojian; Lashchev, Andrei; Patil, Sandeep; Atmojo, Udayanto Dwi; Vyatkin, Valeriy; Department of Electrical Engineering and Automation; Luleå University of Technology; Department of Electrical Engineering and Automation
    This paper presents a swarm-based architecture for the composition of mechatronic systems from smart components complemented by a Virtual Commissioning (VC) environment. The architecture is based on the IEC 61499 standard. The proposed solution enables plug-and-play composition of the system which is ready to operate 'out of the box' right after it was composed without extra programming.
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    X- and Ku-Band SAR Backscattering Signatures of Snow-Covered Lake Ice and Sea Ice
    (MDPI AG, 2024-01) Veijola, Katriina; Cohen, Juval; Mäkynen, Marko; Lemmetyinen, Juha; Praks, Jaan; Cheng, Bin; Finnish Meteorological Institute; Department of Electronics and Nanoengineering; Department of Electronics and Nanoengineering
    In this work, backscattering signatures of snow-covered lake ice and sea ice from X- and Ku-band synthetic aperture radar (SAR) data are investigated. The SAR data were acquired with the ESA airborne SnowSAR sensor in winter 2012 over Lake Orajärvi in northern Finland and over landfast ice in the Bay of Bothnia of the Baltic Sea. Co-incident with the SnowSAR acquisitions, in situ snow and ice data were measured. In addition, time series of TerraSAR-X images and ice mass balance buoy data were acquired for Lake Orajärvi in 2011–2012. The main objective of our study was to investigate relationships between SAR backscattering signatures and snow depth over lake and sea ice, with the ultimate objective of assessing the feasibility of retrieval of snow characteristics using X- and Ku-band dual-polarization (VV and VH) SAR over freshwater or sea ice. This study constitutes the first comprehensive survey of snow backscattering signatures at these two combined frequencies over both lake and sea ice. For lake ice, we show that X-band VH-polarized backscattering coefficient ((Formula presented.)) and the Ku-band VV/VH-ratio exhibited the highest sensitivity to the snow depth. For sea ice, the highest sensitivity to the snow depth was found from the Ku-band VV-polarized (Formula presented.) and the Ku-band VV/VH-ratio. However, the observed relations were relatively weak, indicating that at least for the prevailing snow conditions, obtaining reliable estimates of snow depth over lake and sea ice would be challenging using only X- and Ku-band backscattering information.
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    A new technique for fault diagnosis in transformer insulating oil based on infrared spectroscopy measurements
    (Institution of Engineering and Technology, 2024-01-25) Darwish, Mohamed M.F.; Hassan, Mohamed H.A.; Abdel-Gawad, Nagat M.K.; Lehtonen, Matti; Mansour, Diaa Eldin A.; Benha University; Department of Electrical Engineering and Automation; Egypt-Japan University of Science and Technology; Department of Electrical Engineering and Automation
    Condition monitoring of the insulating system within power transformers has a massive importance according to the electrical utilities. Dissolved gas analysis (DGA) is frequently used for this purpose. However, DGA lacks the necessary level of accuracy to identify all equipment faults, particularly in their initial stages of degradation. Also, it does not have the capability for real-time monitoring and relies on manual sampling and laboratory testing, causing potential delays in fault identification. Additionally, the interpretation of DGA data necessitates specialised expertise, which may pose difficulties for smaller entities that have limited access to resources. Therefore, the contribution of this research is to use infrared spectroscopy measurements as a new effective technique substituting the DGA method for fault diagnosis in insulating oil. The inception faults that were considered in this study were the electrical fault (discharges of high energy) and the thermal fault (300°C < Temperature <700°C). Regarding that, two test cells were crafted especially for serving the simulation processes inside the laboratory for both types of inception faults. Subsequently, six samples of pure paraffinic mineral oil were taken to be degraded in the laboratory. Following that, all of them besides another sample that were not subjected to any kind of faults were taken to be examined by Fourier transform infrared (FTIR) spectroscopy to obtain an overview of the oil's behaviour in each fault case. After that, the FTIR analysis was initially verified utilising the DGA method. Then, for further affirmation, the dielectric dissipation factor (DDF) for all samples was measured. In the final analysis, the verification tests provide experimental evidence about the outperformance of this new optical technique in detecting the transformer's inception faults in addition to proving its potential for being a superior alternative to the well-known traditional diagnostic techniques.
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    Data-efficient Reinforcement Learning for Variable Impedance Control
    (IEEE, 2024) Anand, Akhil S.; Kaushik, Rituraj; Gravdahl, Jan Tommy; Abu-Dakka, Fares J.; Norwegian University of Science and Technology; Department of Electrical Engineering and Automation; Department of Electrical Engineering and Automation
    One of the most crucial steps toward achieving human-like manipulation skills in robots is to incorporate compliance into the robot controller. Compliance not only makes the robot’s behaviour safe but also makes it more energy efficient. In this direction, the variable impedance control (VIC) approach provides a framework for a robot to adapt its compliance during execution by employing an adaptive impedance law. Nevertheless, autonomously adapting the compliance profile as demanded by the task remains a challenging problem to be solved in practice. In this work, we introduce a reinforcement learning (RL)-based approach called DEVILC (Data-Efficient Variable Impedance Learning Controller) to learn the variable impedance controller through real-world interaction of the robot. More concretely, we use a model-based RL approach in which, after every interaction, the robot iteratively learns a probabilistic model of its dynamics using the Gaussian process regression model. The model is then used to optimizea neural-network policy that modulates the robot’s impedance such that the long-term reward for the task is maximized. Thanks to the model-based RL framework, DEVILC allows a robot to learn the VIC policy with only a few interactions, making it practical for real-world applications. In simulations and experiments, we evaluate DEVILC on a Franka Emika Panda robotic manipulator for different manipulation tasks in the Cartesian space. The results show that DEVILC is a promising direction toward autonomously learning compliant manipulation skills directly in the real world through interactions. A video of the experiments is available in the link: https://youtu.be/_uyr0Vye5no
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    Towards migration from IEC 61131-3 to IEC 61499 in process industry: Redesign of visualisation
    (2023) Kothalawala, Hiruni; Jhunjhunwala, Pranay; Vyatkin, Valeriy; Information Technologies in Industrial Automation; Department of Electrical Engineering and Automation; Department of Electrical Engineering and Automation
    This work focuses on reengineering an HMI implemented in a third-party legacy tool to an IEC 61499 implementation. We propose a method to re-engineer the view for a process system and gather relevant information for the migration of the controller to IEC 6499. The proposed methodology has been tested and demonstrated using a lab-scale 'Water process system' that is currently controlled using an IEC 61131-3 compliant PLC with a view and HMI developed on a legacy tool. It is intended to migrate to the IEC 61499-compliant EcoStruxure Automation Expert (EAE) tool.
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    Robust On-Body Antenna Based on Discretized Planar Surfaces
    (2023) Kormilainen, Riku; Langi, Konsta; Lehtovuori, Anu; Viikari, Ville; Department of Electronics and Nanoengineering; Department of Electronics and Nanoengineering
    This paper presents a novel on-body antenna consisting of discretized metallic patch elements connected with lumped components. The antenna is designed to on-body application, and it could be placed, e.g., to the sling of the backpack. Since most of the human body is composed of water, it has a significant effect on antenna performance. The main effects include detuning of the antenna and a deteriorating radiation efficiency. The proposed antenna inherently resists the detuning effects as compared to a conventional antenna. Therefore, the antenna provides a robust operation in various user scenarios. The details of the structure and element values are optimized with a readily available method that enables the optimization of lumped components to improve the total efficiency. The results show that a 400 MHz bandwidth can be achieved with reasonable efficiency.
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    Accurate Estimation of non-Resonant Far-Field Superresolution by a Glass Microparticle
    (2023) Simovski, Constantin R.; Heydarian, Reza; Department of Electronics and Nanoengineering; Department of Electronics and Nanoengineering; Jaworski, Marek; Marciniak, Marian
    In this paper, we theoretically study the spatial resolution granted by a glass microsphere to two pointwise dipoles separated by a tiny gap and located on the sphere surface. This resolution is considered via parameters of the so-called virtual sources effectively shaped by the microparticle of the radiation of the real sources. The geometrical optics qualitatively explains these virtual sources, but only full-wave simulations give a reliable information of their location and sizes. We developed a method for finding these sources from COMSOL simulations. The virtual source is defined as the waist of the wave beam obtained from the imaging beam by its exact inversion performed at a very large distance from the microparticle. We have obtained both pessimistic and optimistic estimates for the ultimate resolution. We found that the novel scenario of the microparticle imaging theoretically revealed in our previous work, promises much finer resolution than the conventional scenario.
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    On the Relationship between Ultra Low Frequency Oscillation and Power System Inertia
    (2023) Seppänen, Janne; Lehtonen, Matti; Kuivaniemi, Mikko; Haarla, Liisa; Department of Electrical Engineering and Automation; Fingrid Oyj; Department of Electrical Engineering and Automation
    In recent years, ultra low frequency oscillation (ULFO) has been observed in various power systems. In the ULFO phenomenon, the frequency of the system continuously slowly oscillates around the nominal value. Such oscillation exists also in the Nordic power system, where the frequency of the system oscillates around the nominal 50 Hz value with a period of around 40 to 90 s. Such oscillation deteriorates the frequency quality of the system, might cause unnecessary control actions and wear and tear of turbines, and potentially even risk the frequency stability of the system. This paper analyses the characteristics of the ULFO during the latest seven years in the Nordic power system. The paper shows, based on measurements, that the ULFO has a relationship with the inertia of the system. More specifically, system inertia is clearly correlated with the frequency of the ULFO. As the inertia of the system increases, the frequency of the ULFO decreases, and vice versa. Furthermore, the amplitude of the ULFO is also correlated with inertia and smaller inertia leads to higher amplitude of the ULFO. These findings give important insight into the characteristics of ULFOs and similar analysis is also recommended for other power systems experiencing an ULFO phenomenon.