[article-cris] Sähkötekniikan korkeakoulu / ELEC

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  • Comparison and Optimization of SLID Bonding for Die-attach on Various Substrate Types in Power Module Packaging Applications
    (2025) Liu, Shenyi; Vuorinen, Vesa; Schuh, Jannis; Albrecht, Jurgen; Paulasto-Krockel, Mervi
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    As advancing power electronic modules increasingly operate at higher power densities, thermally and thermomechanically stable die-attach materials are required to ensure their thermal performance and reliability. This study conducted finite element (FE) simulations and substrates characterization, followed by low-temperature (LT) solid-liquid interdiffusion (SLID) bonding experiments. FE simulations qualitatively compared the thermal performance of Sn-Ag-Cu (SAC) solder, sintered Ag, and Cu-Sn SLID bonds, while additionally analyzing thermomechanical stress development during Cu-Sn SLID cooling phases. Three types of substrates were characterized in terms of warpage and surface roughness. Additionally, Cu-Sn-In LT SLID bonding process was applied to achieve thin die-attach layers. Dummy Si chips of varying sizes were bonded to the direct copper bonding (DCB) and active metal brazing (AMB) substrates, with bonded area percentage quantified by scanning acoustic microscope (SAM). Furthermore, large-sized functional insulated-gate bipolar transistor (IGBT) chips achieved up to 93% bonded area on insulated metal substrates (IMSs) with the LT SLID die-attach process. Microstructure analysis confirmed stable phase and reduced crack formation located at the corners of the LT bonded samples. The simulation and experimental results demonstrate that LT SLID bonding enables a thin die-attach layer with low thermal resistance and high melting point. However, the substrate technology improvements on warpage and surface roughness reduction remain necessary for implementing the new die-attach method in novel power modules.
  • Parallel Kalman Smoothing Augmented Lagrangian Method for Constrained State Estimation
    (2025) Han, Qingwang; Gao, Rui; Särkkä, Simo
    A4 Artikkeli konferenssijulkaisussa
    This paper presents a novel parallel computing framework for solving constrained state estimation problems. The proposed methodology integrates an augmented Lagrangian formulation with a parallel Kalman filter and smoother implementation, specifically designed to optimize the primal update step through efficient parallel computation. Experimental results demonstrate that our approach provides a significant computational time improvement while maintaining the estimation accuracy.
  • Insights into jet–NLR energetics in PMN J0948+0022
    (2026) Dalla Barba, B.; Foschini, L.; Berton, M.; Lähteenmäki, A.; Tornikoski, M.; Sani, E.; Crepaldi, L.; Congiu, E.; Venturi, G.; Hon, W. J.; Vietri, A.
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The analysis of the optical spectra of PMN J0948+0022 showed significant variations in the spectral lines that, when combined with the Fermi γ-ray light curve and radio observations reported by other authors, were interpreted as the result of interactions between the relativistic jet and the narrow-line region (NLR). In this work, we present order-of-magnitude calculations of the energetics associated with this proposed jet-NLR interaction. We demonstrate that the observed outflows are capable of absorbing a fraction of the jet energy and converting it into kinetic energy. This mechanism provides a natural explanation for the optical spectral variability recorded with the X-shooter and Multi-Unit Spectroscopic Explorer (MUSE) instruments. Our results support the scenario in which feedback from the relativistic jet can dynamically influence the circumnuclear gas, offering new insights into the coupling between jets and the NLR in γ-ray-emitting narrow-line Seyfert 1 galaxies.
  • Dynamic Hydrogen Injection in Integrated Electricity-Gas Systems: A PDEs-Embedded Flexible Operation Strategy
    (2026) Zhai, Junyi; Gao, Xianhui; Wang, Sheng; Li, Zhengmao; Wang, Jianxiao
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Integrated electricity-gas systems (IEGS) with hydrogen injection has emerged as a crucial pathway to decarbonize energy systems. By coordinating the flexibility of the gas system and hydrogen blending, the accommodation of large-scale renewable energy integrated in power systems can be further improved. However, the time-varying nature of dynamic hydrogen injection, influenced by stochastic renewable energy, can lead to fluctuations in gas concentrations in the gas network, threatening the secure operation of IEGS. This paper focuses on the low-carbon flexible operation strategy for IEGS with dynamic hydrogen injection. First, a flexible operation strategy of hydrogen-mixed gas turbine is developed with a detailed correlation characterization of carbon emission, combustion thermoldynamics, and chemical reaction kinetics under dynamic hydrogen ratios. Second, an optimal IEGS dispatch strategy is formulated in which the joint dynamics of the system component and the gas flow is captured to accurately track the time-varying concentrations of hydrogen in the gas network. Third, a set of discretized partial differential equations (PDEs) is utilized to model mixed gas flows, leading to a PDEs-constrained optimization model. Finally, Taylor series expansions for PDE linearization and McCormick envelope for bilinear terms are employed, enabling tractable algebraic representations. A sequential linear programming (SLP) algorithm with adaptive penalty factors is developed to drive relaxation tightening more efficiently. Numerical results on a 24-bus-20-node system and a practical 197-bus-171-node system in Northwest China illustrate the effectiveness of the proposed model.
  • Bandwidth Enhancement of Sequential Rotation Arrays based on Characteristic Mode Inspired Dual-Resonant LP Element
    (2026) Pang, Cheng; Wang, Qiming; Sun, Mingyuan; Wang, Yuxuan; Xu, Xinyuan; Wang, Yibo; Yue, Ziang; Wang, Yinghan; Wang, Yuzhong; Sihvola, Ari; Qi, Jiaran
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This communication presents a compact, broadband dual-resonant linearly polarized (LP) element aiming at enhancing the bandwidth of sequential rotation arrays (SRAs). The proposed LP element consists of a center-fed split-ring resonator (SRR) microstrip patch and a C-shaped capacitive slot loading. Characteristic mode (CM) analysis shows that the proposed LP element can excite two TM20 modes in the LP state, resulting in broadband radiation performance across the X-band. Furthermore, by applying different sequential rotations and excitation phases to the element, it maintains broadband radiation performance, making it an ideal building block for SRAs and thus enhancing the array's operating bandwidth. To validate the proposed method, a 16 × 16 SRA prototype for dual-circularly-polarized radiation is designed and fabricated using the proposed LP element. Experimental results demonstrate that the prototype achieves a joint 3-dB gain and axial ratio bandwidth exceeding 46.85% for right-hand circularly polarized (RHCP) radiation and 51.1% for left-hand circularly polarized (LHCP) radiation.
  • Event-Triggered Particle Filter with One-Step Randomly Delayed Measurement
    (2025) Gasmi, Elhadi; Kumar, Kundan; Sid, Mohamed Amine; Särkkä, Simo
    A4 Artikkeli konferenssijulkaisussa
    In this paper, we develop an event-triggered remote state estimator for nonlinear state-space models under network-induced one-step randomly delayed measurements. Adopting event-triggering strategies reduces the transmission burden between the sensor and the estimator while maintaining the estimation accuracy. The developed method employs a particle filter to approximate the posterior distribution using particles and weights. In the non-triggering case, we use the constrained Bayesian estimation to compute the integrals associated with the posterior distribution. We evaluate the performance of the proposed algorithm using a simulated aircraft tracking problem. The results show that the proposed algorithm provides a comparable estimation accuracy to a particle filter without event triggering.
  • Approach to beamforming minimizing the signal power estimation error
    (2025) Ollila, Esa; Mestre, Xavier; Raninen, Elias
    A4 Artikkeli konferenssijulkaisussa
    We study the properties of beamformers, particularly Capon and MMSE beamformers, in their ability to obtain the true signal power of the signal of interest (SOI). A curious feature of these beamformers is their tendency to either overestimate or underestimate the signal power. Consequently, they are not asymptotically unbiased (as the sample size approaches to infinity). To address this issue, we propose to shrink the Capon beamformer by finding a scaling factor that minimizes the mean squared error (MSE) of the signal power estimate. The new beamformer, referred to as the Capon+ beamformer, is evaluated against the Capon and MMSE beamformers in terms of bias, signal power MSE, and signal waveform MSE. The Capon+ beamformer demonstrates a superior balance in both signal power and waveform estimation while exhibiting minimal bias, which approaches zero as the sample size increases.
  • Bridging the Gap in Children’s Speech Recognition: Zero-Speech Approaches with Speech Modifications and ASR architectures
    (2025) Sinha, Abhijit; Singh, Mittul; Kadiri, Sudarsana Reddy; Kathania, Hemant Kumar; Kurimo, Mikko
    A4 Artikkeli konferenssijulkaisussa
    Pretrained end-to-end (E2E) automatic speech recognition (ASR) models, such as Wav2Vec2, HuBERT and WavLM, have achieved near-human performance on adult speech in zero-resource settings. However, their performance in children’s speech remains poor in zero-resource scenarios. To substantially improve performance in children ASR fine-tuning with little in-domain data is required, which might be untenable given the lack of labeled data. In this context, we wonder how without using children’s speech can we bridge the performance gap? In this work, we address this challenge by (1) reviewing modifications applicable in zero-resource scenarios, (2) leveraging in-domain text resources for adaptation, and (3) comparing both E2E ASR architectures and hybrid HMM/DNN Kaldi-based systems. Our observations serve as important takeaways for building children ASR with minimal resources.
  • Analog Sun Sensor Array: 4 Quadrant Sensor for Missions Beyond LEO
    (2026-02-06) Anger, Marius; Nyman, Samuli; Fetzer, Anton; Praks, Jaan
    A4 Artikkeli konferenssijulkaisussa
    The Foresail-2 mission demands an enhanced radiation-tolerant sun sensor to withstand the high-radiation environment encountered in its Geostationary Transfer Orbit (GTO). The Analog Sun Sensor Array (ASSA) is developed to fulfill this requirement, providing attitude determination with an accuracy better than 1°. Traditional analog sun sensors, reliant on position-sensitive diodes (PSDs), encounter challenges such as distinguishing Albedo and celestial bodies like the Moon from the Sun, resulting in an in-orbit accuracy limitation of approximately 5°. ASSA utilizes four individual PSDs, each housed in a separate chamber equipped with its own pinhole to expand the field of view, thereby overcoming previous limitations. This design has built in triple hot redundancy. By using a STM32 controller and sufficient shielding the sensor can be used in high radiation environments.
  • FilMBot: A High-Speed Soft Parallel Robotic Micromanipulator
    (2026) Yu, Jiangkun; Bettahar, Houari; Kandemir, Hakan; Zhou, Quan
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Soft robotic manipulators are generally slow despite their great adaptability, resilience, and compliance. This limitation also extends to current soft robotic micromanipulators. Here, we introduce FilMBot, a 3-DOF film-based, electromagnetically actuated, soft kinematic robotic micromanipulator achieving speeds up to 2117 °/s and 2456 °/s in α and β angular motions, with corresponding linear velocities of 1.61 m/s and 1.92 m/s using a 4-cm needle end-effector, 0.54 m/s along the Z axis, and 1.57 m/s during Z-axis morph switching. The robot can reach ∼1.50 m/s in path-following tasks, with an operational bandwidth below ∼30 Hz, and remains responsive at 50 Hz. It demonstrates high precision (∼6.3 μm, or ∼0.05% of its workspace) in path-following tasks, with precision remaining largely stable across frequencies. The novel combination of the low-stiffness soft kinematic film structure and strong electromagnetic actuation in FilMBot opens new avenues for soft robotics. Furthermore, its simple construction and inexpensive, readily accessible components could broaden the application of micromanipulators beyond current academic and professional users.
  • Mixtures of Gaussian Process Experts with SMC^2
    (2025-10) Harkonen, Teemu; Wade, Sara; Law, Kody; Roininen, Lassi
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Gaussian processes are a key component of many flexible statistical and machine learning models. However, they exhibit cubic computational complexity and high memory constraints due to the need of inverting and storing a full covariance matrix. To circumvent this, mixtures of Gaussian process experts have been considered where data points are assigned to independent experts, reducing the complexity by allowing inference based on smaller, local covariance matrices. Moreover, mixtures of Gaussian process experts substantially enrich the model's flexibility, allowing for behaviors such as non-stationarity, heteroscedasticity, and discontinuities. In this work, we construct a novel inference approach based on nested sequential Monte Carlo samplers to simultaneously infer both the gating network and Gaussian process expert parameters. This greatly improves inference compared to importance sampling, particularly in settings when a stationary Gaussian process is inappropriate, while still being thoroughly parallelizable.
  • Statistical Linear Regression Approach to Kalman Filtering and Smoothing under Cyber-Attacks
    (2025) Kumar, Kundan; Iqbal, Muhammad; Särkkä, Simo
    A4 Artikkeli konferenssijulkaisussa
    Remote state estimation in cyber-physical systems is often vulnerable to cyber-attacks due to wireless connections between sensors and computing units. In such scenarios, adversaries compromise the system by injecting false data or blocking measurement transmissions via denial-of-service attacks, distorting sensor readings. This paper develops a Kalman filter and Rauch-Tung-Striebel (RTS) smoother for linear stochastic state-space models subject to cyber-attacked measurements. We approximate the faulty measurement model via generalized statistical linear regression (GSLR). The GSLR-based approximated measurement model is then used to develop a Kalman filter and RTS smoother for the problem. The effectiveness of the proposed algorithms under cyber-attacks is demonstrated through a simulated aircraft tracking experiment.
  • Comparative Measurement Study of 12-Pulse Thyristor and 2-Level Voltage Source Converter in PEM Water Electrolyzer Test Setup
    (2025) Vuorsalo, Simo; Nikiforow, Kaj; Hiltunen, Jere; Kauhanen, Matti; Niemelä, Markku; Kyyrä, Jorma
    A4 Artikkeli konferenssijulkaisussa
    The need for green hydrogen has created the need to study water electrolysis process in more detail. Current state-of-the-art electrification solutions are thyristor-transformer rectifier systems and the electrolysers are typically connected in parallel or two in series with middle point grounding. In this paper, the electrification and the rectifier is studied. It is imperative to try to increase the voltage instead of current as it is known from the field of power electronics that increasing current increases costs. Therefore, in this paper the parallel and serial connections of PEM electrolyser stack system in the same system is compared with commercial thyristor and PWM-based active rectifiers. The focus is on the electrical quantities and Specific Energy Consumption (SEC). Hydrogen production in both connection methods is measured and reported.
  • Spatio-Temporal Coordinated Operation Strategy of Data Centers Considering Virtual Storage System via Two-Stage Distributionally Robust Optimization
    (2026) Qin, Xuexue; Li, Zening; Li, Zhengmao; Xue, Yixun; Chang, Xinyue; Su, Jia; Jin, Xiaolong; Wang, Peng; Sun, Hongbin
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Energy consumption in data centers is growing rapidly in recent years. To realize the economic and flexible operation of data centers, this paper proposes a spatio-temporal coordinated operation strategy of data centers considering virtual storage system (VSS) via two-stage distributionally robust optimization. First, based on the dynamic thermal network of buildings and differentiated demands of cloud users in data centers, a comprehensive operation model with VSS is proposed. Then, considering uncertainties of general load demands from data centers and outdoor temperatures, a fuzzy set is deduced, using an imprecise Dirichlet model derived from historical data. This ensures confidence level satisfaction for uncertain sets of general load and outdoor temperatures. In addition, the spatio-temporal coordinated operation strategy of data centers considering virtual storage system via two-stage distributionally robust optimization that flexibly adjusts conservativeness through uncertainty regulation parameters. Finally, a Dynamic Accuracy Column and Constraint Generation (DA-C&CG) algorithm is developed for solving the proposed strategy. The results indicate that the proposed strategy can effectively enhance operational economics, computing efficiency and resilience by leveraging flexibility in cooling and cloud user loads within suitable server temperatures. Meanwhile, the DA-C&CG algorithm exhibits excellent solution performance.
  • Virtual Tethered Autonomous Underwater Vehicle for Real-time Underwater Internet of Things
    (2026) Weng, Yang; Chun, Sehwa; Sekimori, Yuki; Yokohata, Hiroki; Matsuda, Takumi; Pajarinen, Joni; Maki, Toshihiro
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The establishment of high-speed optical links between marine platforms for underwater internet of things (UIoT) applications is a rapidly evolving research area. This study investigates UIoT connectivity between mobile marine platforms, focusing on high-definition data retrieval from the ocean. We propose a virtual tethered autonomous underwater vehicle (VTAUV) system, which integrates the advantages of both tethered and untethered architectures to enhance real-time underwater communication. The core of our approach is an acousto-optic tethering method, which enables stable, high-speed optical links between mobile platforms. By leveraging acoustic ranging and a reinforcement learning-based tracking controller, the VTAUV effectively follows an autonomous surface vehicle (ASV). A multi-beam sonar system and a real-time processing algorithm accurately estimate pointing errors, ensuring robust optical link establishment for continuous data transmission. We validate our method through sea trials, demonstrating that the VTAUV can successfully maintain acousto-optic tethering while significantly reducing pointing errors with a moving ASV. These findings mark an important step toward reliable, high-bandwidth underwater communications, paving the way for future scalable UIoT deployments.
  • Ergonomic design of an intelligent robotic walker based on user perception and needs
    (2025-07) Segui-Verdu, Isabel; Dunai, Larisa; Vyatkin, Valeriy; Turcanu, Dinu
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This article presents the development of a robotic walker designed from an ergonomic perspective, focused on users' specific needs. The design integrates a height-adjustable modular system, a rectangular frame with four wheels to ensure stability and manoeuvrability, and a vertical folding mechanism for easy transport. Key components were optimized, such as ergonomic handles with support and grooves for improved grip, an anatomical seat made of memory materials, a durable fabric backrest, and a sports bag-type basket attached by hooks. The 24 mm front wheels are spring-loaded to reduce vibrations, while the 20 mm motorized rear wheels have a non-slip surface for overcoming obstacles. The design incorporates safety features such as reflective strips, integrated lights, and advanced sensors (pressure, TOF, IMU), enhancing user comfort, functionality and safety. This comprehensive approach represents a significant advance in the ergonomics and functionality of robotic walkers.
  • Decentralized Stochastic Optimal Power Flow Problem Considering Prohibited Operating Zones and Renewables Sources
    (2025) Yamaguti, Lucas Do Carmo; Home-Ortiz, Juan M.; Pourakbari-Kasmaei, Mahdi; Mantovani, Jose Roberto Sanches
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The traditional Optimal Power Flow (OPF) problem is formulated in a centralized manner assuming a single operator manager has full access to the system information. However, transmission power systems often consist of interconnected areas controlled by multiple regional operators who can only access local information and must coordinate with neighboring areas, sharing limited data like voltage magnitude and angle at tie-lines. In this work, the decentralized OPF problem is extended by including prohibited operational zones (POZ) constraints of thermoelectrical units and formulated as a mixed-integer nonlinear programming model. Uncertainties in load behavior and renewable energy sources are addressed using a stochastic scenario-based approach. A matheuristic algorithm based on the variable neighborhood descent heuristic method is used to handle the integer variables. The proposed model and solution technique are applied in the IEEE 118-bus system, considering the local weather conditions. The obtained results demonstrate the good quality and performance of the proposed model and solution technique compared with the solution of the OPF problem considering a centralized approach.
  • Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations
    (2026-02-01) Ramakrishnan, Venkatessh; Savolainen, Tuomas; Wiik, Kaj; , Event Horizon Telescope Collaborat
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the significantly enhanced (u,v) coverage in the 2021 Event Horizon Telescope (EHT) observations. The integration of the 12-m Kitt Peak Telescope (USA) and NOEMA (France) stations into the array introduces two critical intermediate-length baselines to SMT (USA) and IRAM 30-m (Spain), providing sensitivity to emission structures at spatial scales of ~250 μas and ~2500 μas (~0.02 pc and ~0.02 pc). Without these new baselines, previous EHT observations of the source in 2017 and 2018 lacked the capability to constrain emission on large scales, where a "missing flux"of order ~1 Jy is expected to reside. To probe these scales, we analyzed closure phases-robust against station-based gain calibration errors-and model the jet base emission using a simple Gaussian component offset from the compact ring emission at spatial separations > 100 μas. Our analysis revealed a Gaussian feature centered at (ΔRA ~ 320 μas, ΔDec. ~ 60 μ as), projected separation of ~5500 AU, with an estimated flux density of only ~60 mJy, implying that most of the missing flux identified in previous EHT studies had to originate from different, larger scales. Brighter emission at the relevant spatial scales is firmly ruled out, and the data do not favor more complex models. This component aligns with the inferred position of the large-scale jet and is therefore physically consistent with the emission of the jet base. While our findings point to detectable jet base emission at 230 GHz, the limited coverage provided by only two intermediate baselines limits our ability to robustly reconstruct its morphology. Consequently, we treated the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate baseline coverage will be essential to constrain the structure and nature of this component with higher precision.
  • Arch bridge-inspired solar evaporator with GO/MXene heterostructure for highly efficient and salt-resistant desalination
    (2025-12-01) Zhong, Pengjie; Hu, Jinhua; Hu, Qinglong; Yao, Xin; Shang, Manyu; Huang, Youqiang; Bai, Gongxun
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The inadequate supply of fresh water in remote and developing regions has spurred growing demand for cost-effective and efficient desalination technologies. Solar-powered interfacial evaporation has emerged as an environmentally friendly desalination method. However, salt precipitation on the evaporation surface largely affects the efficiency and continuity of desalination. In general, salt tolerance is achieved at the cost of substantial heat loss, leading to an inherent trade-off between salt tolerance and thermal localization. Herein, we report a water-bridging solar evaporator (denoted as MXG), where a thin water layer is lifted by capillary forces to form a bridge-like architecture. Notably, the water bridge reduces the diffuse reflection of sunlight and inhibits heat dissipation to the bulk water, while endowing the evaporator with salt resistance. Through a graphene oxide (GO)-assisted process, 2D Ti3C2Tx is cleverly assembled into a 3D bridge device that serves as a stand-alone solar-driven interfacial evaporator for efficient and comprehensive water purification. The MXG evaporators achieves an evaporation rate of 3.14 kg·m−2·h−1 under one sun illumination, surpassing that of many previously reported counterparts. In addition, the MXG evaporator can effectively carry out seawater desalination and wastewater treatment. Our strategy provides an avenue for designing high-performance solar evaporators through the synergistic regulation of water transport, light management, and thermal confinement.
  • Comparative Experiment of Conventional and Dynamic Theories for Wireless Power Transfer
    (2025) Dat Pham, Chi; Ha-Van, Nam; Thuy Le, Minh; Nguyen, Kien; Sekiya, Hiroo
    A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This letter presents a comparative experiment of conventional and dynamic theories for wireless power transfer (WPT) systems. The dynamic theory considers the effect of the intermediate and far-fields on system operation, while it is neglected in the conventional theory. Theoretical analyses reveal that conventional theory cannot accurately predict the system behavior at high frequencies, where the transfer distance is comparable to the wavelength. In contrast, the dynamic theory can be applied over a wide frequency range to determine the optimal frequency. Experimental results demonstrate that a maximum power transfer efficiency (PTE) of 70.4% can be achieved at an optimal frequency of 29.7 MHz for a transfer distance of 3.5 times the antenna radius by applying the dynamic theory. Matching circuits were designed for source and load impedance at the optimal frequency. The system with matching circuits exhibits an overall efficiency of 44% at the same transfer distance.