ABSTRACT. In this study a transient finite element model was developed to examine the temperature evolution of continuous casting blooms during reheating prior to rail rolling. The simulation was carried out using COMSOL Multiphysics, incorporating convective and radiative heat transfer mechanisms under a three-zone furnace (preheating, heating and soaking) temperature schedule. The temperature distribution and soaking uniformity were evaluated over a 7200 s heating cycle. Results indicate that proper adjustment of furnace setpoints enables the bloom center to reach approximately 1220 °C while maintaining acceptable temperature uniformity ∆T≤50 ℃. The study shows how numerical modeling can be used to improve thermal homogeneity prior to hot rolling and optimize reheating furnace performance.

ABSTRACT. This paper aims to provide a numerical estimate of the capacity of a battery in a photo-voltaic system. The battery is calculated numerically based on the current operating state of the load in the grid. This evaluation helps the designer to use the appropriate battery capacity. In the case of a large capacity, part of the battery will not be used, which reduces the efficiency of the photovoltaic design and operation. A small battery capacity will limit the operational use of the photovoltaic system. The analytical assessment and recommendations for the battery condition are made using real data from the operation of the photovoltaic system.

ABSTRACT. This paper considers an optimal energy management problem for a real electric vehicle charging system with photovoltaic power generation, stationary battery energy storage, and an electrical grid. A linear optimization model is formulated with the objective of minimizing the cost of electricity purchased from the grid while satisfying the technical constraints of system components and the energy requirements of electric vehicles. Vari-ous parameters of the system have been evaluated when changing the main variables. From the analysis and comparison of the results, a conclusion has been drawn about re-source savings with appropriate management of energy capacities.

ABSTRACT. Air duct contamination in HVAC systems significantly affects the air quality in buildings and is a major factor in the reduced energy efficiency of the systems. Significant accumulation of dust and other contaminants increases the aerodynamic resistance of the air ducts, which leads to increased pressure drop and reduced heat exchange and air flow, respectively - to increased electricity consumption. This study is a systematic analysis of the causes of contamination in HVAC systems and its impact on the healthy environment in buildings, changes in operating characteristics and energy efficiency. We examined the main types of contaminants - physical, biological, chemical, as well as the mechanisms for their appearance and accumulation. Special attention is paid to methods for inspection and diagnosis of the degree of contamination, which helps facility managers decide in which direction to build the monitoring of HVAC systems. The assessment used five criteria for each of the ten studied methods - Measurement Reliability, Practical Applicability, Inspection Speed, Cost Efficiency, Diagnostic Value. The results show that Optical camera inspection with image processing and Pressure drop measurement are the two methods with the highest score, based on accuracy, speed and applicability.

ABSTRACT. This research explores the application of deep learning architectures to model the relationship between standard ventilator parameters and the resulting mechanical power. This study utilized a one-dimensional (1D) convolutional neural network (CNN) with Bidirectional LSTM layers, and the calculated mechanical power from the equations was used to verify the results. The model achieved a mean absolute error (MAE) of 0.0094 and a Root Mean Squared Error (RMSE) of 0.186. The mean absolute percentage error (sMAPE) was 3.26%, confirming the model’s robustness. The scatter plot shows a near-perfect linear alignment along the identity line, suggesting that this model effectively captures the underlying physical relationship between ventilator parameters and mechanical power in PCV.

ABSTRACT. Hysteresis defines a system’s state as a function of its historical trajectory, manifesting as path-dependent loops where variables diverge based on the direction of change. Intrinsic to both information storage and irreversible thermodynamic dissipation, this phenomenon serves as a critical diagnostic fingerprint for characterizing behaviors and dynamic system responses. The automated classification of dense hysteresis signals is critical for control systems and bio-signals. This study presents a comprehensive comparison between two manifold learning pipelines for clustering 7,736 cycles from patients that have undergone invasive mechanical ventilation. Methodology utilizes a extraction of 27 geometric and spectral features, reduced via Principal Component Analysis (PCA) to be forwarded to powerful clustering algorithms. Methodology II proposes a Deep Learning (DL) framework utilizing a 1D-Convolutional Autoencoder (CAE) for automated feature discovery. Both methods utilize UMAP and HDBSCAN for the final clustering stage. While the classical approach maintains a high Trustworthiness score (0.9841), the CAE approach significantly improves cluster separation, achieving a Silhouette score of 0.6510 and a Davies-Bouldin index of 0.4982.

ABSTRACT. Smart campus development is increasingly associated with the combined use of IoT technologies, artificial intelligence, cloud infrastructures, and large-scale data analytics in higher education. Despite this progress, many existing data models are not well-suited to the educational domain, particularly when interoperability and real-time analytical capabilities are required. To address this limitation, the study proposes a Smart Campus Education Data Model (SCEDM), which can be integrated into any FIWARE-based platform. The model is organized as a layered architecture that includes data acquisition, processing, and storage; analytics and decision support; application presentation; and security. The proposed model is not presented only at a conceptual level, it is also validated in a containerized FIWARE environment built around the Orion-ld Context Broker and NGSI-ld specifications. The SCEDM model is validated in a system that supports real-time state management across multiple campus domains. The model's practical operation is validated across several experimental scenarios, including a simulation of a lecture process, classroom occupancy monitoring, and automated notifications to external platforms. In addition, the study compares five international case studies from different contexts.

ABSTRACT. Governments have a pivotal role in the realm of AI. In order to achieve success within the context of the AI revolution, it is essential to possess robust political intentions. The capacity to exert influence over these entities is predicated on the formulation of policies, rules, and incentives. Furthermore, the capacity to establish collaborative relationships with private enterprises is a strategic asset that facilitates the realization of their objectives. In the global competition to develop AI and data-driven businesses, the EU's investments in public computing infrastructure are a key part of encouraging AI innovation across Europe.

ABSTRACT. This paper presents an experimental framework for synchronizing multi-modal physiological data in a real-world driving environment. Research-grade sensors (Car-dioBAN, respiBAN) were integrated with consumer wearables (Huawei Watch D2, Oura, and Xmart smart rings) to monitor driver heart rate (HR) and respiration rate (RR). A custom MATLAB-based workflow was developed to align disparate data streams, using a nearest-neighbor principle to ensure temporal accuracy. The setup was validated through 60-minute driving sessions, successfully correlating physiological responses with video feeds. Our results demonstrate that, with proper synchronization, consumer wearables can be compared with precise research-grade equipment for continuous driver state monitoring.

ABSTRACT. This paper presents a fractal–multiscale supervisory control strategy for a digitally controlled DC–DC buck converter. A conventional PI controller is augmented with a supervi-sory layer that adaptively modulates the effective control gains based on multiscale error dynamics and oscillation indicators derived from the error and its time derivative. In ad-dition, automatic PI shaping using notch and lead compensators is performed through FFT-based identification of dominant oscillatory modes. The proposed approach is evaluated under load, input voltage, and combined disturbances, as well as robustness and stress-test scenarios. Simulation results demonstrate improved transient performance, reduced oscillatory behavior, and enhanced robustness compared to the baseline PI control.

ABSTRACT. This paper presents a frequency-domain harmonic and modal analysis of a four-bus Zeta DC–DC converter, targeting the identification of resonance phenomena up to 200 MHz. The method is based on nodal admittance modeling and eigenvalue decomposition of the impedance matrix, enabling extraction of modal impedance and dominant resonance modes. Participation factors and sequential current injection at each bus are employed to evaluate spatial sensitivity and voltage amplification under different excitation scenarios. A two-stage frequency sweep, combining a coarse global scan with fine local refinement around detected peaks, ensures efficient and accurate resonance characterization. Simulation results demonstrate strong dependence of resonance severity on the injection location and highlight the dominant contribution of specific modes and reactive elements. The proposed framework provides physical insight and supports resonance-aware design of power electronic converters.

ABSTRACT. A method is proposed for determining the reliability of electrical machines of the most widely used types during their design stage and for verifying this reliability through testing of the components intended for them. The method is invariant with respect to the type of machine and its operating mode. Particular attention is given to its practical application in real engineering practice.

ABSTRACT. An invariant method is proposed for determining the expediency of implementing the structure and organization of an information system (IS). The method is based on determining time losses occurring during the execution of information processes and on assessing the functional reliability of the system with respect to the performed tasks, taking into account the reliability characteristics of the corresponding IS components.

ABSTRACT. The paper presents a simulation and experimental study of a three-phase 12/8 Switched Reluctance Motor (SRM) operating in motoring mode. The operation of the asymmetric bridge converter is analyzed, and the mathematical equations describing the phase-current change under various commutation states in soft-switching mode (modulation of the upper transistors) are derived. An analytical model is used to examine the energy exchange between the battery, the power switches, and the phase inductance. For the purposes of the study, a simulation model was developed in MATLAB/Simulink environment, including models of the battery, the power converter, and the SRM. Simulation studies were conducted under various load conditions and phase current values, yielding time-domain waveforms of the phase currents and voltages, as well as the electromagnetic torque. Experimental waveforms of the phase current and voltage, measured under conditions corresponding to those in the simulation studies, are presented. A comparative analysis was performed between the simulation and experimental results, with tabular and graphical dependencies presented, and the relative error between them determined. The results obtained show good agreement between the simulation model and the experimental setup, which confirms the applicability of the developed approach for the analysis and optimization of SRM drives.

ABSTRACT. The paper presents a review of research on the time mutability method for stochastic objects drift velocity measurement. The method models one-dimensional signals describing the movement of cloud structures with different types of statistical heterogeneity, without considering their evolution in the measurement interval. The main study shows the dependence of the accuracy of the measured speed V ⃗ results on the number of recorded images, the type of statistical heterogeneity of the fields examined, and the measurement conditions. The obtained results are graphically illustrated.

ABSTRACT. The paper presents an analytical study and small-signal modeling of a non-inverting two-switch buck–boost converter based on the LM5118 controller, accounting for parasitic elements. The converter dynamics were analyzed using simulations in the PLECS® environment. Furthermore, the steady-state and dynamic characteristics of the LM5118-based two-switch buck–boost converter were examined using the PSpice for TI® simulator for different input voltages and load conditions. The obtained simulation results show close correspondence with the analytical analysis, confirming the validity of the proposed approach.

ABSTRACT. Rapid expansion of renewable weather-dependent electricity generators has created a challenge in the way grid operators manage electricity dispatch. This article explores the core challenges of managing the instantaneous balance between generation and consumption needed to maintain healthy grid operation. The magnitude of the challenge is depicted clearly from the 2025 Iberian Peninsula blackout, listing as a main contributing factor the large penetration of renewable energy in the grid. The article proposes a model to better forecast photovoltaic production and limit grid entropy. It also outlines how implementation of such model would lead to increased feed-in prices for producers and off-set renewable cannibalization as well as lower end user electricity bill.

ABSTRACT. This article presents a high-performance system for classifying types of solar bursts. The proposed system was installed on Livingston Island, Antarctica. Results from early 2026 are presented - the period of maximum solar activity in the 11-year solar cycle. The results obtained are verified by correlation with satellite X-ray data. Unlike satel-lite data, the ground system obtained data shows how an RF frequency (50-820MHz band) is useful to classify radio bursts of types I, II, III, and IV. Capturing these radio signals simplifies solar burst classification and improves the reliability of space weather research.

ABSTRACT. This study examines the electrical energy performance of a photovoltaic (PV) power plant with respect to the duration of the daylight period. The evaluated indicators are normal-ized to the initially installed capacity of the PV plant. During the operational period, a portion of the photovoltaic panels ceased operation. However, for the subsequent analysis periods, the calculations of the energy and power performance indicators were carried out with reference to the originally installed capacity in order to maintain consistency and comparability of the results. The energy yield and power output of the PV plant were evaluated on both a monthly and a daily basis for the total installed capacity and nor-malized to 1 kWp of installed capacity. Tabulated datasets and graphical representations were used to provide both quantitative and visual comparisons of the PV system perfor-mance over the investigated period.

ABSTRACT. There are many measurements performed using ultrasound in technology. The main advantage of this measurement method is the lack of contact between the measuring element and the object being measured. An application based on the Arduino platform is presented, created for measuring distances with ultrasound. The assessment of the measurement accuracy and the sensitivity of the measuring elements to different types of surfaces will be presented. The compilation of measurement systems and the mathematical processing of the results obtained from repeated measurements will assist students in the learning process in var-ious engineering disciplines related to the measurement of non-electrical quantities and the creation of various systems for automatic control of measurement systems.

ABSTRACT. The article presents a study of the overall brightness of semi-transparent materials used in additive manufacturing. Components with varying thicknesses and diverse types of finished surfaces were manufactured and evaluated under various lighting conditions. The test components were fabricated from a transparent PETG (polyeth-ylene terephthalate) material. The printing parameters were selected based on a series of experiments conducted on this type of material. The measurements were conducted on a specially designed and manufactured stand for measuring overall light transmit-tance. The classical method for evaluating the results according to the normal distribu-tion law was applied. The findings of the studies demonstrate that transmittance is predominantly contingent on the thickness of the component, as opposed to the illu-minance or the nature of the surface.

ABSTRACT. This article presents a new heuristic algorithm for conceptual learning. The process of constructing a decision tree through the minimization of a logical function is explained. This heuristic approach eliminates the need to expand the inverse property space into a full logical expression, which could be computationally expensive for large expressions. Tests were conducted using various sets of standardized machine learning benchmarks. They show that the algorithm operates with high precision and accuracy on noise-free data as well as on datasets that include noise through the presence of mislabeled examples. The algorithm’s ability to capture non-linear patterns was experimentally evaluated, where the target class is defined by the equality of two specified attributes. The application of the proposed algorithm for the creation of hybrid neuro-symbolic architectures is discussed, with the aim of achieving logically grounded and interpretable machine learning and machine reasoning.

ABSTRACT. The increasing demand for speech recognition technologies in Bulgaria has revealed a significant technological gap: currently, no compact Bulgarian speech recognition model is available within the Vosk automatic speech recognition toolkit. Such models are critically needed in domains such as public administration, healthcare, digital education platforms, and IoT environments, where reliable speech interaction must function without cloud connectivity. This paper presents a comprehensive methodology for generating compact Bulgarian Voice Language Models (VLMs) using Vosk-type algorithms derived from the Kaldi automatic speech recognition framework. The proposed pipeline integrates acoustic model training, language model construction, lexicon generation, and decoding graph compilation while imposing strict size constraints suitable for edge devices.

ABSTRACT. This paper evaluates machine learning (ML) classifiers for predicting preeclampsia (PE) and gestational hypertension (PIH) using first-trimester screening data from 1,383 pregnant women in Plovdiv, Bulgaria (2018-2020). Three classifiers - Logistic Regression (LR), Extra Trees Classifier (ETC), and Voting Classifier (VC) - are compared across multiple prediction targets and feature configurations. The impact of SMOTE oversampling strategies on model performance is assessed in the context of a severe class imbalance (2.46% PE prevalence). Logistic Regression achieves the highest AUC of 0.853 for preterm PE prediction without oversampling, while SMOTE significantly improves tree-based models (ETC: +0.058 AUC). A non-screened control cohort of 533 patients is evaluated separately using maternal characteristics alone (AUC 0.751). The ML model demonstrates detection rates comparable to the FMF algorithm for preterm PE in this Bulgarian population, without reliance on proprietary risk-calculation software. These findings support the integration of ML-based tools within AI-powered clinical decision support systems to extend systematic PE screening in resource-constrained settings.

ABSTRACT. The paper presents a RAG-based architecture with a large language model for automated assessment of student responses, in which Bloom's Taxonomy is used to structure an analytical rubric and to form a cognitive profile of the performance. The approach integrates Moodle, n8n, a vector database for semantic extraction of relevant context, and a GPT model for criterion-based assessment. The model solution is formed based on the student response, the RAG-extracted learning context, and a predefined rubric. The system is implemented in a real exam environment in Bulgarian and English. The results of data analysis show differentiated behaviour according to Bloom's cognitive levels, with higher sensitivity in tasks related to memorization, comprehension, and application, and more conservative behaviour in evaluative and critical activities. Expert evaluation of the automatically generated feedback confirms its clarity, usefulness, and correctness.

ABSTRACT. The article describes the process of designing and developing an Android application to support students in a university environment by automating the management of the study schedule, providing navigation to the educational buildings and providing notifications about upcoming classes. The main problem that the development addresses is the lack of a centralized system for timely notifications and the difficulties in navigating university campuses. The solution integrates an automated notification system using Firebase Cloud Messaging, working in real time, and geospatial navigation to the educational buildings. The technology stack includes Django REST Framework for the server part, PostgreSQL for database management and Java for the mobile application, with security guaranteed through JWT authentication and TLS encryption. The result is a comprehensive application that provides users with personalized access to the weekly schedule, information about current commitments and the location of the classrooms. This contributes to better organization, reducing absences and increasing the efficiency of the educational process.

ABSTRACT. Proactive cyber security has moved from being an innovative approach to absolutely es-sential requirement for business success, especially considering the large number of new tools and technologies that organizations need to learn and use. In this paper we discuss and demonstrate the possibility of automating important cyber security and immediate response steps with the use of dedicated artificial intelligence agents. We focus on lo-cal-first solutions that are able to keep sensitive data private and at the same time fit well with existing data protection policies and infrastructure. Using a minimalistic AI agent addressing a local large language model, we experiment with skills aimed at running cyber security tools and processing their output in support of cyber incident response.

ABSTRACT. The goal of the study is to compare the most popular protocols available in IoT (Internet of Things) nodes and how they perform under multiple samples in controlled conditions. The focus is put on local telemetry transmission in which the ESP32-C6 board communi-cates with two other devices - a router and a laptop over the 2.4 GHz band. For the exper-iment were pre-set 3 standard nominal sizes for transmission 16, 64 and 256 bytes. Each one was trialed for 30 samples per each protocol. All transmissions received a 100% suc-cess rate and were received by the end device. Both latency and success rate were used as the main performance indicators. The end results were CoAP with the lowest mean laten-cy of 47.0ms, followed by MQTT with 63.0ms and HTTP with a mean overall latency of 1419.0ms. The results indicate an order of magnitude faster performance of CoAP and MQTT against HTTP in the test setup. The study offers a simple and reproducible setup and benchmarking allowing for practical reference point when selecting the right protocol for a given use-case.

ABSTRACT. This article offers a conceptual overview and functional analysis of the author's web-based application LolyDash, a tool for managing personal finances through structured dashboards for expenses, income, notes, and group fundraising. The main features of the system, its advantages over traditional financial tracking solutions, visual analytical tools, categorization and planning mechanisms, as well as integration capabilities via webhook are described. This article argues for the need for modern digital tools for financial literacy in the context of the growing complexity of personal financial management. This article examines the concept of a web-based financial diary that integrates functionalities for managing expenses, income, financial notes, and group payments within a single information system. The main components of the system, its advantages over traditional financial management methods, and the potential for future development through the integration of artificial intelligence are analyzed

ABSTRACT. This article presents a technology for managing and grouping datasets regarding protected natural areas and species in Bulgaria for the period 2010-2024. The studied indicators include area in hectares and number of the following objects: protected areas, natural landmarks, reserves and maintained reserves, national and natural parks. The other group of considered elements are the number of protected plant and animal species, as well as protected venerable trees. This information is extracted from a created relational database and subsequently it is processed. The use of certain rules and the calculation of a given set of variables leads to the generation of the relevant solutions concerning the studied objects. Linear regression analysis is used to evaluate the examined data related to the protected natural areas in the considered period. This work also applies hierarchical cluster analysis to the mentioned data. The number of protected plant and animal species does not change in the whole examined time interval. During 2024 compared to the first year of the interval (2010), the number of protected venerable trees decreases by about 17.31%. The presented technology can also be applied when studying other economic indicators

ABSTRACT. This paper presents a comparative study of two unsupervised machine learning methods - Isolation Forest and Gaussian Mixture Model, for anomaly detection in Automatic Dependent Surveillance-Broadcast (ADS-B) data. ADS-B messages provide valuable information on aircraft position, speed, altitude, and identification, but their high volume, dynamic nature, and data-quality issues make anomaly detection challenging. To address this, the study applies both algorithms within an ETL-based processing pipeline designed for efficient analysis of large air traffic datasets. The results contribute to evaluating the suitability of lightweight anomaly detection methods for aviation monitoring and support the development of safer and more effective air traffic analysis systems.

ABSTRACT. The study aims to demonstrate the role and significance of product evaluation at the concept stage of its development. It is particularly dedicated to a contemporary product – a hydropower generation system – that is directly related to green renewable energy sources. The assessment of each of the six developed design variants combines the output of the virtual prototyping system (generated power) and an analysis of capital expenditure, both used to assess the return on investment period. The project's financial aspects highly influence final design variant selection, and this is a good example of the possibility of including financial specifics at the early project stage.

ABSTRACT. The paper investigates impact resistance of 3D printed ABS and PETG specimens by a falling dart drop. 25 specimens of each material were manufactured in accordance to ASTM D1709 standard, as well as 25 specimens of notched ABS. The test data is pro-cessed when 10 cracked and 10 uncracked specimens are available. Dart drop tests are typical for specimens that are manufactured in shape of films. The purpose of the test is to perform an impact from a certain height with a controlled velocity of the falling dart drop.

ABSTRACT. Anticipating component degradation before failure occurs has become a cornerstone of intelligent condition monitoring in safety-critical engineering environments. This paper benchmarks three supervised learning algorithms — Linear Regression (LR), Random Forest (RF), and Gradient Boosting (GB) — against each other for the task of Remaining Useful Life (RUL) forecasting on turbofan engines, using the NASA CMAPSS FD001 simulation dataset as the evaluation testbed. The benchmark encompasses 100 run-to-failure training trajectories and 100 test sequences, each characterised by 21 on-board sensor channels recorded over successive flight cycles. Following a systematic preparation stage — which involved discarding uninformative constant-variance channels and applying a piece-wise linear degradation labelling scheme capped at 125 cycles — all three algorithms were trained and scored on normalised feature vectors. Among the three candidates, Random Forest delivered the strongest results (RMSE = 17.09, MAE = 12.10, R² = 0.818), ahead of Gradient Boosting (RMSE = 17.42, R² = 0.811) and the linear baseline (RMSE = 20.60, R² = 0.736). These outcomes confirm that bagging-based ensemble regressors provide a compelling accuracy-deployability trade-off for degradation forecasting, with direct relevance to autonomous scheduling and condition surveillance in industrial automation contexts.

ABSTRACT. Technological progress in metrology provides new opportunities for enhancing the quality and transparency of metrological services, with the Digital Calibration Certificate (DCC) playing a significant role in this development. In contrast to traditional calibration certificates, the DCC enables the transfer of extended machine-readable data and can bring additional technical and metrological information describing the calibration process. This paper focuses on the use of DCC additional information for enhanced expression of calibration results in metrological practice and education. Using the example of pressure transducer calibration, the implementation of extended datasets within the DCC structure and their application in result evaluation are demonstrated, including analytical determination of measurement uncertainty and the application of the Monte Carlo method. The results confirm the contribution of DCC to more transparent interpretation of calibration results and to the support of professional education in metrology.

ABSTRACT. This study examines the influence of the dynamic pressure control parameter in the noz-zle during melt deposition onto the build platform (k-Factor), also known as Linear Ad-vance or Pressure Advance in different firmware implementations, on the quality of parts produced using the Fused Deposition Modeling (FDM) technology. The investigation was conducted based on printed test specimens made from Ingeo Bi-opolymer 4043D, with k-Factor values varied in the range of 0.01 to 0.20 under compara-ble process conditions. Dimensional measurements were performed along the X and Y axes, and visual analysis was carried out to identify defects in the specimens. The results from statistical analysis and visual inspection reveal a clear correlation be-tween the k-Factor value and part quality. An optimal balance between dimensional and geometric stability was achieved at k-Factor = 0.04, identifying it as a suitable value for the material and process conditions used.

ABSTRACT. The paper examines the possibility of compensating one of the significant systematic factors in turning that affects the quality of machined surfaces—namely, the dimen-sional wear of cutting inserts during finish turning. Predictive wear models are devel-oped in which the process is approximated using either a linear or an exponential function. The wear prediction error is determined. Based on data from the authors’ experimental studies and other published sources, the duration and intensity of the in-itial and steady-state wear stages are established. A corrected predictive function is presented, consisting of a nonlinear initial segment and a linear steady-state segment. A design variant of a measuring probe for monitoring the current wear of the insert cutting edge, applicable under production conditions, is also proposed

ABSTRACT. This study investigates the capabilities of modern ultrasonic non-destructive testing (NDT) methods for the detection and sizing of fatigue cracks in hinge bolts used in railway transport. Experimental investigations were carried out using immersion ultrasonic testing with a phased array probe and advanced signal processing techniques, including PAUT, FMC/TFM, and FMC/PCI. Artificially introduced erosion notches with sizes ranging from 0.05 to 2.4 mm were used to simulate fatigue defects. The probability of defect detection was evaluated using Hit/Miss analysis and determination of the a90/95 parameter. The results indicate that the TFM method provides the highest sensitivity for detecting small defects, while the PAUT technique demonstrates the highest accuracy in defect sizing.

ABSTRACT. The presented study is dedicated to a “green” solution for hot beverage packaging – a biodegradable single-use cup. Product development follows a specifically elaborated methodology that involves initial material characterization, followed by adjusted product design. The research and design activities continue toward mold design development and testing. The study concludes with a cost analysis, in which an alternative material composition is also examined. Finalized product is ready for industrialization, reaching TRL 5, with assessed financial aspects. The study is a good demonstration of the application of modern technologies and tools for the development of an innovative product.

ABSTRACT. This study presents a theoretical justification and simulation analysis of a flat wagon with increased load capacity for the needs of intermodal transport. A modification is proposed by replacing the middle two-axle bogie with a prototype three-axle bogie. Preliminary calculations using mechanics of materials demonstrate the possibility of increasing the payload by 12 tonnes per wagon, thereby improving economic efficiency and reducing the carbon footprint. To assess the safety against derailment and the running behaviour, a computational multibody model was developed in the Universal Mechanism software. The simulation results are evaluated in accordance with the requirements of the European standard EN 14363, confirming the operational reliability of the proposed innovative design.

ABSTRACT. The assessment of safety against derailment is a mandatory procedure according to EN 14363:2019. Since practical on-track tests are accompanied by significant difficulties and high costs, current regulations allow the use of validated calculation methods. To intro-duce an objective assessment, the authors apply the theoretical "Horizontal Dynamic Passport" approach to investigate three-axle bogies. The study analyzes the influence of the rigid wheelbase on lateral interaction forces across a wide range of radii. The results demonstrate that the wheelbase length significantly affects the forces in the middle and trailing wheelsets, often altering the kinematic guiding mode. These findings provide ob-jective criteria for defining operational limits and ensuring stability according to Nadal’s criterion.

ABSTRACT. The paper presents a comprehensive review of innovation management tools and methods. It is based on analysis of a specific segment of available specialized and re-search bibliography, i.e., current international standards for innovation management systems that are published as the ISO 56000 series since 2019. The innovation man-agement tools and methods presented in this paper are related to innovation partner-ships, intellectual property management, strategic intelligence management, manag-ing innovation opportunities and ideas, and innovation operation measurements. The innovation management tools and methods are aligned with the clauses of ISO 56001. The findings present opportunities for industrial organizations to implement specific tools and methods and to realize value from innovation processes, activities and initi-atives. The conclusions summarize the results of the analysis and highlight directions for further development and improvement of existing innovation management sys-tems in industry.

ABSTRACT. The threaded connections used in drilling machines operate under high axial loads and torques. These elements ensure both the reliable fastening of the drill heads and their centering relative to the other components of the structure. For fastening drill heads, threads with specific profiles are used, such as trapezoidal profiles with asymmetric flanks. A profile tool is typically used for manufacturing such profiles. This, in turn, limits the flexibility of the process and increases costs in small-batch and repair pro-duction conditions. This study proposes a methodology for machining a buttress thread, with trapezoid side angles of 5° and 45°, using a standard grooving insert. The profile geometry is de-scribed analytically through the height of each pass of the threading cycle and the in-clination angles of the trapezoid flanks, and the implementation is carried out using a macro program based on synchronized G92 cycles. The final profile is formed by the superposition of helical surfaces with a constant pitch. The approach enables the real-ization of non-standard trapezoidal profiles without the need for a specially profiled tool. The proposed model serves as a foundation for subsequent research into the geometric accuracy and strength characteristics of the thread profile obtained by this method.

ABSTRACT. This study represents a comprehensive analysis of the implementation of twin counter–rotating common–axis (coaxial) rotor systems in the design process and the technical application of rotorcraft and Unmanned Aerial Vehicles (UAVs). In detail, the conducted literature review clearly illustrates the already usable vehicles and the existing experimental models. The application of a system of two coaxial rotors one above each other, rotating in opposite directions eliminates the need of a tail rotor for the provision of the directional stability and leads to several additional advantages such as the reduction of the rotorcraft’s weight and enhancement of the directional stability qualities in comparison with the single main rotor configurations. However, the inclusion of two rotors one above each other affects the lift generation, reducing its magnitude on the lower rotor, while the thrust of the upper rotor remains relatively unchanged. The implementation of two rotor systems requires complex algorithms with respect to the cyclic and the collective pitch control. Eventually, the analysed cases indicate the existing gap in the research of coaxial systems concerning the influence of the distance between the two rotors and the collective pitch on the generated thrust and the interferences in cross flow conditions.

ABSTRACT. This study represents the evaluation of the aeroelastic flutter stability characteristics of a helicopter rotor, expressed as correlations between the flutter frequency ratio and the relative center of gravity coordinate, considering the varying distance from the ground surface and the location of the aerodynamic center for a fixed flapping frequency. For every constant aerodynamic center’s coordinate, a decrease in the relative distanced from the ground surface produces a reduction of the flutter stability zone for the given helicopter rotor. Conversely, moving the center of gravity coordinate in the backward direction from 〖ξ_A〗_1=0.1 to 〖ξ_A〗_3=0.8 leads to a rise in the flutter stability zone and then improves the stability characteristics. A nonlinear representation of the lift force coefficient variations with the angle of attack, when the reduced frequency changes from k_1=0 to k_6=10 is conducted with the incorporation of the Theodorsen’s function. Eventually, an increase in the reduced frequency magnitude leads to a rise in the produced nonlinearities in the lift force coefficients, when the angle of attack changes.

ABSTRACT. This research examines the fundamental autonomous inertial navigation formulas for aircraft. The study aims to identify analytical errors arising from the use of approximate gravity field models and proposes corrections for horizontal inertial components relative to changes in flight altitude. GPS measurements of ground speed and its total and relative derivatives are trans-formed into compensations for Coriolis and centrifugal accelerations with flight altitude taken into account. This type of compensation corresponds to a precisely defined gravitational field model, assumed to be accurate to the second degree of eccentricity.

ABSTRACT. The safe crossing of pedestrians at traffic light-regulated intersections is ensured by the permissive and prohibitive signals provided for them and by certain priority rules that drivers in conflicting traffic flows with pedestrians must comply with. This in turn leads to the occurrence of traffic delays, which are inevitable under certain traffic conditions. The present study focuses on determining the values of traffic delays when pedestrians cross at a traffic light-regulated intersection in the city of Sofia, Republic of Bulgaria. The inter-section was selected due to the high intensity of pedestrian flows established in prelimi-nary random observations, which is provoked by its location. The study includes deter-mining the traffic delays for a period of 12 hours with full readings of this indicator for each cycle of the traffic light system during the morning and evening peak periods and partially, for 15 minutes for each cycle of the remaining hours of the study period. The re-sults show the lack of a relationship between the waiting time of vehicles and the number of pedestrians crossing. Such an influence can be sought in the behavior and type of pedestrians and their interval of entering the crosswalk.

ABSTRACT. One of the primary responsibilities of local authorities is to develop infrastructure that meets the needs of its users. Roundabouts are widely used worldwide; however, clear cri-teria for selecting roundabouts instead of conventional four-way intersections are often lacking. In different countries, guidelines regarding their applicability exist, but despite this, numerous examples of problematic locations with traffic congestion caused by roundabouts can still be observed. In this study, a comparative analysis is conducted using the specialized traffic simulation software PTV Vissim. The analysis examines a real roundabout that has been imple-mented in place of a previously existing three-way intersection, comparing the capacity of the two configurations. Additionally, an analysis of the measures introduced after the construction of the roundabout to improve its capacity is also carried out.

ABSTRACT. Traffic accidents involving pedestrians during the hours of darkness pose a serious threat to road safety due to reduced visibility and drivers’ delayed perception of the traffic situation. Accurate estimation of the pedestrian detection distance is essential in the reconstruction of traffic accidents. The present study analyzes the relationship between driver visibility, environmental conditions and the ability to detect pedestrians in a timely manner during nighttime driving. The study examines the main factors influencing the driver’s “perception–reaction” process, including the illumination provided by the vehicle’s headlights, the illumination of the road environment, the contrast and reflective properties of the pedestrian’s clothing, as well as the driver’s level of attention. Using a graph-analytical method, the detection distances for pedestrians under nighttime conditions are estimated. A real-life accident scenario was reconstructed to determine whether the driver had sufficient time and distance to perceive the danger and take action to avoid a collision. The results show that pedestrian visibility depends on lighting conditions, which directly affect the driver’s reaction time. These findings contribute to the refinement of the methodological approach to reconstructing traffic accidents and can assist experts in conducting automotive technical examinations.

ABSTRACT. The development of internal combustion engines aims to maximize the combustion process energy, achieve the highest possible efficiency, and reduce harmful emissions released into the environment, while also improving power output. In this regard, here we have presented two pairs of vehicles, which have mechanically similar internal combustion engines but either different environmental or dynamic performance char-acteristics. The first example is of upgrading an internal combustion engine from Euro 4 to Euro 5. The second example demonstrates a 23% increase in power while main-taining the same emission parameters.

ABSTRACT. This study investigates the stagger of tram overhead lines and its effect on pantograph carbon strip wear. Using a specialized measurement vehicle equipped with a stagger decoder, field data were collected along selected tram routes. The measured wire ge-ometry was compared with the original design documentation to assess deviations and their impact on energy collection efficiency and strip longevity. Results highlight areas where design tolerances differ from reality, providing insights for infrastructure opti-mization and maintenance planning. The findings demonstrate the importance of pre-cise monitoring of overhead line alignment to ensure uniform wear and improve the reliability of tram systems.

ABSTRACT. This study presents a systematic design and feasibility assessment of retrofitting a pneumatic suspension system into a vehicle originally equipped with a conventional passive suspension system. The research aims to evaluate the structural compatibility, dynamic performance improvements, and techno-economic feasibility of the proposed conversion. A conceptual retrofit architecture was developed, including air springs, compressor unit, air reservoir, solenoid valves, electronic control module, and associated mounting adaptations. Vehicle dynamic performance was investigated through analytical modeling and simulation-based analysis, focusing on ride comfort, suspension deflection, natural frequency variation, and load distribution characteristics. A comparative assessment between the baseline conventional suspension and the proposed pneumatic configuration was conducted under various loading and road excitation scenarios. The results indicate that the pneumatic suspension system significantly improves ride comfort by reducing vibration transmissibility and enabling adjustable ride height control. However, the retrofit introduces additional system complexity, weight increase, energy consumption, and higher initial implementation cost. A multi-criteria feasibility evaluation incorporating technical performance, cost analysis, maintainability, and operational flexibility suggests that the retrofit solution is viable for specific application domains such as commercial, adaptive-load, or performance-oriented vehicles. The findings provide a structured framework for evaluating suspension system conversion projects in automotive engineering applications.

ABSTRACT. This report presents an experimental study focused on parametric optimization of an electric vertical take-off and landing (eVTOL) wing–propeller lifting system. The experi-ments were conducted in a wind tunnel equipped with a Particle Image Velocimetry (PIV) system, and a wing–propeller thrust and power measurement test stand was used. The total mission flight energy was evaluated and compared for three different propel-ler-to-wing gross area ratios and three mission profiles. Two principal configurations of the wing–propeller lifting system were considered, corresponding to the hovering and cruising stages of flight. In these configurations, both the propellers and the tilting wing sections were oriented according to the requirements of hover and cruise operation. The total flight energy was adopted as the figure of merit and was calculated for all design points. The figure of merit was then analyzed as a function of the propeller-to-wing gross area ratio. The results allowed the determination of optimal configurations for different hover times. Finally, the total flight energy obtained from the experiments was calculated and compared with the corresponding simulation results.

ABSTRACT. This work presents an analysis of the methods and means for determining the technical condition of brake fluid in the conditions of technical operation of vehicles. The study is motivated by the fact that hygroscopic glycol-ether brake fluids degrade over time due to moisture absorption, which leads to a decrease in the boiling point, an increase in electrical conductivity and a change in their chemical properties. Within the experimental part, measurements of three diagnostic parameters were carried out: boiling point, electrical current/conductivity and moisture tester readings at different controlled water contents in several types of brake fluids. The results obtained show a clear relationship between moisture content and a decrease in the boiling point, as well as a nonlinear increase in electrical conductivity with increasing water percentage. It was found that moisture tester provides a quick but limited accurate assessment. Based on the comparative analysis, conclusions have been formulated regarding the reliability, applicability and limitations of different diagnostic methods in service and operational conditions. The results support the optimization of technical control procedures and increase the reliability of brake systems in real operation.

ABSTRACT. This paper presents the results of identification of the natural frequencies of a MacPherson suspension using FFT, FRF, and spectral analysis. Experimental investigations are conducted to determine the accelerations of a quarter-car MacPherson suspension system of an Audi A3 passenger car at a tire inflation pressure of 0.22 MPa. The investigated suspension control arm was equipped with one original equipment manufacturer (OEM) bushing and one polyurethane bushing. The paper also presents the results obtained using a set of MATLAB scripts developed for processing the vibration measurement data acquired from the components of the vehicle’s quarter-car suspension system, with each script corresponding to a specific analysis approach. The developed scripts implement several methods for analyzing the recorded acceleration signals, including the Fast Fourier Transform (FFT), the frequency response function (FRF), and spectral analysis. The results obtained by applying these three methods to the experimental data are compared and discussed.

ABSTRACT. The first natural frequency is most important vibration parameter during the design of suspensions. It has major impact on vehicle ride comfort and stability. This paper presents the results of the effects of different bushings elastic characteristics of the natural frequencies of a front-independent MacPherson suspension system. The natural frequencies and mode shapes were obtained from theoretical and experimental studies. A simulation study was conducted, taking into account the elastic characteristics of bushings and analysis with two rubber bushings within the mounting of the arm (Case I), and a rubber bushing and a polyurethane bushing (Case II). The natural frequencies were determined by experiment using a suspension tester and a measuring system. Frequencies analysis of the suspension was performed using the SolidWorks 2023. The results were compared and analyzed.

ABSTRACT. The present study investigates the physicochemical degradation and wear metal evo-lution of low-viscosity SAE 0W-20 engine oil during the first service interval of a mod-ern gasoline internal combustion engine. Two oil samples were analyzed: fresh lubri-cant and used oil collected after approximately 13,000 km of vehicle operation. The analysis included determination of kinematic viscosity at 100 °C (ASTM D445), total base number (ASTM D2896), FT-IR spectroscopic indicators of chemical degradation (ASTM E2412), and elemental analysis of wear and additive metals using ICP-OES (ASTM D5185). The results show a viscosity reduction from 8.5 to 7.01 mm²/s and a de-crease of the alkalinity reserve to 3.7 mgKOH/g, indicating progressive lubricant aging. FT-IR analysis revealed moderate oxidation, nitration, and sulfation processes, while elemental analysis identified Cu, Fe, and Al as the dominant wear metals. The observed changes correspond primarily to normal oil aging and engine running-in processes. The results demonstrate the effectiveness of combined oil analysis techniques for monitor-ing lubricant degradation and early engine wear.

ABSTRACT. This article is devoted to the development of an effective technological scheme for the enrichment of copper ores from the Yoshlik deposit. The studies were conducted using a set of ore samples from the Yoshlik deposit. The study showed that chalcopyrite is the main sulfide mineral in the copper content. In addition, the presence of additional ore minerals, including bornite, pyrite, magnetite and hematite, was detected. Quartz, sericite, feldspar, chlorite and carbonates were observed as non-metallic minerals. It was found that a high content of sericite and chlorite has a detrimental effect on the processing properties of the ore. The intergrowth of useful minerals is characterized by a minimum degree of separation, which requires the implementation of gentle crushing methods to separate them from each other and from the matrix of surrounding rocks. In order to obtain commercial copper concentrates, a series of tests were conducted on single-stage and two-stage flotation schemes. As demonstrated in Figure 1, the two-stage scheme yielded extraction rates of copper into copper concentrate of 86.0%, 91.5%, 93.0%, 90.2% and 89.3%, respectively, for samples No. 1, 2, 3, 4 and 5. Consequently, a technological scheme for the flotation of copper ores was developed.

ABSTRACT. Current scientific article deals with the study of the relationships between material and iconic image in fine graphics. In this specific case, these relationships relate only to letter-press printing. Of interest are non-standard materials, which, depending on their physical and chemical indicators, lead to different results in the final artistic image. The aim of the article is to establish the dependencies: material - manipulation - result - effect/defect. Specific "effects" are sought in the final version of the image, in which the work of fine art - graphics, is of high artistic value. For this purpose, experiments were conducted with the same images, but on different material carriers, with different inks and types of paper, which practically establish the physical and technological indicators and results. From these conditions it is established what artistic effect or defect is realized. The parameters presented in the comparative tables are intended to orient the user as to what base, ink and paper to choose for a specific creative manifestation.

ABSTRACT. The present article aims to explore the feasibility of manufacturing multi-material components from PETG and TPU using FFF/FDM 3D printing. The mechanical characterization includes Shore hardness measurements of the individual materials and Izod impact strength testing of samples produced with different structural configurations. Furthermore, a test part based on a rover wheel is fabricated using a PETG/TPU multi-material structure to validate the practical applicability of the proposed multi-material concept.

ABSTRACT. The present article considers the feasibility of using 3D printed patterns for sand casting applications. The patterns have been produced by FDM/FFF technology, and the casting process has been simulated in the specialized CAE software product ProCAST. A prototype series of castings has been produced for the purpose of experimentally validating the applicability of 3D-printed patterns in the sand casting process. The results obtained demonstrate the significant potential of 3D printed patterns to streamline the techno-logical process in the manufacture of foundry tooling equipment. The use of additive manufactured patterns reduces the time required for their design and production and increases flexibility in the manufacture of prototypes or small series products. The result is more efficient production planning and a reduction in the overall time required to produce sand castings.

ABSTRACT. In this work, composites based on thermosetting polymers (epoxy, polyester and vinylester resins) are presented, in which carbon particles and cenospheres are embedded. The influence of combinations of multi-walled carbon nanotubes (MWCNTs) and cenospheres, as well as amorphous carbon and cenospheres on the mechanical properties and electromagnetic attenuation in the X-band range (8-12 GHz) of the developed composites has been studied. It has been established that the used combinations of carbon particles and cenospheres have the greatest positive effect on the mechanical properties of the composites based on vinylester resin. The developed materials demonstrate effectiveness for electromagnetic protection in the X-band range. The combination of a polymer matrix and appropriate fillers leads to significant attenuation of the radio frequency signal. The presented composites are suitable materials for integration into various defense systems - from coatings to structural elements with functional purpose.

ABSTRACT. This paper presents an experimental study on the amount of formaldehyde released during a fire in apartments, focusing on the flooring most commonly used in Bulgaria. Formaldehyde is a potentially hazardous chemical known to be carcinogenic in large amounts. It often causes respiratory problems and skin irritation. People prone to al-lergies are especially sensitive to formaldehyde. These health risks depend on the dura-tion and level of exposure. The experiment tested seven types of laminate samples and six types of natural parquet samples at three different combustion temperatures in a room. Each sample was placed in a muffle furnace, heated to the specified tempera-tures of 150, 250, and 400 °C, respectively.

ABSTRACT. The present study examines the influence of plastic deformation on the corrosion behavior of aluminum alloy EN AW-2024 (AlCu4Mg) in an acidic medium (0.5 M H₂SO₄). Specimens without deformation and with a degree of plastic deformation of 25%, 33% and 50% were studied. Corrosion tests were carried out by gravimetric method for a period of 72 hours. The inhibitory effect of environmental inhibitors – ascorbic acid (vitamin C) and citric acid – was also evaluated. The results show that the degree of plastic deformation has a significant influence on the corrosion rate of the studied alloy. The lowest corrosion rate is observed at 25% deformation, while at higher degrees the resistance decreases. It was found that citric acid exhibits the highest inhibitory effect at 33% deformation (77.27%), while vitamin C shows a better effect at 50% deformation. The results obtained show that the optimal degree of deformation in combination with a suitable inhibitor can significantly improve the corrosion resistance of EN AW-2024 alloy in acidic environments.

ABSTRACT. This article is reporting the results of a simulation modelling of a closed-die cold forging process used to manufacture a part made of S235JR. All tools and parts were developed with SolidWorks CAD software. Two simulation runs, with and without lubricant, were carried out by use of the QForm simulation software. The simulation parameters and re-sults are described in the article. The analysis showed that the simulated process could certainly be used for manufacturing of quality products with high dimensional precision and improved strength suitable for use in various industries. It further proved the QForm professional software’s suitability for use in the above industries for precise simulation of various cold and hot working processes.

ABSTRACT. The paper deals with the experimental evaluation of the influence of infill direction (raster angle) on the tensile strength of 3D-printed ABS (Acrylonitrile Butadiene Styrene) samples, produced at high speed (200 mm/s infill) and 100% infill density. The used printing method is FDM (Fused Deposition Modeling). The printing material was dried before printing and then stored in a low-humidity en-vironment filled with desiccant in order to minimize the moisture absorption during the experi-ments. The testing and conditioning of the samples was performed according to the EN ISO 527 standard. The results revealed that the highest average tensile strength of 39.3 MPa is obtained with the aligned rectilinear pattern at 0° infill direction, while the lowest of 34.6 MPa is reached with the same infill pattern at 90° infill direction relative to the load direction. The tested material demon-strated moderate tensile strength anisotropy in the direction of the load. Moreover, productivity when printing ABS can significantly be increased when the correct conditions are available, as the resulted average tensile strength is comparable and higher than the one given by the filament’s producer which recommends printing at low to moderate speeds.

ABSTRACT. The publication focuses on the additive technology PolyJet and more specifically the photopolymer Digital ABS Plus. After a thorough analysis, it was concluded that this technology is suitable for both rapid prototyping of parts and rapid small-scale pro-duction of various products. The resulting parts can be implemented in a production process with different operating conditions. That is why it is interesting to investigate the Digital ABS Plus material at elevated temperatures. The temperatures at which the tests were performed are consistent with the values for heat deflection temperature (HDT) of the Digital ABS Plus material, described in the manufacturer's technical data sheet, as well as with the results of high-temperature tests of parts obtained using Fused Deposition Modeling (FDM) technology. The investigated test pieces are sub-jected to annealing in order to increase their tensile strength and temperature re-sistance. The process is carried out in an oven with digital temperature control with a thermal profile according to a procedure approved by the manufacturer Stratasys. The obtained data from the mechanical properties before and after annealing of the Digital ABS Plus material are compared and depicted in a diagram.

ABSTRACT. The present work is based on experimental determination of the mechanical properties of test bodies made using PolyJet technology from the two-component material Digital ABS Plus. The parameters of 3D printing of the test bodies and their dimensions are presented. The testing methods and equipment for the studied parameters of hardness and roughness are selected. The roughness studies were conducted in different posi-tions and orientations of the test bodies. Hardness studies were carried out on the Shore D scale for three types of test bodies: after 3D printing of their surface; inside the test body and after applied thermal treatment of test bodies and on the Rockwell M scale. These indicators are extremely important in the evaluation of the details and the possibility of their implementation in the production process.