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Abstract: The paper deals with the dynamics of a resistance spot welding system. At the core of this system is a transformer, which is powered on the primary side by a pulse-width modulated inverter and has a full-wave output rectifier on the secondary side that provides a direct welding current. The entire system is nonlinear, due to magnetic hysteresis and electronics. The electronics prevent the current from flowing in all parts of the welding transformer at separate time intervals during the voltage supply period; therefore, not all the parameters affect the dynamic of currents and voltages all the time so the system is also time-variant. To design a high-performance welding system and to predict the maximum possible welding current at a specific load, it is necessary to know the welding and primary currents. The leakage inductances of the system can reduce the maximum welding current significantly at higher frequencies and the same load. There are several methods to determine these currents, each with its drawbacks. Measurements are time-consuming, using professional software is expensive and requires time to learn and free open-source software has many limitations and does not guarantee the correctness of the results. The article presents a new, fourth option—a theoretical derivation of analytical expressions that facilitate straightforward and rapid calculation of the welding and primary currents of the resistance spot welding system with symmetrical secondary branches. The derivation of the mathematical expressions is based on the equivalent circuits that describe the system in different operating states. The results of the numerical simulations confirmed the derived expressions completely.
Keywords: DC–DC converters, pulse width modulation (PWM), welding transformers, center-tapped transformers, full-wave rectifier, resistance spot welding (RSW), leakage inductance, analytical modeling, time-variant system, equivalent circuit
Published in DKUM: 08.08.2024; Views: 77; Downloads: 22
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Abstract: Background and purpose: Induced travel demand (ITD) is a phenomenon where road construction increases vehicles’ kilometers traveled. It has been approached with econometric models that use elasticities as measure to estimate how much travel demand can be induced by new roads. However, there is a lack of “white-box” models with causal hypotheses that explain the structural complexity underlying this phenomenon. We propose a system dynamics model based on a feedback mechanism to explain structurally ITD. Methodology: A system dynamics methodology was selected to model and simulate ITD. First, a causal loop diagram is proposed to describe the ITD structure in terms of feedback loops. Then a stock-flows diagram is formulated to allow computer simulation. Finally, simulations are run to show the quantitative temporal evolution of the model built. Results: The simulation results show how new roads in the short term induce more kilometers traveled by vehicles already in use; meanwhile, in the medium-term, new traffic is generated. These new car drivers appear when better flow conditions coming from new roads increase attractiveness of car use. More cars added to vehicles already in use produce new traffic congestion, and high travel speeds provided by roads built are absorbed by ITD effects. Conclusion: We concluded that approaching ITD with a systemic perspective allows for identifying leverage points that contribute to design comprehensive policies aimed to cope with ITD. In this sense, the model supports decision- making processes in urban contexts wherein it is still necessary for road construction to guarantee connectivity, such as the case of developing countries.
Keywords: induced travel demand, system dynamics, decision-making, dynamic modeling
Published in DKUM: 23.01.2018; Views: 1477; Downloads: 175
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Abstract: The system dynamics approach is a holistic way of solving problems in real-time scenarios. This is a powerful methodology and computer simulation modeling technique for framing, analyzing, and discussing complex issues and problems. System dynamics modeling is often the background of a systemic thinking approach and has become a management and organizational development paradigm. This paper proposes a system dynamics approach for studying the importance of infrastructure facilities on the quality of primary education system in a developing nation. The model is built using the Cross Impact Analysis (CIA) method of relating entities and attributes relevant to the primary education system in any given community. The CIA model enables us to predict the effects of infrastructural facilities on the community's access of primary education. This may support policy makers to take more effective actions in campaigns that attempt to improve literacy.
Keywords: developing countries, system modeling, cross impact analysis, simulation, system dynamics, primary education
Published in DKUM: 28.11.2017; Views: 1302; Downloads: 387
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Abstract: The fruit production in Bosnia and Herzegovina (BIH) is slowly returning to normality after Balkan wars. Due to favorable climatic and soil conditions it presents a clear business opportunity for family farms. The decision which fruit species/variety to grow is a complex decision and should be made on the basis of sound empirical analysis. This paper present a multi criteria model for planning and decision making methodology based on expert system DEX-i. In the first stage the technological and financial cost benefit analysis were conducted for each fruit production alternative. The results were further evaluated with expert system DEX-i considering all possible criteria influencing the fruit grower decision. In the case of a sample family farm the plum yielded with best multi criteria evaluation, followed by apple and pear, while sour cherry gave unsatifactory outcome.
Keywords: cost benefit analysis, fruit production, multi attribute decision modeling, expert system DEX-i
Published in DKUM: 10.07.2015; Views: 1559; Downloads: 38
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