Edvard Govekar - Academia.edu (original) (raw)

Papers by Edvard Govekar

International Journal of Bifurcation and Chaos, Jun 1, 2011

We examine the dynamics of laser droplet generation in dependence on the detachment pulse power. ... more We examine the dynamics of laser droplet generation in dependence on the detachment pulse power. In the absence of the detachment pulse, undulating pendant droplets are formed at the end of a properly fed metal wire due to the impact of the primary laser pulse that induces melting. Eventually, these droplets detach, i.e. overcome the surface tension, because of their increasing mass. We show that this spontaneous dripping is deterministically chaotic by means of a positive largest Lyapunov exponent and a negative divergence. In the presence of the detachment pulse, however, the generation of droplets is fastened depending on the pulse power. At high powers, the spontaneity of dripping is completely overshadowed by the impact of the detachment pulse. Still, amplitude chaos can be detected, which similarly as the spontaneous dripping, is characterized by a positive largest Lyapunov exponent and a negative divergence, thus indicating that the observed dynamics is deterministically chaotic with an attractor as solution in the phase space. In the intermediate regime, i.e. for low and medium detachment pulse powers, the two chaotic states compete for supremacy, yielding an intermittent perioddoubling to amplitude chaos transition, which we characterize by means of recurrence plots and their properties. Altogether, the transition from spontaneous to triggered laser droplet generation is characterized by a chaos-to-chaos transition with an intermediate dynamically nonstationary phase in-between. Since metal droplets can be used in various industrial applications, we hope that the accurate determination of the dynamical properties underlying their formation will facilitate their use and guide future attempts at mathematical modeling.

Advanced modelling for grinding - from friction to ploughing and dynamic chip formation with temperatures

Procedia CIRP, 2021

Laser droplet weld: an innovative joining technology opens new application possibilities

Proceedings of SPIE, Jul 18, 2003

ABSTRACT "Laser Droplet Welding" is an innovative joining technology. The weldi... more ABSTRACT "Laser Droplet Welding" is an innovative joining technology. The welding is realised by a laser generated liquid metal droplet which is deposited onto the parts to be joined. The raw material is a metal wire. In conventional laser welding a gap between the parts worsens the quality of the laser welded joint substantially. Contrarily a droplet offers sufficient material to bridge gaps. Even different gap sizes can be bridged by a suitable selection of the droplet size. A further advantage is the controllable heat transfer, only given by the heat content of a single drop that is sufficient to produce a high-temperature weld. The droplet heating provides the opportunity to weld small devices, thin coatings and even heat sensitive components without negative influence on their mechanical and electrical function. It is also possible to interconnect different materials by the addition of material supplied in form of drops. With the Laser Droplet Weld it is furthermore possible to join high reflective materials. This article describes the process and the system technology as well as achieved results. It will mainly focus on the droplet detachment which influences the complete process, e.g. the heat quantity or weld splashes.

The influence of cryogenic cooling on process stability in turning operations

CIRP Annals, 2011

ABSTRACT This paper presents results of the influence of cryogenic machining on the process stabi... more ABSTRACT This paper presents results of the influence of cryogenic machining on the process stability. The stability diagrams are obtained experimentally using the coarse-grained entropy rate estimator for chatter detection from measured cutting forces. In comparison with conventional machining, enlarged stability windows are observed for the case of cryogenic machining. Based on the defined specific force models in turning operations, it is shown that a higher machining stability is achievable in cryogenic machining due to the reduction of specific cutting force components, in comparison with dry machining.

Mechanical Systems and Signal Processing, Sep 1, 2017

Semi-supervised vibration-based classification and condition monitoring of the reciprocating comp... more Semi-supervised vibration-based classification and condition monitoring of the reciprocating compressors installed in refrigeration appliances is proposed in this paper. The method addresses the problem of industrial condition monitoring where prior class definitions are often not available or difficult to obtain from local experts. The proposed method combines feature extraction, principal component analysis, and statistical analysis for the extraction of initial class representatives, and compares the capability of various classification methods, including discriminant analysis (DA), neural networks (NN), support vector machines (SVM), and extreme learning machines (ELM). The use of the method is demonstrated on a case study which was based on industrially acquired vibration measurements of reciprocating compressors during the production of refrigeration appliances. The paper presents a comparative qualitative analysis of the applied classifiers, confirming the good performance of several nonlinear classifiers. If the model parameters are properly selected, then very good classification performance can be obtained from NN trained by Bayesian regularization, SVM and ELM classifiers. The method can be effectively applied for the industrial condition monitoring of compressors.

Sciyo eBooks, Aug 18, 2010

the presentation and discussion of practical forecasting results, obtained through the several ye... more the presentation and discussion of practical forecasting results, obtained through the several years of operation, and the last section 6 draws some conclusions. 2. Forecasting requirements The forecasting requirements that motivate the forecasting efforts of energy distribution companies vary from country to country and are also subject to modifications according to the legislation. Slovenia's economic incentive model is briefly described in this section. Currently, companies in Slovenia are motivated to accurately forecast their daily gas consumption for the next "gas consumption" day. Slovenia's gas consumption day is defined as the period from 8.00 AM until 8.00 AM the following day. The forecast should be delivered approx. at 9.00 AM for the gas consumption day starting tomorrow, therefore the forecasting horizon is H = 2 days. Currently, the incentive model only refers to forecasting on daily resolution, therefore hourly forecasting is only required by the company for online optimization of their resources and operation strategies. Both, daily and hourly forecasting results are considered in this chapter. Daily forecasting requirements are shown in Fig. 1, and expanded hourly forecasting requirements are plotted in Fig. 2. How to reference In order to correctly reference this scholarly work, feel free to copy and paste the following:

Journal of Materials Processing Technology, Aug 1, 2021

This paper experimentally studies the influence of conventional process parameters (laser beam po... more This paper experimentally studies the influence of conventional process parameters (laser beam power, P, workpiece, and wire feeding velocities), and the nominal workpiece irradiation proportion WIP N parameter, on ALB-WC process stability and generated clad geometry. The study was based on an extensive set of single clad experiments using stainless steel AISI 316L wire of 0.6 mm diameter and a stainless steel AISI 304 workpiece. The results show that the laser beam power required for a stable process increases non-linearly with WIP N , and this relationship can be described by quadratic regression. An increase in either the workpiece or wire feeding velocity respectively causes a reduction or enlargement of the stability region in the P-WIP N stability diagrams. The influences on stability diagrams are explained by means of the increase in the nominal WIP N to the effective WIP E, as a result of the clad height generated. With respect to clad geometry, it was shown that an increase in the WIP N causes a decrease in the dilution ratio, which is linearly correlated with the workpiece and melt pool temperatures, while clad width and height are not influenced by the WIP N. The paper further explains the complex influences of the process parameters observed on process stability and generated clad geometry using a phenomenological description of the ALB-WC process at a low and high nominal WIP N. For this purpose, heat transfer, Marangoni flow, and laser beam reflections are considered qualitatively and supported by IR and visual images of the wire-end and melt pool as well as the workpiece and melt pool temperatures.

Journal of Materials Processing Technology, 2016

In the paper a novel system for drop-on-demand (DoD) generation from a metal wire is presented, w... more In the paper a novel system for drop-on-demand (DoD) generation from a metal wire is presented, whose main component is a newly developed laser droplet generation head, consisting of annular laser beam shaping optics and a wire feeding system. In the pendant droplet formation phase of the DoD generation, a laser pulse is used to melt the wire-end, which is fed into the focus of an annular laser beam. The formed pendant droplet is then detached by means of a detachment pulse, which induces Rayleigh-Plateau instability of the molten column of wire above the neck of the pendant droplet. The main process parameters, including the laser pulse and wire feeding parameters as well as the additional parameters which influence particular phases of the DoD generation process, have been identified. The empirical correlations between the influencing process parameters and the droplet characteristics, including droplet diameter and temperature, were determined, based on the analysis of high speed IR records of the process, images being acquired by an optical microscope and temperature data being acquired by pyrometers. As an example, DoD generation from a commercially pure 99.6 % Ni wire (Nickel 200) of 0.6 mm diameter is considered. It is shown that droplets with diameters ranging from 0.85 to 1.25 mm can be generated, with a resolution of 50 µm and a standard deviation of 15 µm. The temperature of the detached droplet remains above the melting point of the Ni wire, and increases with the droplet diameter within the range from 1650 °C to 1750 °C. Some examples of Ni droplets deposited on a Ti sheet surface are presented, with the aim of demonstrating the capability of the proposed system, and motivating further applications in which drops on demand having a high temperature and a precisely defined diameter need to be generated, while limiting the thermal loading of the surroundings.

Building and Environment, Aug 1, 2018

Applied Acoustics, Mar 1, 2014

In the paper chatter detection in band sawing is considered as a signal processing and classifica... more In the paper chatter detection in band sawing is considered as a signal processing and classification problem. A multi-sensory experimental setup was established on an industrial band saw including sound, acceleration and cutting force, and measurements. Based on an experimental analysis sound signal is shown to be the most appropriate for chatter detection, therefore a sound-based online chatter detection method is proposed. The method consists of a sound signal pre-processing with Short-Time Fourier Transform, extraction of features in frequency space with optimal threshold and application of Quadratic Discriminant Analysis for chatter detection. The proposed method tested with twofold cross validation yields over 96% success of chatter detection.

Estimation of drill wear from AE signals using a self-organizing neural network

NDT & E international, 1994

Analysis and Short-Term Forecasting of Highway Traffic Flow in Slovenia

Lecture Notes in Computer Science, 2011

Analysis and short-term forecasting of traffic flow data for several locations of the Slovenia hi... more Analysis and short-term forecasting of traffic flow data for several locations of the Slovenia highway network are presented. Daily and weekly seasonal components of the data are analysed and several features are extracted to support the forecasting. Various short-term forecasting models are developed for one hour ahead forecasting of the traffic flow. Models include benchmark models (random walk, seasonal random walk, naive model), AR and ARMA models, and various configuration of feedforward neural networks. Results show that the best forecasting results (correlation coefficient R > 0.99) are obtained by a feedforward neural network and a selected set of inputs but this sophisticated model surprisingly only slightly surpasses the accuracy of a simple naive model.

A Comparison of Spectral and Chaotic Analysis of Electrochemical Noise

ASTM International eBooks, Mar 4, 2009

Potential and current fluctuations spontaneously generated by corrosion reactions are known as el... more Potential and current fluctuations spontaneously generated by corrosion reactions are known as electrochemical noise. In certain cases, good correlation can be obtained between the results of the spectral analysis of electrochemical noise and corrosion rate and type. However, because of the chaotic nature of corrosion processes, a special mathematical treatment may be needed. In the present study, the electrochemical noise measured on various metals was treated by methods known from the theory of chaos, and the results were compared with the results of spectral analysis. It has been shown that the chaotic characteristics of electrochemical noise are related to corrosion type, whereas the rate of corrosion has no influence on the fractal dimensions of the noise.

Detection of Corrosion by Analysis of Electrochemical Noise

Fractals, Jun 1, 1994

Stochastic fluctuations of the corrosion potential and the current generated by corrosion reactio... more Stochastic fluctuations of the corrosion potential and the current generated by corrosion reactions are known as electrochemical noise. These fluctuations can be measured in freely corroding systems, therefore the characteristics of electrochemical noise are influenced only by the type and rate of corrosion. The classical spectral analysis of electrochemical noise in the frequency domain achieve good correlation to corrosion rate and type; however, the chaotic nature of corrosion processes requires different mathematical treatment. In this paper self-similarity and fractal dimensions of electrochemical noise are examined in order to explain its mechanism and improve the corrosion monitoring system. Capacity and correlation fractal dimensions of voltage and current-noise, measured on various metals, are calculated and compared to the results of the classical spectral analysis. Relations between different rates and types of corrosion (passivation, local, uniform) and the fractal characteristics of electrochemical noise are established. The analysis of spontaneous electrochemical voltage and current fluctuations is confirmed as a rich source of information in corrosion processes.

CIRP Annals, 2019

In this paper the stability of the annular laser beam (ALB) direct wire-deposition process, which... more In this paper the stability of the annular laser beam (ALB) direct wire-deposition process, which enables process symmetry and a well-defined ALB workpiece irradiation proportion (WIP) and related energy input onto the workpiece and the wire surface is considered. Various initial process phase strategies with respect to different initial wire-end positions and WIPs were analysed based on the process visualization and outcome, and the melt pool temperature. It was shown that in addition to a precise synchronization of the mutually time-dependent ALB power, wire and workpiece feeding velocity, the fastest and the most robust transition into a stable stationary process could be achieved with the initial position of the wire-end on the workpiece surface. Additionally, the WIP was shown to have a strong and nonlinear influence on the process stability.

CIRP Annals, 2015

The paper presents recurrence plot based stability analysis of the horizontal band sawing process... more The paper presents recurrence plot based stability analysis of the horizontal band sawing process of structural steel profiles. The analysis is performed in the parameter space defined by the cutting speed, the distance between the blade supports, and the feed rate. The corresponding stability diagrams have been constructed using the recurrence plot characteristic, the determinism of the sound pressure emitted by the process, which quantifies the process predictability. The topology of the experimentally obtained stability diagrams revealed non-linear nonmonotonic dynamic behaviour, which made two different chatter avoidance strategies possible by cutting speed variation. Cutting, Chatter, Band sawing 2.1. Experiments In order to characterize the band sawing process and construct corresponding stability diagrams, a set of 21 cutting experiments were performed during which the cutting force and sound Contents lists available at SciVerse ScienceDirect

Analysis of traffic dynamics on a ring road-based transportation network by means of 0–1 test for chaos and Lyapunov spectrum

Transportation Research Part C-emerging Technologies, Nov 1, 2013

ABSTRACT This paper considers the dynamics of traffic on a ring road-based transportation network... more ABSTRACT This paper considers the dynamics of traffic on a ring road-based transportation network around a major city, via traffic flow time series analysis and characterization. In particular, three traffic flow time series are examined. Two of the time series are acquired from measurement stations located on highways, while one is from a station on the ring road around Ljubljana city. For the analysis and characterization of time series the novel test called 0–1 test for chaos is applied. Based on the outputs of the test it is concluded that the observed traffic dynamics is inherently chaotic. Additionally, a more detailed characterization of traffic dynamics is carried out on Lyapunov spectrum basis, which reveals that traffic dynamics on the highway is quantitatively quite different from the traffic dynamics on the ring road.

Raziskava in razvoj empiričnih modelov za napovedovanje odjema toplote v vročevodnem sistemu

Instantaneous Frequency-Based Analysis and Characterization of Laser Generated Droplet Sequence Dynamics

Advances in Adaptive Data Analysis, Apr 1, 2013

In the laser droplet generation process three different dripping regimes are experimentally obser... more In the laser droplet generation process three different dripping regimes are experimentally observed in dependence on the detachment pulse power. Besides being nonlinear, the process is also inherently nonstationary. In order to consistently analyze all the dripping scenarios based on an experimental time series, time-frequency analysis by means of instantaneous frequency is used. For the calculation of instantaneous frequency, the most recent developments of the Hilbert–Huang transform are applied, i.e. ensemble empirical mode decomposition, empirical amplitude/frequency modulation decomposition, and direct quadrature. In time-frequency spectra specific patterns are associated with corresponding dripping regimes. By means of a detailed inspection of patterns, the influence of the detachment pulse power on dripping dynamics is characterized.

Global Attractors of High-Speed Milling: Analyses, Numerics and Experiments

Global Attractors of High-Speed Milling: Analyses, Numerics and Experiments. [ASME Conference Pro... more Global Attractors of High-Speed Milling: Analyses, Numerics and Experiments. [ASME Conference Proceedings 2003, 2231 (2003)]. Gabor Stepan, Robert Szalai, Brian P. Mann, Philip V. Bayly, Tamas Insperger, Janez Gradisek, Edvard Govekar. Abstract. ...