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 non-monotonic dynamic behaviour, which made two different chatter avoidance strategies possible by cutting speed variation. Presented results are important from the point of view of assuring the stability of band sawing process which is reflected in chatter phenomena and related low quality of the cut surface or tool brakeage. The results are basis for further research and development of a system for an automatic detection of chatter onset by which a higher productivity and quality of the band sawing could be achieved.
COBISS.SI-ID: 13966107
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 0.050 mm and a standard deviation of 0.015 mm. 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. In the developed system a novel concept of annular laser beam is applied, which assures process symmetry and stability. The presented results are important for further development of novel droplet based technologies including micro joining of temperature resistant joints in electronic and electrical industry and 3D metal printing technology. In addition to this, the annular laser beam concept can be adapted for laser direct deposition of metal wire or powder.
COBISS.SI-ID: 14120731
The subject of this study is the vertical mass-spring-like oscillation of a pendant droplet and its resonant detachment, which was experimentally observed in the process of laser droplet generation from a metal wire. The process was characterized by various time series, which were generated from a sequence of infrared intensity images of the process. Following a visual inspection of pendant droplet images and an analysis of a wavelet based time-frequency map of the droplet’s vertical displacement time series, the pendant droplet’s oscillation is described by a time-variable mass-spring system. Based on the characteristics of the time-frequency map, the resonant nature of the pendant droplet detachment was demonstrated. Additionally, an algebraic expression was formulated, which can be used to predict the detached droplet’s diameter as a function of the laser pulse frequency. The presented results are important for application of the laser droplet generation process in the fields of droplet joining and 3D structuring technologies. The theoretical description of the process dynamics by a time-variable mass-spring system enables selection of process parameters values in advance for generation of metal droplets with a selected diameter.
COBISS.SI-ID: 14623515
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. Importance of the achievement is reflected in the introduction of the latest modern non-linear machine learning methods in the field of classification and diagnosis of industrial production of household appliances. The achievement will have effects in improved quality of classification and fault detection in industrial production.
COBISS.SI-ID: 15296539
This paper presents a high-speed thermal imaging method using a visible light camera, with application to the laser droplet generation process (LDG). In the experiment, a nickel wire-end was exposed to a collimated laser beam, and the subsequent process of wire melting, pendant droplet formation and its detachment were recorded by a high speed camera. Instantaneous temperature fields of the metal surface were calculated from the imaging data and were characterized by a very good spatial and temporal resolution (200 x 400 pixels at 13,837 frames per second). The droplet temperature could be accurately calculated between the melting point of nickel (1455 °C) and approximately 1950 °C, where image saturation started to occur. The remaining pendant droplet was shown to cool much more rapidly than the detached droplet, which is due to the heat conduction to the solid wire. Except for the time immediately after the droplet separation, the temperature distribution across the melt droplets was found to be quite uniform. Apart from the possibility of temperature field calculation, it was also demonstrated that the high-speed images of the LDG process can accurately capture contours and oscillation dynamics of melt droplets. Presented results are important for development of laser droplet generation process applications in the field of droplet joining and 3D structuring technologies where the detached droplet temperature is one of the most important parameters that influences the quality of joint between the deposited droplet and the workpiece. At the same time, a cost-effective method of measurement of the droplet temperature field and its contour with a high temporal and spatial resolution was demonstrated which opens the possibilities of further investigations of the laser droplet generation process temperature field and dynamics that would not be achievable employing the infrared camera.
COBISS.SI-ID: 14815515