1,721,020 research outputs found
Synthesis of Vibration Signals with Prescribed Power Spectral Density and Kurtosis Value
No full textIn the field of vibration qualification testing, random excitations are typically set as input in terms of a PSD profile. The physical motion at the shaker head is obtained through the application of the Inverse Fourier Transform in combination with randomized phases. The overall probability distribution of the input signal tends toward Gaussian, whereas distinctive peaks are often present in real-life random excitations, causing the probability distribution to be non-Gaussian. The parameter known as kurtosis is usually exploited to quantify the feature of non-Gaussianity. Several methods have been proposed to control kurtosis, still maintaining the desired PSD profile, in order to synthesize more realistic signals. However, kurtosis control implemented by some of these methods may be ineffective. In fact, in some cases, the response of a lightly damped system can prove closer to Gaussian than the applied excitation. This work presents two novel algorithms to effectively control kurtosis in random vibration tests are proposed
Advanced procedures for accelerated vibration-based durability tests
No full textAmong vibration qualification tests, durability testing is aimed at verifying components’ endurance against the fatigue damage induced by environmental vibrations. For the sake of practical feasibility, it is often required to accelerate the test duration with respect to the expected lifetime of the component still preserving the entire fatigue damage experienced by the latter during its operational life.
The spectral function named Fatigue Damage Spectrum (FDS), which is assumed to estimate the fatigue damage potential of (measured) environmental excitations, is commonly used to synthesize vibratory signals that will then be used as input profiles in the accelerated tests. However, the standard practice presents some limitations: in fact, the synthesized signals are in the form of a Power Spectral Density, thus characterized by a Gaussian signal distribution. This could be a strong limitation given the recurring non-Gaussianity of signals measured in real applications. The statistical parameter known as kurtosis is usually employed to account for the deviation from Gaussianity and a number of kurtosis-control algorithms are proposed in the literature as a solution to make accelerated tests more realistic. Yet, some possible problems associated with these techniques should be highlighted: in particular, the kurtosis is only a global metric that roughly estimates the non-Gaussianity of the input signal neglecting the response of the system, on which – conversely – the calculation of the FDS is based.
This presentation proposes new methodologies that are based on original algorithms able to synthesize test signals by controlling simultaneously the FDS in conjunction with other target parameters. Each algorithm can control different features of the signal to be synthesized and its selection depends on user demands. The common outcome of these algorithms is the synthesis of non-Gaussian signals, which aim to extend the standard procedures in order to make the accelerated tests more realistic and reliable
Analysis of synthesized non-Gaussian excitations for vibration-based fatigue life testing
Get PDFCertain applications require that some critical components must undergo vibration qualification tests to check their suitability with respect to dynamic excitations. The common procedure is to consider measured field data as reference for the synthesis of random stationary signals to be used as input excitations for shakers or slip-tables. The current synthesis procedures usually generate test profiles in terms of PSD, corresponding to stationary random processes with Gaussian probability distribution of values. Such signals may be unrealistic in representing the characteristics of the reference data if the latter are not Gaussian. The Kurtosis parameter is often used to synthetically represent the amount and the amplitude of signal peaks. Its value is 3.0 for Gaussian signals, whereas higher values hold for signals featuring non-negligible high peaks, for example due to micro-shocks. In case of accelerated fatigue life tests, the synthesized signal must induce, in a limited duration, the same fatigue damage which the reference signal cause on the tested component throughout the expected lifetime of the latter. The Fatigue Damage Spectrum (FDS) is generally used to quantify the fatigue damage potential associated with the excitation. The test signal is synthesized targeting the same FDS computed for the reference profile. This paper presents two kurtosis-control algorithms for the synthesis of test profiles in combination with a technique able to match the prescribed FDS
Elastodynamic analysis of the desmodromic valve train of a racing motorbike engine by means of a combined lumped/finite element model
No full textA combined lumped/finite element model of a portion of the desmodromic valve train of a racing motorbike engine was developed and validated in order to simulate the elastodynamic behaviour of such a particular timing system. The model includes the lumped parameter model of the belt transmission that drives the camshafts, the finite element model of the camshafts, and the lumped parameter model of two cam-valve mechanisms (one for each camshaft). The procedure to validate the model, based on experimental tests carried out on a test bench described here, is presented and discussed. The comparison between the numerical results and the experimental data shows that the effectiveness of the model is satisfactorily achieved. It will be possible, in a further study, to add the other cam-valve mechanisms and the missing external forces, in order to obtain a complete system model. Some possible applications of the presented model are provided in order to show how the overall model could be employed to perform both design optimisation and diagnostic
Numerical and experimental analyses to enhance the vibration response of rotary transfer machines
No full textThis work deals with the elastodynamic analysis of rotary transfer machines. Such systems are machine tools conceived for the mass-production of families of components. They typically feature a rotary indexing table carrying the workpieces and multiple machining units operating simultaneously. Cutting forces between tools and workpieces and rapid motion of both the indexing table and the machining units can excite the system resonances in a wide frequency range and trigger elastodynamic phenomena possibly detrimental for both the quality of machined surfaces and the tool life. Assessing and solve potential vibration issues is therefore essential to ensure the correct operation of the machine tool.
The study focuses on a new machine tool designed for machining components for the lock&keys industry. It features ten functional stations and fifteen CNC machining units (nine located on the main structure, six on satellite supports). Hydraulically-actuated stiffeners engage the clamping elements during the machining processes to limit their static and dynamic deformations. The manufacturer aims at further reducing the vibration levels by improving the current design of the machine tool. Two strategies are investigated to attain the goal. The first (more conventional) approach consists in optimizing the vibration response of the system through changes of the geometry of its structure and/or subassemblies. A Finite Element model of the whole machinery was implemented to evaluate the effects of possible modifications (e.g. additional stiffeners), by performing numerical modal analyses. The model was validated through Experimental Modal Analysis (EMA) to achieve an adequate reliability. EMA was carried out by considering about 30 measuring points (acceleration signals) as well as a highly redundant dataset. Indeed, numerous signals were required to correctly identify the mode shapes, due to the high modal density of the system. Moreover, redundant data were exploited for comparing the results provided by different subsets, in order to determine the most effective subset for estimating the modal parameters (since exciting properly the system proved a challenging task, due to its complexity).
The second strategy is based on the use of polymer concrete for filling some of the primary structures of the machine tool to increase their damping. The study (still at an early stage) started with experimental tests to assess the effects of polymer concrete on the damping parameters of simple profile bars. Further experiments with more complex components of the real machine tool are being conducted
Structural and elastodynamic analysis of rotary transfer machines by Finite Element model
Get PDFVibration monitoring and control are central topics for machine tools, since high vibration levels reduce the quality of machined surfaces and shorten the tool life. In order to predict potential vibration issues since the early design stage, it is necessary to implement ad hoc numerical models for modal analysis. This requires significant efforts and possible conflicts with tight production scheduling of companies. This work focuses on a specific family of rotary transfer machines for the manufacturing of parts related to lock & keys industry. It investigates the possibility to achieve an acceptable estimation of the elastodynamic behavior of the machine tools through limited modifications of the Finite Element (FE) models used for structural analysis, which are generally available in the early phases of the design process. The structural FE model of a new machine tool is implemented and validated through experimental tests performed on a prototype. Then, the elastodynamic FE model is derived and simulated. The numerical results are consistent with the data provided by Experimental Modal Analysis (EMA). Hence, the proposed approach is confirmed viable and will be integrated in the company workflow of future machine designs
Definizione, acquisizione sperimentale ed elaborazione di traiettorie di riferimento della mano umana per la sintesi di architetture protesiche di arto superiore
No full textThis paper reports an essential part of a wider research activity, which entails the development of a procedure for the Determination of the Optimal Prosthesis Architecture (DOPA) for a given upper limb amputee. A fundamental algorithm of the DOPA procedure performs the kinematic analysis of several prosthetic arm models (also with less than the six degrees of freedom normally required to correctly execute a generic manipulation task). The algorithm must simulate the execution of important daily living activities performed by a prosthesis and thus it requires reference trajectories of the hand. By means of experimental analysis, 59 trajectories of the hand of an able-bodied subject were acquired to identify a modality to correctly perform the corresponding tasks. This paper illustrates in detail the stages of task analysis, experimental acquisition and data processing in order to define the required reference trajectories. The obtained reference trajectories are a temporal succession of the hand pose (position and orientation). A customized algorithm automatically selects the most relevant poses to be considered for the definition of the reference trajectory. The hand pose is reported in the Cartesian Space by means of Natural Coordinates. In order to correctly execute a given task the pose error admitted for the end-effector of the different architectures is associated to each trajectory. In particular, the critical problem to express the orientation error is solved by means of the use of Spherical Rotation Coordinates
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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