Janko Slavič

Damping identification using a continuous wavelet transform: application to real data

A continuous wavelet transform (CWT) based on the Gabor wavelet function is used to identify the damping of a multi-degree-of-freedom system. The common procedures are already known, especially the identification with a Morlet CWT. This study gives special attention to the following: a description of the instantaneous noise, the edge-effect of the CWT, the frequency-shift of the CWT, the bandwidth of the wavelet function and the selection of the parameter σ of the Gabor wavelet function of the CWT. The procedures are demonstrated using several numerical examples and on signals acquired from the lateral vibration of a uniform beam. The study demonstrates the advantages of using the amplitude and phase methods, both of which provide information about the instantaneous noise. The procedures presented are appropriate for automating the identification process.

Typical Bearing-Fault Rating Using Force Measurements-Application to Real Data

In contrast to the commonly used acceleration measurement, this research discusses the use of force measurements to identify bearing faults. A force sensor is fixed between the rigid surroundings and the bearing to measure all of the reactive forces due to the vibration excitation. Using a force measurement, systematically prepared samples with the five typical faults that can occur during the assembly process (axial, radial, bending moment, contamination and shield defect) were investigated. The samples were prepared with low, medium and high fault ratings. The force measurement, with its relatively simple signal processing based on an envelope detection, was shown to be successful in correctly identifying both the fault rating and the fault type. The presented approach was successfully applied to high-series assembly production and is relatively easy to apply to similar applications.

A comparison of strain and classic experimental modal analysis

This research is focused on a comparison of classic and strain experimental modal analysis (EMA). The modal parameters (the natural frequencies, the displacement mode shapes (DMSs) and the damping) of real structures are usually identified with classic EMA, where the responses are measured with motion sensors (e.g. accelerometers). Strain EMA is a special approach in the field of EMA, where the responses are measured with strain sensors. Classic EMA is the preferred method, but strain EMA offers advantages that are important for particular applications: for example, the direct identification of strain mode shapes (SMSs), which is important in the vibration-fatigue and damage-identification models. The next advantage is that strain EMA can sometimes be used, for experimental/geometrical reasons, where classic EMA cannot. There are also drawbacks: for example with strain EMA only, the mass-normalization of the DMSs and SMSs cannot be performed. This study researches the theoretical similarities and differences of both EMA approaches. Furthermore, the accuracy of both approaches for the case of a free–free supported beam and a free–free supported plate is investigated. Classic and strain EMA were performed with a piezoelectric accelerometer and the piezoelectric strain gauges, respectively. The results show that the accuracy of strain EMA results (the natural frequencies, DMSs and the damping) is comparable to the accuracy of classic EMA.

The use of strain sensors in an experimental modal analysis of small and light structures with free-free boundary conditions

Small and light structures have distinctive features, which cause difficulties in the measurement of their modal parameters. The major issues are the mass, which is added to the measured structure by sensors, and the very high resonant frequencies. Those difficulties occur with a measurement of the excitation force. An innovative procedure for the experimental modal analysis of small and light structures was developed in this study. This procedure involves a measurement of the excitation force, which was performed by a piezo strain gauge that enables an analysis of the aforementioned structures with free–free support. The main advantage of this sensor in comparison with other devices used for force measurements is that it adds a very small mass to the measured structure (≈0.4 g) but at the same time enables an accurate measurement of the modal parameters in a wide frequency range (up to 20 kHz). This makes it suitable for a measurement of the frequency-response functions of light structures that have high resonant frequencies. Consequently, an experimental modal analysis can be performed. The presented approach was experimentally tested on a sample with small dimensions and mass. The results of the experiment (modal parameters) were compared with the results of the numerical model. The good agreement between the results indicates that this procedure can be used on other similar structures.

Measurement of the bending vibration frequencies of a rotating turbo wheel using an optical fibre reflective sensor

An optical fibre reflective sensor was used to analyse the vibrations of a rotating turbo wheel up to 20 400 rpm. The measured signal required correction because of the natural unevenness of the turbo wheel and because of the variable deflection. Because the turbo wheel was rotating the signal became distorted and so we used a special method to extract the frequencies of the vibrations from the power spectra. The analysis showed increased intensity of the first three natural frequencies with an increased speed of rotation. The experimental results match very well with those obtained by numerical computation.

A Generalized Magnetostrictive-Forces Approach to the Computation of the Magnetostriction-Induced Vibration of Laminated Steel Structures

This research introduces a new, numerical and experimental approach to the analysis of the vibration of laminated structures resulting from magnetostriction. The focus is on the in-plane magnetostriction of electrical steel and its transmission into the out-of-plane direction, in which laminated structures (e.g., transformer cores, stators, and rotors) exhibit the greatest vibration. A finite-element magnetostriction model is developed on an experimental basis and enables a general, in-plane and out-of-plane assessment of the magnetostrictive response. The magnetostriction model is compatible with various finite-element structural models and is incorporated into a structural model, updated based on experimental data, representing a clamped laminated structure. An experiment employing the operating-deflection-shapes method is used to assess the presented approach under various operating conditions.

Non-linearity and non-smoothness in multi-body dynamics: Application to woodpecker toy

Abstract The introduction of the article gives a short historical overview of the modelling of multi-body dynamics with unilateral contacts. The unilateral contacts formulation as introduced by Pfeiffer and Glocker is adapted to discretely defined body shapes. By using two-step collision detection, a fast and exact collision detection is achieved. The procedures are tested on a numerical example of the woodpecker toy and the results are compared with those of other authors who used a simpler mathematical model.

Frequency Characteristics of Magnetostriction in Electrical Steel Related to the Structural Vibrations

The ac magnetostriction in electrical steel is commonly characterized in the time domain (e.g., the peak-to-peak, zero-to-peak amplitude) and also in the frequency domain (e.g., a harmonic analysis). However, due to the dynamical coupling of the test sample with the experimental setup, the characterization of the magnetostriction (especially the one in the frequency domain) can give the wrong result. This research focuses on an experimental frequency characterization of magnetostriction and gives the theoretical background of the test samples dynamical coupling with the experimental setup. The discussed natural dynamics of the test sample from the point of view of the different boundary conditions that can be used at the experiment gives a clear picture of the dynamical coupling. Besides the theoretical background, a detailed experimental approach is presented. This paper theoretically and experimentally shows that the dynamical coupling of the sample can result in incorrect characterization of the magnetostriction. However, with the theoretical guidelines presented, the dynamical coupling can be completely avoided, which results in an accurate characterization of the magnetostriction.

Fatigue Damage for Sweep-Sine and Random Accelerated Vibration Testing

The vibration testing of components in the automotive industry requires long testing times and the use of expensive facilities. To shorten these testing times an accelerated vibration-testing approach is usually applied. This research tries to shorten these testing times by considering the parameters that define vibration-testing techniques. With special attention to the excitation types sweep-sine and random, a damage-based approach is applied. The phenomenon of fatigue damage is closely observed from the frequency-domain point of view but also by considering the relationship between the time and the frequency domains. The Palmgren-Miner cumulative rule is applied to calculate the fatigue life. Two case studies of measured responses are used to compare the times to failure. The results show that proposed predictions can be used to compare different testing techniques, but they are not so accurate when predicting the actual times to failure.

A Sequential Approach to the Biodynamic Modeling of a Human Finger

In an effort to understand the vibration-induced injuries incurred by manual workers, mechanical models are developed and used to predict the biodynamic responses of human body parts that are exposed to vibration. Researchers have traditionally focused on the arms and hands, but there has been only limited research on finger modeling. To simulate the accurate response of a single finger, a detailed mechanical model based on biodynamic finger measurements is necessary. However, the development of such models may prove difficult using the traditional one-point coupling method; therefore, this study proposes a new approach. A novel device for single-finger measurements is presented and used to expose the finger to a single-axial broadband excitation. The sequentially measured responses of the different finger parts are then used to identify the parameters of a multibody mechanical model of the index finger. Very good agreement between the measured and the simulated data was achieved, and the study also confirmed that the obtained index-finger model is acceptable for further biodynamic studies.