Experimental Transfer Function Research Articles

Cross-Coupled Dynamics and MPA-Optimized Robust MIMO Control for a Compact Unmanned Underwater Vehicle

A compact, 3-degrees-of-freedom (DoF), low-cost, remotely operated unmanned underwater vehicle (UUV), or MicroROV, is custom-designed, developed, instrumented, and interfaced with a PC for real-time data acquisition and control. The nonlinear equations of motion (EoM) are developed for the under-actuated, open-frame, cross-coupled MicroROV utilizing the Newton-Euler approach. The cross-coupling between heave and yaw motion, an important dynamic of a class of compact ROVs that is barely reported, is investigated here. This work is thus motivated towards developing an understanding of the physics of the highly coupled compact ROV and towards developing model-based stabilizing controllers. The linearized EoM aids in developing high-fidelity experimental data-driven transfer function models. The coupled heave-yaw transfer function model is improved to an auto-regressive moving average with exogenous input (ARMAX) model structure. The acquired models facilitate the use of the multi-parameter root-locus (MPRL) technique to design baseline controllers for a cross-coupled multi-input, multi-output (MIMO) MicroROV. The controller gains are further optimized by employing an innovative Marine Predator Algorithm (MPA). The robustness of the designed controllers is gauged using gain and phase margins. In addition, the real-time controllers were deployed on an onboard embedded system utilizing Simulink′s automatic C++ code generation capabilities. Finally, pool tests of the MicroROV demonstrate the efficacy of the proposed control strategy.

One-fault-shot learning for fault severity estimation of gears that addresses differences between simulation and experimental signals and transfer function effects

Gearboxes are integral elements in rotating machines and have a high tendency to fail due to their operation in harsh conditions. A robust method to estimate the fault size of gears is desirable for a successful prognostic process, which is, to date, still unavailable in the literature. The fault size can be estimated by vibration analysis, using signal processing and machine-learning tools. However, the availability of labeled or unlabeled vibration signals from faulty rotating machinery components is rare, making it challenging to apply machine-learning algorithms. Therefore, some physical pre-knowledge should be incorporated in the model for a successful learning process. This can be done by exposing the learning model to simulated data, and by a physical pre-processing procedure. This paper suggests a novel algorithm to overcome the lack of faulty data (labeled and unlabeled), and it is trained on a combination of simulated data and some real data. The algorithm tunes the differences between simulation and experiment using one faulty experimental example, and transfers knowledge from simulation to reality by addressing the transfer function effects. It addresses the transfer function by spectrum background estimation and minimum phase estimation while also selecting features that are invariant to the unmitigated effects of the transfer function. The new algorithm is demonstrated on simulated signals and measured transfer function, and on experimental signals with known fault sizes.The codes and the data of the study are available via the link: https://github.com/omriMatania/one_fault_shot_learning_for_gears_fault_severity_estimation.

Development of experimental Vibro-acoustic transfer function for a system with impact

Systems with combined sliding-rolling contacts such as cam-follower, clutches, and gearbox are prone to have clearance(s) as per the design and/or due to manufacturing imperfections or wearing as a result of sliding between components. This clearance non-linearity results in an impact between components, which in turn generates a significant rattle during operation. The noise generation mechanism in rattle is a strong function of impact-velocity and contact forces. The objective of this study is to develop an experimental transfer function to quantify the vibro-acoustics of a cam-follower system with clearance non-linearity. Contact between the cam and the follower is lubricated to minimize the sound generated due to friction. Follower acceleration, reaction forces, and acoustic pressure are measured on a cam-follower setup with combined rolling-sliding contact during impacts under various conditions. Velocity and contact forces are back-calculated using measured signals and the analytical model dynamics. Transfer functions relating the acoustic pressure to velocity and contact forces are estimated in the frequency domain. These transfer functions provide insights into the vibro-acoustic system and can be directly used in conjunction with dynamics models. The output of the study can be used in designing quieter systems with impact.

Characterization of Tensile and Fatigue Damages in Composite Structures Using Lamb Wave for Improved Structural Health Monitoring

The majority of ultrasonic characterizations are done on thermoplastics, with just a few articles available on the characterization of thermoset resin characteristics. A non-destructive methodology for monitoring fatigue and static deformation induced by mechanical loading on a fiber-reinforced plastic is presented. However, these materials’ dynamics of elastic waves are considerably more complicated. A large part was devoted to the calculation of dispersion curves of guided waves in composites. Therefore, this study presents a thorough description of the Glass/Epoxy system by comparing ultrasonic and mechanical data. Ultrasonic wave propagation at high frequencies, functioning as a dynamic mechanical deformation, may be utilized to calculate longitudinal and shear moduli during static and dynamic loading. The evolution of attenuation and velocity during loading is linked to the significant changes that occur during the aging process. The experimental transfer function is determined by the Fourier transform of all the obtained ultrasonic echoes.

Shaped input for reducing crosstalk of two-axis MEMS scanners

We have proposed a pre-shaped input method to minimize the crosstalk of two-axis microelectromechanical system (MEMS) scanners. The mechanical and geometrical crosstalk of a two-axis MEMS scanner was estimated to mitigate the total crosstalk of the scan pattern in an image plane. The desired scan pattern was converted into a required mechanical scan angle to compensate for the geometrical crosstalk. The device used in this study was a two-axis gimbal-less MEMS scanner, whose experimental transfer functions were obtained in terms of the mechanical scan angle. The method was evaluated for one-axis driving, reducing the total crosstalk by an average of 23.1 and 2.78 times along the x- and y -axis, respectively. The method was extended to two-axis driving, reducing the total crosstalk by an average of 18.1 and 2.49 times along the x- and y -axis, respectively. The lower reduction in the y -axis can be explained by the fact that the geometrical crosstalk appears much more on the y -axis than x -axis in the present measurement setup. In conclusion, the high suppression of crosstalk experimentally indicates that this method can effectively mitigate the total crosstalk and will be helpful to improve the performance of MEMS scanners. • Pre-shaped input was applied to obtain desirable output for two-axis MEMS scanners. • Two-axis pre-shaped input was carried out to reduce the crosstalk in MEMS scanner. • Scan pattern was improved by removing optical and mechanical crosstalk.

Transfer function model-based Bayesian estimation of dissipation and sound speed in impedance tube measurements

The air dissipation, mainly due to the boundary effect at interior walls of impedance tubes, is noteworthy for normal incidence measurements of acoustic materials using the transfer function method. This work proposes a transfer function model-based Bayesian approach to estimate the dissipation factors. Other parameters, such as the sound speed and the microphone positions, are also critical, therefore, estimated as well, along with the dissipation. The measurement of the empty impedance tube with a rigid termination applies the two-microphone transfer function method. A hypothetical air layer assumed in front of the rigid backing enables the numerical comparison between the air layer’s theoretical and experimental transfer function values. Experimental results incorporating the estimated dissipation coefficient validate the accuracy of the proposed approach. Furthermore, the estimated parameters are incorporated into further evaluations of material properties using the transfer function method, which effectively reduces the residual absorption error of the impedance tube measurement.

Structure-borne noise of marine diesel engines: Dynamic characterization of resilient mounts

This paper presents an experimental procedure to measure the dynamic properties of resilient mounting elements, commonly used in the marine industry to isolate the marine diesel engine from the supporting structure. First, the transfer mobility function of the isolator is measured experimentally. A numerical model, simulating the modal response with a finite-element model is then created. Using Response Surface Methodology (RSM), the frequency-dependent values of the damping ratio of the numerical model are optimized in order to maximize the fit to the experimental transfer function. This analysis, performed on two resilient mounting elements for marine applications, with different shape and size, is used to outline an experimental methodology for the dynamic characterization of a vibration isolator. The analysis presented in this paper builds upon the outcome of a previous work, where the coefficients of the hyperelastic constitutive relationship of the rubber material of the isolator, used in this paper, are estimated with RSM.

Development of experimental vibro-acoustic transfer function for a system with combined rolling-sliding motion

Combined rolling-sliding contact is present in popular non-linear systems such as cam-follower, gears, clutches, and brakes. These systems produce significant noise due to complex contact between the components during operation. The noise generated is a strong function of the contact parameters and excitation to the system. The objective of this study is to develop a transfer function to quantify the vibro-acoustic noise for various contact conditions. Acceleration, reaction forces, and acoustic pressure measurements are made on a cam-follower setup with combined rolling-sliding contact. Experiments are performed under different conditions of friction, lubrication, load, and speed. Contact forces are back-calculated using the kinematics. The transfer function relating the acoustic pressure to different forces is estimated. It is observed that the contact parameters govern the transfer function and hence the vibro-acoustic systems. The developed transfer function is useful in designing better sub-systems with combined rolling-sliding contact to reduce noise exposure, as a direct technique to relate the contact parameters to the noise does not exist. This study can be extended to other complex systems such as gears and clutches.

Experimental investigation of a closed vertical cylinder-shaped fish cage in waves

Wave-induced response of a closed floating fish cage consisting of a vertical circular cylinder with an external toroidal floater is studied theoretically and experimentally. A main purpose was to investigate how the internal sloshing would influence the global response of the cage, the interior wave elevation and also the mean drift loads. An ocean basin laboratory was used in order to minimize tank wall effects. An optical system involving eight markers was used to obtain experimental values for radial elastic deformations of the cylindrical part. The closer a coupled natural period between body motions and sloshing is to a corresponding natural sloshing period, the more nonlinear sloshing can be. In the examined case, the highest coupled natural period between surge, pitch and sloshing is 0.8 times the highest natural sloshing period. Linear potential flow theory can, in general, explain experimental transfer functions of rigid-body motions and sloshing due to rigid-body motions obtained by both regular wave and truncated white noise tests. Theoretical second order mean wave drift forces based on a rigid body and potential flow agree also well with experimental results. Resonant 3D sloshing was excited due to ovalizing hydroelastic structural modes in both regular waves and white-noise tests. The closeness between the corresponding structural and sloshing frequencies caused large response. A linear hydroelastic analysis based on WAMIT and LS-DYNA could partly explain the response. Parametric resonant pitch response occurred experimentally at large wave periods partially due to in and out of water motion of portions of the floater and was explained by treating the pitch as uncoupled motion modeled by the Mathieu equation. The pitch motion predicted by linear potential flow theory was, in general, unsatisfactory when the floater was partly out of the water occasionally.

Solution of Thermoacoustic Eigenvalue Problems With a Noniterative Method

Abstract Gas turbine combustors are prone to undesirable combustion dynamics in the form of thermoacoustic oscillations. Analysis of the stability of thermoacoustic systems in the frequency domain leads to nonlinear eigenvalue problems (NLEVP); here, “nonlinear” refers to the fact that the eigenvalue, the complex oscillation frequency, appears in a nonlinear fashion. In this paper, we employ a noniterative strategy based on contour integration in the complex eigenvalue plane, which returns all eigenvalues inside the contour. An introduction to the technique is given, and is complemented with guidelines for the specific application to thermoacoustic problems. Two prototypical nonlinear eigenvalue problems are considered: a network model of the classical Rijke tube with an analytic flame response model and a finite element discretization of an annular model combustor with an experimental flame transfer function (FTF). Computation of all eigenvalues in a domain of interest is vital to assess stability of these systems. We demonstrate that this is generally challenging for iterative strategies. An eigenvalue solver based on contour integration, in contrast, provides a reliable, noniterative method to achieve this goal.

Three-dimensional effects on the transfer function of a rectangular-section body in turbulent flow

This paper investigates the influence of three-dimensional effects on the transfer function of a rectangular-section body in turbulent flow. The dimensionless factor$\unicode[STIX]{x1D713}$, as derived by Liet al.(J. Fluid Mech., vol. 847, 2018, pp. 768–785), is adapted to evaluate this influence. The calculation of$\unicode[STIX]{x1D713}$requires the spanwise influence term. For this purpose, an adapted form of the lift coherence function is derived, enabling the use of the measured lift coherence for the estimation of the spanwise influence term. Three rectangular models with different cross-sections (chord-to-depth ratios of 3, 5 and 10) are chosen for testing, and a NACA 0015 airfoil model is tested for comparison. Using the measured spanwise influence terms, the dimensionless factors of these models are then numerically calculated under different ratios of the turbulent integral scale to the chord$\unicode[STIX]{x1D6FE}$and aspect ratios$\unicode[STIX]{x1D703}$. It is shown that the dimensionless factors of the rectangular models increase as$\unicode[STIX]{x1D6FE}$and$\unicode[STIX]{x1D703}$increase, which are similar to the dimensionless factor of the airfoil model. If$\unicode[STIX]{x1D6FE}$and$\unicode[STIX]{x1D703}$have suitable values, the strip theory could be applicable to the rectangular-section body. It is also found that the dimensionless factors of all the rectangular models are larger than the dimensionless factor of the airfoil model under the same parameters. The smaller the chord-to-depth ratio is, the larger the dimensionless factor is. Using the strip theory to calculate the lift response of the rectangular-section body may provide more accurate estimation. Additionally, the one-wavenumber transfer functions of these models are determined under the consideration of the three-dimensional effects. The results show that the experimental transfer functions of the rectangular models cannot be captured by the Sears function. They are larger than the Sears function at lower frequencies, while falling at a faster rate as the frequency increases. For bluff bodies with separated flow, the modified transfer function presented here appears to be an appropriate approach.

Experimental Assessment of the Performance of CECO Wave Energy Converter in Irregular Waves

The performance assessment of a wave energy converter (WEC) is a key task. Depending on the layout of the WEC system and type of power take-off (PTO) mechanism, the determination of the absorbed power at model scale involves several challenges, particularly when the measurement of PTO forces is not available. In irregular waves, the task is even more difficult due to the random character of forces and motions. Recent studies carried out with kinetic energy harvesters (KEH) have proposed expressions for the estimation of the power based only on the measured motions. Assuming that the WEC behaves as a KEH at model scale, the expressions for power estimation of KEHs have been heuristically adapted to WECs. CECO, a floating-point absorber, has been used as case study. Experimental data from model tests in irregular waves are presented and analyzed. Spectral analyses have been applied to investigate the WEC responses in the frequency domain and to derive expressions to estimate the absorbed power in irregular waves. The experimental transfer functions of the WEC motions demonstrated that the PTO damping is significantly affected by the incident waves. Based on KEH approach's results, absorbed power and PTO damping coefficients have been estimated. A linear numerical potential model to compute transfer functions has been also implemented and calibrated based on the experimental results. The numerical results allowed the estimation of combined viscous and losses effects and showed that although the KEH approach underestimated the absorbed power, qualitatively reproduced the WEC performance in waves.

Input Shaping Based on an Experimental Transfer Function for an Electrostatic Microscanner in a Quasistatic Mode.

This paper describes an input shaping method based on an experimental transfer function to effectively obtain a desired scan output for an electrostatic microscanner driven in a quasistatic mode. This method features possible driving extended to a higher frequency, whereas the conventional control needs dynamic modeling and is still ineffective in mitigating harmonics, sub-resonances, and/or higher modes. The performance of the input shaping was experimentally evaluated in terms of the usable scan range (USR), and its application limits were examined with respect to the optical scan angle and frequency. The experimental results showed that the usable scan range is as wide as 96% for a total optical scan angle (total OSA) of up to 9° when the criterion for scan line error is 1.5%. The usable scan ranges were degraded for larger total optical scan angles because of the nonlinear electrostatic torque with respect to the driving voltage. The usable scan range was 90% or higher for most frequencies up to 160 Hz and was drastically decreased for the higher driving frequency because fewer harmonics are included in the input shaping process. Conclusively, the proposed method was experimentally confirmed to show good performance in view of its simplicity and its operable range, quantitatively compared with that of the conventional control.

Direct measurement of the Sears function in turbulent flow

The applicability of the strip assumption in the estimation of the unsteady lift response of a two-dimensional wing in turbulent flow is investigated. The ratio between the lift spectrum calculated from the two-wavenumber analysis and the lift spectrum calculated from the strip assumption is used to evaluate the accuracy of the strip assumption. It is shown that the accuracy of the strip assumption is controlled by the ratio of the turbulence integral scale to the chord and the aspect ratio. With an increase of these two parameters, the ratio for evaluating the accuracy of the strip assumption increases, the one-wavenumber transfer function obtained from the strip assumption approaches the Sears function gradually. When these two parameters take suitable values, the strip assumption could be applicable to the calculation of the unsteady lift on a wing in turbulent flow. Here, the term aspect ratio refers to the ratio of the specified span (an finite spanwise length of the two-dimensional wing) to the chord, the unsteady lift is calculated over this specified spanwise length. The theoretical analysis is verified by means of force measurement experiments conducted in a wind tunnel. In the experiment, a square passive grid is installed downstream of the entrance of the test section to generate approximately homogeneous and isotropic turbulence. Three rectangular wings with different aspect ratios ($\unicode[STIX]{x1D703}=3$, 5 and 7) are used. These wing models have an NACA 0015 profile cross-section and a fixed chord length$c=0.16~\text{m}$. The testing results show that, at a fixed ratio of turbulence integral scale to chord, the deviation between the experimental one-wavenumber transfer function obtained from the strip assumption and the Sears function is reduced with increasing aspect ratio, as expected by the theoretical predictions. However, due to the effect of thickness, the experimental values at high frequencies cannot be captured by the Sears function which is derived based on the flat plate assumption. In practical applications, the effect of thickness on the transfer function should be considered.

2D site response analysis of a cultural heritage: the case study of the site of Santa Maria di Collemaggio Basilica (L’Aquila, Italy)

The Santa Maria di Collemaggio Basilica is an important cultural heritage site and exemplifies Romanesque-Gothic art in the Abruzzo region (central Italy). Erected in the second half of the XII century, the Basilica was severely damaged during the April 6, 2009 L’Aquila earthquake (MW 6.1). In particular, the area of the transept collapsed causing the dome to fall. A refined two-dimensional (2D) geotechnical model was built representing a section that includes the Basilica, in order to better understand the soil response of the Basilica site. The subsoil model was constrained using the geophysical and geotechnical data collected from the seismic microzonation studies, the reconstruction of private damaged buildings and other technical and scientific studies realized in the L’Aquila basin and in the area of the Basilica before and after L’Aquila earthquake. 2D site response analyses were performed to verify the presence of local site effects by comparing simulated versus experimental transfer functions. Moreover, a frequency–wavenumber (f–k) analysis was executed with the aim of evaluating the occurrence of surface waves generated within the basin. 2D seismic effects involve significant amplification in the period range of engineering interest, therein providing an appropriate elastic response spectrum for the restoration of the Basilica.

In-plane vibration analysis of heavy-loaded radial tire utilizing the rigid-elastic coupling tire model with normal damping

Theoretical modeling, parameters identification and vibration characteristic of heavy-loaded radial tire is investigated with rigid-elastic coupling model with normal damping. The normal damping, including structural damping of flexible carcass and proportion damping of distributed sidewall element is added to enrich the flexible beam on modified elastic foundation tire model. The rigid-elastic coupling tire model with normal damping is investigated and derived with finite difference method. The mass, stiffness and damping matrixes of the proposed tire model are analytically related with the structural and geometrical parameters of heavy-loaded radial tire. Taking the error between the analytical and experimental transfer function as the object value, Genetic Algorithm (GA) is utilized to identify the damping coefficients of flexible carcass and distributed sidewall element. The influence of modal order and tire damping parameters on the in-plane transfer function is discussed. The theoretical and experimental results show that the rigid-elastic coupling tire model with normal damping can achieve the sectional feature of in-plane transfer function resulting from the coupling characteristic between the flexible carcass and distributed sidewall element within the frequency band of 300 Hz.

Performance evaluation of a circular electrical aerosol classifier (CirEAC)

Two circular electrical aerosol classifiers (CirEACs), i.e., one in large diameters (large version, LV) and the other in small diameters (small version, SV), were designed and their performance was evaluated in this study. The design of CirEACs stems from the demand to have an extended particle size classification range for a miniature electrical mobility classifier while keeping it compact in the package. A CirEAC is constructed by two coaxially aligned cylinders. The particle classification channel is established in the annular spacing defined by two cylinders. Aerosol stream flows into the classification channel via a one-end-closed tube with a series of side holes for reducing the particle loss and keeping particles away from both channel sidewalls. Clean sheath flow is distributed to the entire width of CirEAC channel via a tube similar in design to the aerosol inlet one. The total flow exits the CirEAC from a tube in the same type as the aerosol inlet one. The effects of total flow rate, aerosol-to-sheath flow rate ratio, and particle size on the penetration curves of CirEACs were studied. It is found that the optimal performance of CirEACs was observed when operated at the total flow rate of 1.8 lpm and the aerosol-to-sheath flow rate ratio of 0.2. A 2-D model is also presented to derive the transfer function of CirEACs. The experimental transfer function of CirEAC LV agrees well with that estimated theoretically using the working sheath flow rate (defined as the portion of total sheath flow rate underneath the aerosol slit opening). In comparison, the CirEAC LV possesses a steeper penetration curve, wider total flow rate range (1.2 – 3.6 lpm), and broader particle size range (5 – 300 nm) than the CirEAC SV. The CirEAC SV however has a higher transmission efficiency than the CirEAC LV because of its shorter particle classification length.

Experimental Validation of an Aerodynamic Sectional Modeling Approach in Fixed-Wing Unmanned Aerial Vehicles

This paper presents an experimental validation of the aerodynamic sectional modeling approach as a general methodology to model the aerodynamic effects on aerial vehicles. The introduced modeling scheme also takes into consideration important effects such as engine load torque and landing gear contact. The dynamic model is derived after Kirchhoff’s formulation expressed as a function of the extended momentum vector. The effective forces and torques are included as the addition of independent sources. The aerodynamic effects are computed as the summation of forces and torques over each body section on the airplane, and the engine thrust and load are included in simulation by means of experimental transfer functions. The experimental validation is conducted using a RC-controlled fixed-wing airplane PT-40 instrumented with the proper avionics to measure its pose (position and attitude) for trajectory tracking purposes. A comparative analysis shows that the analytical model and the experimental setup describe similar qualitative behavior in an open-loop maneuver and reveals that the propeller load torque produces significant aerodynamic effects on both longitudinal and lateral-directional stability of the vehicle.

Assessment of railway ground vibration in urban area using in-situ transfer mobilities and simulated vehicle-track interaction

ABSTRACTThis article proposes an alternative approach to the well-known Federal Railroad Administration method to evaluate ground vibrations induced by the passing of railway vehicles. The originality lies on the excitation mechanisms that occur in urban areas. A common source of railway-induced ground vibrations is local defects (rail joints, switches, and turnouts) which cause large amplitude excitations at isolated locations along the track. To analyse such situations, a combined numerical-experimental study is developed, based on the use of numerical train/track results and experimental mobility transfer functions. The influence of building foundation type, vehicle, defect type, and size and location is evaluated through experimental data collected in Brussels (Belgium). The results show that it is possible to assess vibrations from light rapid transit systems in the presence of local rail defects and unknown soil conditions.

Eigenvectors phase correction in inverse modal problem

The solution of the inverse modal problem for the spatial parameters of mechanical and structural systems is heavily dependent on the quality of the modal parameters obtained from the experiments. While experimental and environmental noises will always exist during modal testing, the resulting modal parameters are expected to be corrupted with different levels of noise. A novel methodology is presented in this work to mitigate the errors in the eigenvectors when solving the inverse modal problem for the spatial parameters. The phases of the eigenvector component were utilized as design variables within an optimization problem that minimizes the difference between the calculated and experimental transfer functions. The equation of motion in terms of the modal and spatial parameters was used as a constraint in the optimization problem. Constraints that reserve the positive and semi-positive definiteness and the inter-connectivity of the spatial matrices were implemented using semi-definite programming. Numerical examples utilizing noisy eigenvectors with augmented Gaussian white noise of 1%, 5%, and 10% were used to demonstrate the efficacy of the proposed method. The results showed that the proposed method is superior when compared with a known method in the literature.