1,720,991 research outputs found
Rotational degrees of freedom data synthesis based on force excitation
No full textIn many structural dynamic tools, mobility matrices manipulations are foreseen. It’s understood that the full matrices, containing translational and rotational degree of freedom data are requested in order to obtain meaningful and correct results. Unfortunately the latter data that comprehend mobility function either with rotational responses or angular excitations are neglected due to the difficulties related to their measurements. In this article a method to derive RDOF data will be presented. The proposed technique relay on response data obtained with standard force excitation and on numerical matrix expansion techniques.
The approach is based on the assumption that, in the surrounding of the point of interest, within the frequency range of interest, the structure behaves as a rigid body. Under this condition, if reciprocity holds, some relations between linear and rotational motions can be established and the RDOF functions due to angular excitation can be derived
Optimal parameters for experimental dynamic testing
No full textModal testing is normally performed by experienced test engineers and technicians. On the base of their expertise, they define the best set up for extracting all the requested information from a vibrating structure. They decide how to suspend it and where, how many sensor and which kind of exciters have to be used and where they have to be placed, they select the best acquisition parameters and so on. In most of the cases this sensibility leads to excellent results, still sometimes, once the measurements are done, the quantity and the quality of the acquired data is not sufficient for the tasks ahead. Several parameters have been defined in the previous years to help the experimenters defining the best set-up. Often the interpretation of these indexes is subjective and applicability is limited due to the test pieces’ geometry. This paper presents two new pre-test indicators focused on the optimal sensor and exciter positioning that reduce the personal intervention. These indexes provide a large spectrum of solutions, supplying more possibilities for the experimenter to acquire a complete and reliable database
Modelling, testing and modification on a high performances ski
No full textSkis are very common sport tools. The competition among the different producers to promote and sell their products is very harsh and every seasons new technologies are introduced to improve ski performances and safety characteristics. Even if ski are quite simple in the geometry definition, their internal sandwich structures and the large number of materials and layers makes them somehow problematic to model. In this work, the results of the correlation analysis between the finite elements and the experimental models are presented. The results of the sensitivity analysis performed on the updated model are shown as well. The influence of different material characteristics and various structural modifications have been compared to suggest the best location and properties of a damper element used to reduce the ski vibration. Comparisons between different dampers technologies will be shown as well
Test planning techniques for improved vibration measurements and model correlation
No full textThis paper proposes a procedure for optimising sensors placement during experimental testing. The selection of those degrees of freedom (DOF) aim at improving measurements quality and delivering the best possible data set for further correlation analysis between experimental and numerical models of the structure under in- vestigation. The procedure is based on two selection algorithms: one direct, based on the manipulation of the numerical mode shapes of the structure, the second, iterative, based on the optimisation of the experimental auto-MAC matrix. The paper will present both the theory and some examples
Rotational degrees of freedom : an historical overview on techniques and methods
No full textMost of the tools used in the analysis of vibro-acoustic systems are based on the manipulation of frequency response function (FRF) matrices. The different procedures foresee the use of complete matrices where both translational and rotational degrees of freedom (TDOF and RDOF) information are contained. Unfortunately the second group of data is very often neglected with substantial errors in the final results. In order to overcome this problem, known since the early sixties, many solutions have been tried using existing sensors, developing new ones, defining different methodologies and approximations.
This article will give a short review of the different techniques used to measure and excite accurately the RDOF data. Following a historical path, the different procedures will be presented, the finite difference methods, the t-block approaches, the multi-point approximations, the single beam LDV, the multiple laser systems, the scanning laser procedure, the magnetostrictive moment shaker, the pure moment exciter and so on. This work will hopefully help the researchers involved in dynamic modelling to select the most appropriate tools and procedures
A novel updating technique for large finite element models
No full textTo obtain reliable and accurate finite elements models, engineers often relay on model updating procedures. These techniques modify the original numerical model on the base of experimentally obtained information, like for example natural frequencies or mode shapes. In most cases, the updating procedures are developed and tested on simplified test structures, where either the dimension of the model or the number of finite elements types are limited to contain the computational costs and elaboration time. Unfortunately, when the same pro- cedures are applied to real industrial cases the updating operations tend to become lengthily and numerically unstable. In this work a novel updating procedure is presented. It consists of two different phases: one devoted to the matching of the natural frequencies between experimental and numerical databases, the second, focused on the matching of the FRFs amplitudes. In both steps, great care has been taken to minimise the computational burden and the operator interference
Implementation of a DRC (Delayed Reference Control) for Contact Force Control in Robotic Application
No full textInstantaneous Angular Speed: comparisons between Torsional Laser Vibrometer and Encoder-Counter estimations
No full textIn recent years, some issues of the condition monitoring of rotating machinery have been tackled with the Instantaneous Angular Speed (IAS) analysis. In fact, from the variation of the IAS during the machine loads’ cycle it’s possible to identify defects and faults. The current work focuses on the estimation of the IAS through the Elapsed Time (ET) method and compares the results with the measurements of a B&K torsional laser vibrometer type 2523, allowing to highlight the differences between the methods and the influences of the processing parameters.
The ET is based on the measurement, with a counter, of the time elapsed between the pulses of an encoder. Obviously, both encoder’s and counter’s characteristics, (the number of pulses/revolutions and the counter frequency), influence the range of applicability and the quality of the estimation. The B&K torsional laser vibrometer instead relies on the lasers’ signal correlation and the angle resampling technique. The measurements are performed on an unloaded asynchronous four poles electrical motor. This is driven by an adjustable frequency AC drive system to simulate different working speed conditions. The IAS is estimated between the 150rpm and the 1500rpm range. The study shows the advantages and disadvantages of the two measurements approaches, reports their limitations and suggests a novel procedure for deriving the IAS form encoder/counter signals elaboration
Transfer Path Analysis of the Agusta Westland AW-109 performed by means of in-flight data and POLYMAX synthesized FRFs.
No full textThe TPA presented in this paper has been performed on the Agusta Westland AW-109 helicopter. A
combination of laboratory measured FRFs and in-flight operational data has been gathered: while the
structural and acoustic FRFs have been collected in a measurement campaign held at Politecnico of Milan
employing the helicopter mock-up, the operational data, i.e. the paths acceleration, were obtained on the
actual helicopter in a succeeding session that took place in the Agusta Westland factory.
Since the data analysis has brought to light some corrupted data concerning the set of the structural FRFs,
an attempt to perform the TPA using POLYMAX synthesized FRFs has been brought to completion with
the aim of verifying at first the reliability of such an approach and then to assess whether this technique
could be employed to substitute corrupted data with synthesized ones when the database occurred to be
unreliable
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